V. PATHOGENIC MICROORGANISMS
A. Epidemiology - This is the study of the occurrence of disease in
populations. It deals chiefly with the incidence, control, and
prevention of disease in populations.
l. Public Health Service - Epidemiological studies are the function of the Public Health Service in the United States. Certain diseases such as AIDS, syphilis, etc. must be reported to the PHS. All of the information which this agency gathers is fed into its nerve center for analysis. This is the Centers for Disease Control (CDC) located in Atlanta, Ga. During the outbreak of typhoid fever which occurred here in San Antonio a few years back, it was the CDC which sent in epidemiologists who eventually traced the outbreak to its source.
2. World Health Organization - Internationally, epidemiology is the function of the World Health Organization (WHO), a subdivision of the United Nations. WHO works in cooperation with the PHS and equivalent agencies in all of the nations of the world. As infectious disease does not recognize national borders, international cooperation is essential for effective control to take place.
B. Definitions of disease prevalence
l. Endemic diseases - Those which infect a relatively small and constant percentage of a given population. African sleeping sickness is a good example.
2. Sporadic diseases - Those in which localized, unpredictable outbreaks occur. Typhoid fever is an example.
3. Epidemic - Whenever a disease increases significantly above its normal percentages. Currently both genital Herpes and AIDS are considered epidemics. Note that to be considered epidemic a disease does not have to infect a large percentage of the population, only increase significantly above its norm.
4. Pandemic - This is the term given to an epidemic that moves across national borders. Influenza is a good example.
C. Disease transmission - Diseases that can be transmitted from host to host are said to be communicable. Most microbial disease (but not all) are communicable. Requirements for a communicable disease include the following.
l. Reservoir - This is the source of the infection. The common
reservoirs are listed below.
a. Human beings - This is the most important reservoir. Humans may be infective (actively shedding the organism) while they have no symptoms of the disease. They may also be carriers, apparently healthy individuals who harbor and shed pathogenic organisms.
b. Animals other than humans - These represent reservoirs of
zoonoses, diseases which are transmissible to humans. Plague and rabies are two examples.
c. Arthropods - These included the insects and arachnids. They lot only serve as reservoirs for disease, but also act as vectors (agents which spread the disease) as well. The spread of malaria by mosquitos is the most famous example.
d. Soil - Certain infectious agents live and multiply in the soil. Most pathogenic fungi fall into this category as well as the bacterial disease, tetanus.
e. Food - Fresh, raw food is generally safe. If it comes from diseased organism then it can become a primary reservoir. Most food borne disease comes from improper handling and subsequent contamination. Under these circumstances, food is a secondary reservoir.
f. Water - Only a very few pathogens (Pseudomonas aeruginosa, Flavobacterium meningosepticum multiply well in water. However, due to large scale pollution of water, it serves as an important secondary reservoir.
2. Transmission - This is the movement of infectious organism from one host to the next. Transmission mechanisms include the following.
a. Contact transmission - This is spread by actual contact with the organism. It may include the following methods.
(1) Direct contact - This is person to person. Kissing, sexual contact, or touching are major methods. Direct contact can also transmit disease from reservoir to humans. Rabies is a case in point.
(2) Indirect transmission - This is brought about by means of a nonliving object usually termed a fomite. Cups, pencils, bedding, etc are all examples.
(3) Droplet infection - This is spread by droplets of saliva or mucous which result from sneezing or coughing. Generally these droplets travel less than one meter.
b. Common vehicle transmission - This refers to the transmission by a common (group) inanimate reservoir. These would include food, water, drugs, blood, etc. These are responsible for outbreaks of disease, such as food poisoning.
c. Airborne transmission - Refers to spread by air borne droplet nuclei or dust at a distance of more than one meter from the reservoir to the host. Such droplets are very small and usually evaporate in the air, leaving a nuclei that contains an organism. Spores of fungi are frequently transmitted this way.
d. Vectors - These are animals that transmit disease from one host to another. The most important ones are the arthropods (insects, ticks, lice) which transmit disease. They may transmit the disease mechanically, attached to the body parts such as legs or wings, or biologically in which the organism has part of its life cycle within the arthropod.
3. Portal of entry - The third requirement is some way to invade. Most organisms have a normal portal of entry and if they enter other than by this portal, they may not be pathogenic. Important portals of entry include the following.
a. Breaks in the skin.
b. Respiratory tract.
c. Eyes and ears.
d. Digestive tract.
e. Urogenital tract.
4. Portal of exit - This is the last requirement for a communicable disease, a way of getting out of the body. The portal of exit for a given organism may be the same as its portal of entry, or completely different. They generally include the same types of areas as the portals of entry.
D. Prevention of transmission - This is the major goal of public health. Control is based upon a detailed knowledge of portals of entry, exit, vectors, and reservoirs. By managing one or more of these areas the transmission cycle may be stopped. Some of the major means of control which are commonly used include the following.
l. Safe water - This includes proper analysis and treatment of all drinking and recreation waters.
2. Sewage disposal - Proper treatment and disposal of all human and animal sewage. This includes both biological and chemical treatment.
3. Safe food - This includes vaccination and disease control in herds, proper handling of food, pasteurization of milk, proper canning, and other appropriate preservation techniques.
4. Control of human reservoirs - This includes quarantine, identification of carriers, treatment of infections, and follow up on post infections.
5. Vaccination - This is the decrease in susceptibility of the population to a disease by promoting active immunity by means of injection. Some of the most spectacular success of public health, such as the eradication of small pox has been due to vaccination of populations.
6. Controlling animal disease - This involves vaccination of animals as well as eradication of animal reservoirs such as rats.
7. Controlling arthropods - In those diseases where an arthropod is the vector, it is frequently easiest and cheapest to control the vector. Mosquito control is an example.
E. Mechanisms of pathogenicity
1. Infection - This is invasion of the body tissues by a parasitic organism. The infection may cause no obvious symptom or it may adversely affect the host in which case it is termed pathogenic.
a. Noscomial infection - These are hospital acquired infections. This is a highly significant source of infections.
(1) The CDC estimates that 5 to 15% of hospital patients acquire an infection.
(2) In the US, 2 million people obtain a noscomial infection each year. Of those, 100,000 die!
(3) Reasons for the high incidence of infections include a wide variety of microorganisms in the hospital environment, weakened status of patients health, and chain of transmission (contact with staff, invasive procedures, catheters, etc).
(4) According to the CDC, hand washing by hospital personnel is the most important means of prevention of infection. Studies have shown that in many hospitals, care givers only wash their hands about 25% of the time before interacting with patients.
2. Factors in infection
a. Dose - This is the number of organisms that enter the body. It is important in establishing infection as only a few invading organisms will usually be stopped immediately by the defense system and no infection will result.
b. Adherence - This represents the attachment of the organism to the host tissues. Generally adherence is necessary if pathogenicity is to occur. Pathogens have surface molecules termed adhesins (ligands) which interact with receptors on the host cell membrane.
c. Virulence - This is the ability of an organism to cause disease. In a given pathogenic species there may be several strains of differing virulence. Low virulence strains will cause only mild disease while high virulence strains will cause severe disease. Virulence depends upon several two major factors.
(1) Aggressiveness - This is the ability of the organism to reproduce and spread in the body. Highly aggressive organisms evade the defense systems and spread rapidly throughout the body. Aggressiveness is dependent upon several factors.
(a) Capsules - These help bacteria and fungi to resist and evade phagocytosis.
(b) Cell wall components - These are chemicals that help the organism resist body defenses. The waxes in the wall of Mycobacterium is a case in point. The M protein of Streptococcus pyogenes is another example.
(c) Enzymes - These are extracellular enzymes that aid pathogens in the destruction of body tissues.
(d) Antigenic variation - This is where certain pathogenc alter their surface antigens which aids in evading the immune system. Some have several sets of genes that control surface antigens that can be utilized. By the time one immune response has been mounted, the organism produces a new set of antigens that require a new immune response.
(e) Penetration of the cytoskeleton - Many organisms produce proteins that rearrange the filaments of the cytoskeleton of the attacked cell. These so called invasins allow the organism to manipulate the cytokeleton to bring the organism into the cell and then move it around in the cell.
(2) Toxigenicity - This is the ability to produce toxins. These are chemical substances which damage the tissues. Organisms which are not very aggressive but which are highly toxic can be very virulent and cause severe disease. There are two major classes of toxins produced by microbes.
(a) Endotoxin - These are integral parts of the cell structure. They are not released until the microorganism dies and breaks up. In G- organisms the endotoxin is part of the cell wall, specifically, a lipopolysaccharide.
(b) Exotoxin - These are protein toxins which are synthesized by the organism and then released. There are three principal types.
/1/ A-B toxins (type III)- These have two parts. The B part binds the toxin to the cell and the A part is the enzyme that causes the effect. Most toxins are of this variety. Diphtheria is one example.
/2/ Membrane disrupting toxins (type II) - Cause lysis of host cell by disrupting plasma membranes. S. aureus and C. perfringins are examples.
/3/ Superantigens (type I) - These are bacterial proteins that provoke a highly intense immune response. This results in high levels of cytokines that give rise to symptoms like fever, nausea, vomiting, diarrhea, and even cardiac shock and death. Examples include Staph antigens that cause food poisoning and toxic shock syndrome.
Exotoxins are often given descriptive names such as neurotoxins which attack nerve tissue and enterotoxins that attack the lining of the digestive tract. Others are named after the organism that produces them botulinum toxin. Others after the disease they cause, the diphtheria toxin for example or the tetanus toxin.
(3) Pathogenic effects of viruses - All viruses interfere with cellular metabolism by seizing the genetic and metabolic machinery of the host cell. This interference usually produces cytopathic effects (CPE). These include cell death, blockage of mitosis, host cell fusion, chromosomal changes, inclusion bodies, and destruction of the cells by the host immune system.
(4) Variations in virulence - The virulence of an organism can be increased or decreased over several generations by selecting the environment in which it must live. Growing a pathogen in an unfavorable environment generally leads to a reduction in virulence, a fact first discovered by Pasteur, and still utilized today in vaccine production. On the other hand, growing an organism in a succession of hosts can lead to an increase in virulence.
F. Survey of microbial pathogens - When surveying disease there are two principal approaches. One is by organism group, i.e., Gram + caused disease. The second is by examining body systems, for example, all diseases associated with the digestive tract. As this is how a disease is usually seen in a clinical setting, it will be the approach which we follow.
G. Infections of the skin - The skin, being the barrier between the sterile internal environment of the body and the microbe laden external environment frequently becomes infected.
l. Normal flora - The skin has a number of organisms that call it home. These range in numbers from about l000 per square cm of skin on the back to about l0 million per square cm on the skin of the scalp and armpit. Although many different species are found on the skin, five groups account for the majority of organisms.
a. Staphylococci - G+ facultative aerobes found on most of the skin. They inhabit the skin surface. Some species such as S. aureus are opportunistic pathogens.
b. Micrococci - G+ aerobes that often contain bright pigments.
c. Diphtheroids - Found principally in moist areas. These G+
metabolize lipids and in doing so release fatty acids that inhibit many pathogens. One group, the propionibacteria are G+ anaerobes that live in the sebaceous glands and produce propionic acid which keeps the pH of the skin between 3 and 5.
d. Yeasts - These are found on many parts of the skin where they utilize lipids. Pityrosporum is a common example. Some species are opportunistic pathogens.
2. Bacterial diseases
a. Folliculitis [Boils (furuncles) and carbuncles] - These are usually caused by Staphylococcus aureus The organisms penetrate into the dermis, usually via a hair follicle, where they induce a severe inflammatory response. There is a great deal of swelling, pain, and much pus. The pressure usually causes the boil to erupt, draining to the surface. Carbuncles are particularly deep penetrations. When the pus moves towards the surface it drains via several different hair follicles. Carbuncles are potentially dangerous because they may rupture inward establishing secondary abscesses all over the body. Invasion of an eyelash hair follicle produces a similar condition termed a sty.
b. Impetigo - This is a superficial skin infection. Bacteria grow beneath the dead layer of the epidermis. The result is a thin walled blister that breaks and is replaced by a crust. There is usually no pain or fever. The cause is usually either S. aureus or Streptococcus pyogenes As will be seen, both of these organisms can cause a wide range of problems.
c. Scalded skin syndrome - This is caused by a toxin produced by certain strains of S. aureus. It begins as a bright read area around nose and mouth which rapidly spreads. Within 48 hours the skin begins to peel off in sheets when touched. This is a serious disease, more frequent in children under two, which requires intensive antibiotic treatment.
d. Toxic shock syndrome - This is a life threatening disease involving fever, vomiting, and red rash which are followed by shock. It is caused by a toxin produced by a lysogenically converted strain of S. aureus. It was originally associated with the use of tampons during the menses. Now most cases are associated with absorbent packing used following surgery and with women following birth. Scalded skin syndrome may appear in the later stages of toxic shock.
e. Erysipelas - This is a disease caused by Streptococcus pyogenes. It is an infection of the dermal layer of the skin that reuslt in reddish patches with raised margins It can result in tissue destruction and even sepsis. High fever is common. Controlled by antibiotics. As will be seen, S. pyogenes is associated with a number of different diseases. One strain produces an exotoxin that serves as a superantigen. This precipitates destruction of tissues, the so called "flesh eating bacteria." This disease is highly dangerous and mortality rates of 40% occur.
f. Pseudomonas dermatitis - This is a rash that lasts about two weeks. If has been associated with hot tubs and saunas. The condition is self limiting. The cause is usually Pseudomonas aeruginosa which has multiple resistance factors. This organism also causes a condition in competition swimmers known as otitis externa, or swimmers ear, a painful ear ache in the outer canal of the ear.
d. Acne - This is the most common skin disease known. It begins with blockage of the sebaceous glands which causes their secretion, sebum, to accumulate. This leads to "blackheads." The bacterium Propionibacterium acnes metabolizes sebum into fatty acids that trigger an inflammatory response. It is the inflammatory response that leads to the tissue damage and subsequent scars. Frequent cleansing of the skin and medications containing benzoyl peroxide are used. In severe cases antibiotics such as tetracycline may be prescribed. Isotretinoin (Acutane) may also be used in severe cases. This blocks sebum production but it has potentially severe side effects (teratogenic and depression). A newer treatment exposes the skin blue light which seems to help.
3. Viral infections
a. Herpes simplex - This is a DNA virus with two major serological types, designated I and II. Type one causes fever blisters and cold sores. These are lesions that occur on the lip, gum, or tongue. They are usually recurrent. Almost everybody has been exposed to type I but not everyone develops fever blisters. A great deal of research is currently being expended in a search for drugs and preventatives for the herpes viruses. Direct contact and droplets are the means of transmission.
b. Chicken pox (varicella) and shingles - Both of these diseases are caused by the virus known as herpes zoster. Chickenpox is usually a childhood disease. Following exposure, which is usually respiratory, the virus multiplies in respiratory epithelium. It then moves via the blood to the skin where it replicates. A rash of small lesions develops. These lesions may rupture and then crust over. In children this is a mild disease that rarely has complications. A vaccine is available and recommended.
Shingles normally occurs in adults. It apparently is due to the virus from a childhood case of chickenpox becoming latent and then reactivating later. Lesions appear on the skin in patterns that follow the peripheral nerves. Nerve ganglia become inflamed. Intense burning pain with fever is experienced. Healing can take several weeks. Persons under severe stress, undergoing cancer treatment, or in an immunodeficient state are particularly susceptible.
Reye's syndrome is a rare complication of chicken pox and other viral diseases such as influenza. Vomiting and brain dsyfunction are symptons. This occurs almost exclusively in children and teenagers. This condition is dangerous and can be fatal. Aspirin seems to increase the chances of its occurence.
c. Measles (rubeola) - The causative virus is a paramyxovirus, an envelope-bearing RNA virus. This is a highly contagious disease, with respiratory transmission, similar to chickenpox in that the skin breaks out in a red rash. Measles can have severe side effects including encephalitis. Secondary bacterial infections can cause pneumonia. Prevention is by means of vaccination. The rubeola vaccination is recommended for all individuals.
d. German measles (rubella)- This is causes by a large RNA virus found in the togavirus group. The disease is usually very mild, and nearly 30% of those infected show no symptoms at all. It can cause serious fetal malformations during the first trimester of pregnancy. For this reason, all child-bearing age women should
be vaccinated against the disease.
e. Smallpox (variola) - This viral disease is now a historical curiosity, but at one time it was the scourge of the world. Before the days of vaccination, 80 out of l00 people could expect to contract the disease and of these, 25% would die. By mass vaccination all over the world, smallpox has been eliminated. The last known natural case was apparent in Somalia in 1977. The eradication of
smallpox is considered the major triumph of public health.
f. Warts - These are caused by papilliomaviruses. The virus causes an uncontrolled growth of skin cells. The growth is benign and stops. The most common treatment is removal by chemicals, freezing, or surgically.
g. Roseola - This is childhood disease which is quite common. High fever followed by a red rash are usual symptoms. Caused by the herpes type 6 virus.
4. Fungal diseases - The skin is subject to a number of fungal
infections, most of which are relatively harmless.
a. Candida albicans - This organism is commonly found among the normal flora but in some people it invades the nails and skin causing infection.
b. Dermatophytes - This is a general term for organisms belonging to the genera Trichophyton, Microsporum, and Epidermophyton. These organisms keep to the surface of the skin and cause a number of infections commonly known as ringworm, jock itch, and athletes foot. They are generally referred to as tineas, for example, tinea capitis, ringworm of the scalp. Most of the unpleasant effects are due to secondary bacterial infections and body allergic responses.
c. Sporotrichosis - This is a subcutaneous mycoses and is more serious that the cutaneous dermatophytes. It is caused by the dimorphic fungus Sporothrix schenckii. It is often obtain by thorns from plants and is common in gardeners. Once in the body, it gets into the lymphatic system resulting in lesions on the surface tracing the lymphatic drainage. It is treated by ingesting a dilute solution of potatassium iodide.
4. Eye infections - Although not part of the skin, the eye is exposed to many of the same organisms on its surface and does have several common infections.
a. Conjuunctivitis (Pink eye) - Inflammation of the cunjunctiva usually caused by Haemophilus influenzae. A wide variety of bacteria and viruses can cause the same condition. This condition has increased with the increased use of contact lenses.
b. Trachoma - A serious infection caused by Chlamydia trachomatis, an obligate intracellular parasite. This is the greatest cause of infectious blindness in the world. World wide there are estimated 500 million active cases and 7 million blinded victims. In the US, it occurs primarily in the southwest, especially among native american people. It is transmitted by hand contact or sharing personal items. Treated with antibiotic ointments.
H. Infections of the nervous system - The CNS (brain and cord) is one of the best protected parts of the body. It is surrounded by bone and inside of the bone are a series of protective membranes termed the meninges. Consequently infections that reach the CNS must do so by moving up the peripheral nerves, from the circulation, or by breaks in the protective barriers.
l. Bacterial infections
a. Hemophilus meningitis - This is the most common cause of bacterial meningitis, an inflammation of the meninges. The causative organism is Hemophilus influenzae type B, a Gram negative coccobacillus. It affects primarily children under five and is not pathogenic in adults. The disease often begins as a middle ear infection. An early sign is a painful stiff neck. This is followed by high fever and convulsions. Without prompt antibiotic treatment mortality is high. A vaccine now exists and is recommended for all children at 24 months. Since introduction of this vaccine, the incidence of this disease has decreased 99%.
b. Meningococcal meningitis - This is caused by Neisseria
meningitidis. This is a Gram-negative diplococcus. The
principal signs are severe headache, stiff neck, and high levels of PMNs in the cerebrospinal fluid. Development of the disease is frequently rapid. Patients may be dead within 24 hours of having felt fine. The disease is highly contagious and is characteristic of crowded conditions such as military barracks and college dormitories. Spread is by air borne droplets. Treatment
is by massive doses of antibiotics, especially penicillin. Life support systems may also be necessary. Prompt treatment is a must as, left untreated, fatality rates are high. A vaccine exists for high risk groups and for use during epidemics, but it is not effective against serotype B which causes about 50% of the cases.
c. Pneumococcal meningitis - This is caused by Streptococcus pneumoniae. It is most likely in young children and old people. It was the rarest of the meningeal bacterial infections, but since the introduction of vaccination against Hib, it has become the most common. It also has the highest fatality rate (30% to 80%). A vaccine is now available and recommended for children under two.
d. Leprosy (Hansen's disease) - This is a disease of the peripheral nerves caused by Mycobacterium leprae. This organism has a very long generation time (about l2 days) which accounts for the long incubation period in humans (a minimum of two years and up to l0 years). There are two distinct clinical forms of the disease.
(l) Tuberculoid - This is the more benign form of the disease. Infection is limited to the peripheral nerves and in most cases the disease arrests itself in a few years. Progressive nerve damage may lead to deformity.
(2) Lepromatous - This involves most of the tissues of the body in addition to the nerves. It is in this form that personal contact can lead to transmission. Blindness and severe deformity can result if this form is not treated.
(3) Treatment - Antibiotic treatment (dapsone and rifampin) stop the spread of the infection and render the patient non-infective in about three to six months. Consequently the long isolation of the leper colony is a thing of the past. Treatment must continue for years and perhaps even life. A vaccine is currently being tested. This vaccine was developed from armadillos in which the organism will
(4) Incidence - There are only about 5000 cases in the U.S. (Texas is a major leprosy center). There are about l2 million cases world wide, especially in underdeveloped countries.
e. Listeriosis - This is a form of meningitis caused by the gram-positive rod Listeria monocytogenes. The organism rarely causes obvious disease in healthy adults, but causes serious meningitis in immunodeficient individuals such as cancer patients. In pregnant women it causes spontaneous abortion and stillbirth. It is fecal transmitted both from animals and humans. In human outbreaks it is usually food borne, especially from dairy products. A few years back there was a major outbreak due to the ingestion of Mexican cheese which had been made from non-pasteurized milk.
f. Tetanus (lockjaw - This is one of the most dangerous diseases resulting from anaerobic infection. Fatality is high, even when treatment is prompt and extensive. The disease is caused by C. tetani, Gram positive endospore forming rod that lives in the soil, especially those rich in organic matter such as barn yards or gardens. The organism is not aggressive and usually does not proceed past the small anaerobic wound area. It releases a toxin which is absorbed into the blood and disables certain pathways in the CNS. The result is uncontrollable contractions of the skeletal muscles resulting in spastic rigidity. Control is by immunization with the DPT shot.Once the disease has been contracted it is treated with antitoxin, but that cannot effect toxin that has already bound to the nerve cells.
g. Botulism - This is the most deadly food poisoning known. It leads to paralysis and death. It is caused by the anaerobe Clostridium botulinum which produces an exotoxin that is considered to be the toxic substance on earth. It blocks the release of acetylcholine by the nerve endings and therefore results in paralysis, the opposite of tetanus. It is usually found in improperly processed canned foods. If its endospores are not killed during processing they can germinate in the anaerobic environment created by the canning process. Treatment is by administering antitoxin early after poisoning has occurred. The toxin may be neutralized by boiling suspected foods for l5 minutes. Suspicious foods should never be eating even after boiling.
(1) Infant botulism - This sometimes occurs in infants less than one year old. The infants ingest spores which grow in their intestine which is largely anaerobic. This does not happen in adults because the clostridia cannot compete with the normal flora. This is however undeveloped in infants. About 30% of the reported cases are associated with ingestion of honey. Honey frequently shows positive for botulism endospores. Infants under one year should not be given honey.
2. Viral diseases
a. Poliomyelitis (Infantile paralysis) - The early symptoms of this disease are similar to those of meningitis, pain, fever, and a stiff neck. Later there is paralysis and wasting away of paralyzed muscles. The poliovirus is enteric and only secondarily attacks the motor nerves. Of people infected, only about 10% ever show any symptoms, and in only 1% of the cases does it go into the paralytic form. In its enteric form it causes only a mild gastroenteritis. Control is by vaccination. The killed and injected Salk vaccine was largely supplanted in this country by the oral Sabin vaccine which is a live attenuated virus. Newer versions of the injected vaccines are lower risk than the Sabin and in 2000, the CDC recommended that IPV, injectable polio vaccine be used for all routine immunication.
b. Rabies - This is one of the most feared of all diseases, not only because of its agonizing manifestations, but because it is also almost always fatal. It is caused by an rhabdovirus. It is an acute infectious disease of the central nervous system and is almost always transmitted by the bite of a warm blood animal. Once in the body the virus passes slowly to the CNS via the peripheral nerves. The incubation is 30 to 60 days.
(l) Diagnosis - Examination of smears of brain tissue from animals or persons suspected of having the disease is the usual procedure. Infected nerve cells when stain show the presence of viral inclusions called Negri bodies.
(2) Control - The reservoir for rabies is wild animal populations. In the U.S. it is chiefly skunks, bats, racoons, and foxes. Most people contract the disease from the bite of a domestic animal that has in turn been bitten by a rabid wild animal. Consequently human populations can be best protected by vaccinating all of the domestic animals. Currently, experiments are being conducted with rabies vaccine laced baits for wild animals. It is hoped that this may eventually vaccinate most wild animal populations as well.
For people who have been exposed there is a relative safe and effective vaccine.
c. Encephalitis - This is a viral induced inflammation of the brain. Mosquitos serve as vectors for the arbovirus (arthropod borne virus). The mosquito genus known as Culex is the most common vector. Hosts include not only man, but many species of birds and mammals as well. For example equine encephalitis infects horses and men, causing death, mental disorder, or motor nerve impairment. A more recent strain, the west Nile virus, first appeared in 1999 in Zoo birds in New York City and has moved across the country. Most case are mild or subclinical but some have proved fatal. Virus reservoirs include populations of wild birds. Control is by mosquito eradication although some vaccines are also available.
3. Fungal infections
a. Cryptococcoses - The organism Cryptococcus neoformans can cause a severe form of meningitis. This is a soil organism and thrives in soils contaminated with pigeon droppings. It is frequently found in urban areas where pigeons roost. Inhalation causes an initial lung infection that can spread to other parts of the body including the meninges and brain. Left untreated it is usually fatal. Treatment is usually with amphotericin B and flucytosine.
4. Protozoan infections
a. Naegleria microencephalitis - This disease is caused by the amoeba Naegleria fowleri. In the U.S. most cases are found in the southern tier of states, including Texas. The organism is usually encountered by swimmers in ponds or streams, especially those which are stagnating. The amoeba crawls up the victims nose, enters the olfactory nerves, and proceeds to the brain. Fatality rate is about 100%. Fortunately, the disease is rare.
5. Prion infections - Prions are abnormally folded proteins that can induce a change in the normal shape of a protein, causing them to clump. These infections have long incubation periods (years), and CNS damage is slow, progressive, and ultimately fatal. These diseases are collectively termed transmissible spongiform encephalopathies, or TSE.
a. CJD (Creutsfeld-Jakob disease) - This is a disease similar to
sheep scrapie. Over a period of months, there is a gradual loss of motor control and death. Transmitted by corneal transplants, and other exposure to infected tissues such as autopsies. Incineration of infected tissues is the only control.
B. BSE (bovine spongiform encephalopathy, also known as mad cow disease. So named because infected animals become unmanageable and must be destroyed. In Great Britain there was a major outbreak which was followed by cases in humans of a variant of CJD (vCJD). It is suspected this came from eating infected beef. Because of the feed route, the use of cattle feeds that contain processed cattle body parts has been banned. Only one case of BSE has been detected in the USA, and that was from a cow that came in from Canada.
I. Infections of the blood vascular and lymphatic systems
l. Bacterial infections
a. Endocarditis - This is an infection of the lining of the
heart, and is usually localized in one of the valves. It most often occurs in an abnormal heart. The disease typically begins with fever and a loss of energy that continues for several months. Diagnosis is by means of blood cultures as the organisms are usually shed from the valves into the blood. Most infections are due to invasions from the normal flora, especially Staphylococcus and Streptococcus. Control is by means of antibiotics.
b. Septicemia - This is the so called "blood poisoning" and is associated with microorganisms and their products circulating in the blood.
(l) Manifestations - Fever and dropping blood pressure are characteristic. The drop in pressure is probably due to heart and vessel damage. In severe cases the damage to blood vessels may be irreversible. If the pressure drops too low then adequate circulation will not be maintained and shock will result.
(2) Causes - This is usually the result of an infection somewhere else in the body. Gram negative and positive bacteria as well as fungi can cause the condition. Septicemia due to Gram negative rods is a devastating disease almost always causing shock. About half of these types of cases are fatal. Gram
negative organisms cultured from the blood in these cases usually show E. coli, Enterobacter aerogenes, Serratia marcescens, or Proteus mirabilis.
(3) Sequence of events for Gram negative septicemia - The exact mechanism is not known, but the probable events are as follows.
(a) Bacteria enter the blood from a focal infection.
(b) Endotoxin is released.
(c) Endotoxin acts on blood vessels causing dilation.
(d) Dilation of blood vessels results in blood pressure drop.
(e) Decreased pressure result in inadequate blood flow and tissue damage.
(f) Death results in many cases, even with antibiotic use, because of the circulatory damage done by endotoxin.
c. Puerperal fever (childbirth fever) - The principal cause of this disease is Streptococcus pyogenes. Transmission is by direct contact at childbirth with contaminated instruments or hands. Septicemia is a frequent outcome.The Hungarian physician, Semmelweiss, was the first to make the connection between dirty hands and this disease. He worked in a hospital in Vienna, Austria around the middle of the l9th century (before the germ theory of disease). By instituting disinfection of instruments and hands he managed to cut the rate from l2% to l%. Modern sanitation has made this disease almost non-existent although outbreaks still occur.
d. Rheumatic fever - This is caused by strain of S. pyogenes. It can spread from the throat to the joints and heart causing the disease rheumatic fever. It is considered to be an autoimmune problem, although its exact mechanism is not known. Heart failure and death may be the result but the disease normally subsides with medication. Often the heart valves are left permanently damaged and the result is a heart murmur.
e. Tularemia (rabbit fever) - This disease is characterized by high fever and swelling of the lymph nodes. Before the days of antibiotics it was often fatal. The causative organism is known as Francisella tularensis. It is principally a disease of wild animals and arthropods. Skinning wild rabbits has been a source and hence the name. Out west the disease is most often contracted from the bite of a tick. Typically an ulcer appears at the site of infection, lymph node in the area enlarge and fill with pus, and the organism spreads throughout the body via the blood and lymph. The disease is similar to bubonic plague but milder. Treatment is by means of antibiotics. Although this disease is rare, it has been placed on the list of reportable diseases by the CDC because of potential use as a bioweapon.
f. Brucellosis (Undulant fever) - This disease is contracted
primarily from cattle, pigs, and goats. In these animals the bacteria cause chronic infection of the mammary glands and uterus. This results in milk contamination and abortion.
(l) Manifestations in humans - Fever, which typically spikes each evening, body aches, and weight loss are characteristic. These symptoms usually subside but may appear several more times in the next few weeks or months. In the U.S. 60% of the cases involve people who work in the meat industry. Antibiotics knock out the disease readily. Without treatment the disease is debilitating but rarely fatal.
(2) Causative organism - Brucella abortus , a small gram negative rod, is most common in cattle but there are five other species each with its own host preference.
g. Anthrax - This is primarily a disease of domestic grazing animals caused by the gram positive endospore forming Bacillus anthracis. This is soil dwelling organism. In humans the major risk is to people who handle animal products such as hides and wool. There are three forms
(1) Cutaneous anthrax - Brought about by contact with endospores. 90% of naturally occurring cases are of this type. Ulceration at the site of infection with a black scab are common. Low grade fever and malaise are the other symptoms. In most cases this does not enter the blood but if it does then the disease becomes serious.
(2) Gastrointestinal anthrax is due to ingestion of undercooked food contaminated by endospores. Nausea, abdominal pain, and blood diarrhea are symptoms. Mortality rates reach 50%. Fortunately this is a rare form of the disease.
(3) Inhalation anthrax - The most serious form of the disease caused by inhalation of the endspores. Initial symptoms are mild, fever coughing a chest pain. Once bacteria enter the blood then septicemia results that kills within 24 to 36 hours. At this stage mortality rate approaches 100%. This is the form which is most feared as a bioweapon. Weapons grade endospores are treated in such a fashion as to make then disperse in the air and be subject to inhalation.
Antibiotics are effective if administered in time. Ciprofloxacin (Cipro) and doxycycline are effective. They may be used prophylactically if one has been exposed to endospores. There is a vaccine but it requires six booster shots and has limited duration.
h. Gas gangrene - This disease is caused by Clostridium. perfringens. This organism invades dirty wounds where it produces gas bubbles and toxins. The bubbles press against the blood vessels blocking blood flow which leads to tissue death and increases the amount of anaerobic tissue in which the organism can live and multiply. By this method the infection can spread rapidly throughout the body destroying tissue as it moves. There is a large amount of vile smelling exudate. Treatment in hyperbaric chambers has proven useful in halting the spread of the disease. Prevention involves cleaning wounds promptly and using only sterile dressings and instruments.
i. Animal bites - Various infections can be transmitted to wounds caused by animal bites or scratches. The most feared is rabies but other diseases (including tetanus) can also result from these types of wounds.
(1) Pasteurella multocida - This the most common pathogen in most animal bites. It is a Gram negative rod closely related to the organisms that causes plague and tularemia. Infections can be severe, but respond rapidly to antibiotics.
(2) Cat scratch fever - This is a mild disease that usually causes swelling and infection of the lymph nodes surrounding the wounded area. The causative organism appears is a Gram negative bacillus, Bartonella henselae. There are 22,000 cases in the US each year.
j. Bubonic plague (black death) - This disease has been a scourge since classical times. During the middle ages fully 25% of Europe's population was killed by repeated epidemics of this disease. The causative organism is Yersinia pestis, a Gram negative rod belonging to the enterobacterial group. Plague is mainly a disease of rodents and is transmitted to humans by the bite of a flea which has fed from an infected rodent. Once infected, the organisms are carried to the lymph glands causing enlargements known as "bubos." Large numbers of organisms are released into the blood and become distributed all over the body. Bleeding beneath the skin may occur and this gives the patient a dark or dusky appearance, hence the black death name. Control is by antibiotics and control of wild animal reservoirs, especially rats in urban areas. A vaccine of limited duration is available for high risk individuals.
(1) Pneumonic plague - This occurs when the bacteria enter into the respiratory tract and enter respiratory secretions. This form does not require a flea bite, but may be transmitted from person to person. Left untreated the fatality rate approaches l00%. Isolation of plague victims and prompt antibiotic treatment is always called for. Anyone coming in contact with someone with this form should receive prophylactic antibiotics.
k. Relapsing fever - This is a spirochete disease caused by Borrelia recurrentis. Transmission is by tick or louse. The disease is named for the frequent and recurring fevers. Relapse occurs at intervals of 5 to l5 days. Prevention is by vector control. Treatment is with antibiotics.
l. Lyme disease - The causative organism is Borrelia burgdorferi and the principal vector is the deer tick, Ixodes dammini. The tick actually contracts the spirochete from the white footed mouse on which the tick larva feed. The disease is centered in the northeast and midwest. In about 75% of the cases the first symptom will be a red dot at the site of the bite with rings around it, giving the impression of a bull's eye. It usually expands until it is about 15 cm in diameter. Flu like symptoms follow within a couple of weeks. A second phase may occur in which heart problems and neurological problems occur. Months or even years later a severe arthritis may ensue. The disease responds well to antibiotics, especially if they are administered in the early phases.
m. Epidemic typhus - This is caused the rickettsial organism, Rickettsia prowazekii. It is spread from person to person by the body louse, Pediculus humanus. This disease tends to occur under crowded unsanitary conditions such as those that occur during war, and has caused more deaths during wartime than armaments. Patients develop fever abruptly, followed in a few days by rash and mental confusion. Damage to small blood vessels occurs resulting in hemorrhage and gangrene beneath the skin. During epidemics mortality ranges from l0 to 40%. Control is by insecticides that kill the louse. Vaccines are available.
n. Rocky mountain spotted fever - This is caused by the organism Rickettsia rickettsia. The reservoir for this disease is wild rodents and the vector is the tick Dermacentor andersoni in the west, and D. variablis, (dog tick) in the east. This is a serious disease which resembles typhus. A vaccine is available and broad spectrum antibiotics are effective. In spite of the name, the incidence of this disease is greatest in the mid Atlantic states of the east coast.
o. Erlichiosis - This is caused by Ehrlichia, gram negative, rickettsia-like, obligately intracellular bacteria that multiply in white blood cells. Once a disease only of dogs, cattle, and sheep, ehrlichyiosis is now an emerging human disease. The vector is a tick. The disease has two forms with two different tick vectors. It is a mild flu-like disease, but can have a significant fatality rate( 5%) especially in immunocompromised patients.
2. Viral infections
a. Myocarditis - This is an inflammatory disease of the heart muscle. It may be caused by bacteria, fungus, or protozoa, but most commonly is caused by viruses. Infection may cause only slight pain and electrical irregularities, but severe cases can lead to heart failure. In most cases the virus is a coxsackie form. Only a few individuals infected with coxsackie viruses ever develop myocarditis.
b. Mononucleosis - This is a disease of the lymph nodes. It is most common among upper middle class people between the ages of l5 and 25. Fever, sore throat and enlargement of lymph nodes and spleen are the usual symptoms. Because kissing seems to be an important means of transmission this is sometimes termed the "kissing disease." Causative organism is the Epstein-Barr virus. This virus has also been associated with type of cancer known as Burkitt's lymphoma.
c. Yellow fever - This viral disease is also transmitted by the bite of a mosquito. Characteristically fever, aches, and vomiting occur. Bleeding for the nose and GI tract also occur, along with jaundice. Mortality rates during epidemics reach 40%. Control is by means of mosquito eradication and a live virus vaccine.
d. Dengue (breakbone fever) - Similar to yellow fever but milder. This is also transmitted by the bite of a mosquito. The mosquito which transmits both dengue and yellow fever is the infamous Aedes aegypti. This mosquito is largely confined to tropical regions of the world. Recently several Aedes albopictus mosquitos were discovered in Harris county, Texas. These are from Asia and can efficiently transmit dengue as well as yellow fever and encephalitis. Because of this, public health officials are carefully tracking this mosquito..
e. Viral hemorrhagic fevers - These are a series of viruses that cause very serious and frequently fatal disease. Most seem to come out of Africa and have such names as Lassa fever, Marburg viral fever, Congo hermorrhagic fever, and Ebola hemorrhagic fever. Headache, pain, internal hemorrhaging, and shock are characteristic. Transmission is by direct contact with body fluids. Although these disease are rare in the U.S. small outbreaks have occurred with importation of monkeys and with people returning from regions where the diseases are endemic. Because of their contagiousness and high fatality rate, they are carefully monitored.
f. Cytomegalovirus (CMV) - This is a large herpes virus that infects nearly everyone. It remains latent in white blood cells like the EBV. Infection is usually asymptomatic or very mild but in immune compromised individuals it can be life threatening. If qcauired duirng pregnancy, serious damage to the fetus can result. Transmission if by body fluid contact, kissing etc.
3. Protozoan infections
a. Toxoplasmosis - This is caused by the sporozoan protozoan Toxoplasma gondii. It can invade a wide range of both mammals and birds, sometimes causing expensive epidemics among commercially important species. Although human infection is common, overt illness is not. Three main patterns are found.
(1) Fetal infection from an asymptomatic mother. The baby may be have convulsions, brain damage, blindness, and die.
(2) Infections of adults, especially those with serious underlying illness. Under these circumstances it may manifest itself as myocarditis or general system involvement.
(3) A mild infection of young adults resembling mononucleosis. The disease seems to be spread by improperly cooked meat and by close association with cats who are commonly infected.
Because of the potentially serious fetal complications, pregnant women are usually tested for the organisms presence. A special test called the TORCH (toxoplasmosis, rubella, cytomegalovirus, and herpes) test will test for this problem as well as several others.
b. Malaria - This is a protozoan disease caused by several species of the genus Plasmodium. These include vivax, falciparum, malariae, and ovale. The parasites invade the red blood cells where they multiply. The most characteristic feature of malaria is the spasms of fever that occur at l to 3 day intervals. The corresponds to the multiplication time of the parasite. All of the red cells harboring the reproduced organisms rupture at once, releasing organisms which then infect other cells. It is the release period that corresponds to the fever period. The destruction of red cells leads to severe anemia which may be fatal in itself or cause a general debilitation which permits secondary infection. It is currently estimated that 300 to 500 million persons worldwide are infected with malaria. There are 2 to 4 million deaths each year. In Africa, one child dies every 30 seconds from Malaria.
(l) Control - In the past control has been primarily by mosquito control. However during the last few years many pesticide resistant forms of the mosquito have evolved leading to a resurgence of malaria. Malaria may be prevented by taking chloroquine one week prior to, during, and for 6 weeks following a visit to a malaria area. Extensive work is taking place on the development of a vaccine. This has proved to be difficult but researchers seem to be close.
J. Infections of the respiratory tract
l. Normal flora - The respiratory system opens to the outside and provides a warm, moist environment. This provides a good environment for many different organisms. The upper respiratory tract (above the larynx) has a rich normal flora, but they do not penetrate below the larynx into the lower respiratory tract, largely due to efficient removal systems. Diphtheroids, micrococci, staphylococci, streptococci,
and small numbers of environmental organisms such as Bacillus
are commonly found.
2. Upper respiratory tract infections
a. Bacterial infections
(1) Strep throat (Streptococcal pharyngitis) - This is the most important infection of the upper tract. The causative organism is a serotype of Streptococcus pyogenes and organism that is the cause of several other diseases. Strep throat is a relatively mild disease from which most people recover in about a week. Probably only about 50% of the cases of "strep throat" are really caused by strep, the others being different bacteria or viruses. Strep throat can become more serious as may be seen in diseases which will be discussed next.
(a) Differentiation of strep species - All streptococcal organisms can be placed into one of three different groups based upon their growth on blood agar medium.
/1/ Alpha hemolytic type - Produce a brown to green zone around their colonies on blood agar. The zone of discoloration is from one to several millimeters wide.
/2/ Beta hemolytic type - The produce clear, colorless zones around their subsurface colonies. The zones are due to hemolysin secreted by the organisms. This is where the strep throat organism is found. The pathogenic forms belong to the group A family of serotypes. There are 55 serotypes in this one group. Immunity is type specific and therefore to be completely immune to "strep throat" would require immunity to all 55 serotypes, an unlikely proposition.
/3/ Gamma type (non-hemolytic) reactions - These produce no visible change in blood agar. Most are not pathogenic.
(2) Scarlet fever - Certain strains of S. pyogenes produce an erythrogenic toxin which is absorbed and carried to the skin by the blood. The result is a red rash and thus the disease name. The toxin is under the control of a temperate phage which infects the bacterium.
(a) Control of strep infections - Adequate ventilation and the avoidance of crowds helps control the spread. Antibiotics can clear diagnosed cases. The incidence of rheumatic fever has dropped in recent years, possibly due to the treatment of strep throats with antibiotics.
(3) Diphtheria - This disease is caused by Corynebacterium diphtheriae. It usually begins with a mild sore throat and a slight fever, accompanied by a great amount of fatigue and malaise. Swelling of the neck may be dramatic and a whitish gray membrane forms on the tonsils, throat, or in the nasal cavity. Heart and kidney failure as well as paralysis may follow, due to the toxin produced by the bacterium. Lysogenic conversion of the bacterium is necessary for toxin production. The toxin blocks protein synthesis in the cells which absorb it. The organisms have very little invasive ability. Transmission is air borne.
(a) Control - Vaccination of a toxoid results in production of antibodies. The childhood shot is usually the DTaP (diphtheria - tetanus - acellular pertussis), all three vaccines being given at once. An outbreak of diphtheria occurred in San Antonio in l970. Sixty-six cases and three death occurred, all in people who had not been immunized.
(4) Ear ache (Otitis media) - This is due to an infection of the middle ear derived from the throat via the eustachian tubes. Inflammation and pus build up resulting in pressure behind the ear drum which in turn causes the throbbing ache. A variety of organisms cause ear ache. The most common bacterial causes include Streptococcus pneumoniae, S. pyogenes, and Haemophilus influenzae. About 70% of the cases are due to bacterial infection.
b. Viral diseases
(1) Common cold - These disorders can be caused by any one of approximately 200 different viruses. About 50% are caused by rhinoviruses, small RNA viruses. Another 20% are due to coronaviruses, and the remainder to several other viral groups. The virus invades the cells that line the upper tract where the irritation causes increased nasal secretions, sneezing, and swelling of the mucosal and nasal tissue resulting in partial or total blockage of the airways. The infection is usually limited but can extend into the ear, sinuses, and lower respiratory tract. Transmission seems to be mainly by direct contact, usually with hands.
2. Infections of the lower respiratory tract
a. Bacterial infections
(1) Whooping cough - This is primarily a childhood disease which at its onset has nasal congestion and a mild cough as the principal symptoms. After about two weeks, these symptoms are replaced by spasms of violent coughing followed by loud gasping sounds as the patient draws a breath. The causative organism is Bordetella pertussis a small, encapsulated, Gram negative rod. Treatment of existing cases is by means of antibiotics and respiratory support. Prevention was by means of the DPT shot. The pertussis vaccine was a crude bacterial extract which had side effects. It was replaced in 1996 by an acellular vaccine DTaP which is safer.
(2) Tuberculosis - This is a chronic infection of the lungs manifested by fever, weight loss, cough, and sputum production. It lasts for months or even years. In some patients the disease spreads to the bone, meninges, and kidneys. The disease is caused by the organism Mycobacterium tuberculosis a slender, acid-fast, irregular shaped rod.
(a) Pathogenesis - Bacilli are inhaled in air which has been contaminated. They lodge in the lungs, triggering an inflammatory response. They are ingested by macrophages but may not be destroyed, and in fact can actually multiply inside of the macrophages. After about two weeks a CMI response develops. The bacteria are surrounded by macs, some of which coalesce to form large multinucleated cells. T-cells collect around these multinucleated cells and wall them off from the surrounding tissues. This type of CMI response is termed a granuloma and is typical for infections that resist digestion and removal. The granulomas formed during TB are termed tubercles. Inside the bacteria continue to live but do not multiply. Tubercles may rupture and release organisms to other parts of the lung.
(b) Diagnosis - The tuberculin skin test is the principal screening test. A strong positive test is thought to indicate the presence of living organisms in the body. Chest X-rays may show tubercles.
(c) Control - Identification of persons with TB and treatment with antimicrobial drugs (rifampin and isoniazid) is the principal control method. Because of the resistance of the organism, treatment may have to continue for one or two years. World wide, TB is the number one bacterial killer. The emergence of AIDS has increased its presence dramatically as AIDS patients are highly susceptible to TB. It is estimated that currently over 2 million people have both conditions. There has also been a resurgence in the United States , especially in Texas which leads all states except California and New York in TB cases. A vaccine known as the BCG is available and used widely in Europe but not in the United States, except for high risk individuals.
(3) Bacterial pneumonia - Pneumonia is an acute infection of the lungs. If one or more entire lobes of the lungs are involved it is termed lobar pneumonia but if it is scattered in patches it is termed bronchopneumonia. Lobar is by far the most serious and in the days before antibiotics frequently resulted in death. There are several different types of bacterial pneumonia.
(a) Streptococcus pneumoniae (Pneumococcal pneumonia) This diplococcus is the most frequent cause of lobar pneumonia. They exist in a virulent (encapsulated) form and a non-virulent (non-encapsulated) form. Both forms are frequently found among the normal flora. When virulent forms enter the alveoli they multiply and elicit an inflammatory response. Serum and phagocytic cells pour into the alveoli resulting in difficulty in breathing and sputum production. Sometimes the pneumococcus attaches to the pleural membranes which surround the lung and cause pleurisy. They may also enter the blood and cause endocarditis or meningitis.
/1/ Control - In the early stages antibiotics such as penicillin and tetracycline effectively control the infection. Advanced cases are more difficult to treat and can still prove fatal. A vaccine exists for high risk individuals.
(b Haemophilus penumonia - This is caused by the gram-negative coccobacillus known as Haemophilus influenzae. Patients in poor health, alcoholics, poor nutrition, cancer, or diabetes are prime candidates. Treated with antibiotics, especially second generation cephalosporins.
(c) Mycoplasma pneumoniae - This disease is often referred to as primary atypical pneumonia. Because the pathogens lack cell walls, penicillins and other similar acting antibiotics have no effect. Tetracyclines are effective. The disease is similar to the pneumococcal form but not so acute. Because of its milder symptoms this is sometimes referred to as walking pneumonia.
(d) Legionellosis - This is a particularly virulent form of pneumonia usually called Legionnaire's disease because of the major outbreak in l976 which took place at an American Legion convention. Twenty nine people died. The causative organism is Legionella pneumophila and other members of the same genus which are Gram - rods. The disease may be controlled with antimicrobial drugs especially erythromycin. The organism is normally found in soil and water. It also multiplies in standing water such as that often found with large air conditioners. It becomes airborne in vapor or dust and can then be inhaled.
(4) Ornithosis - This pneumonia like disease is caused by Chlamydia psittaci. Fever, headache, and chills are characteristic. Stress seems to increase the probability of developing a clinical level case of the disease. Transmission is by the inhalation of dried bird droppings. Many species of fowl are susceptible to this disease. Antibiotic treatment is effective
(5) Q fever - This is another rickettsial disease caused by Coxiella burnetii. Clinically it resembles influenza and is therefore different than most rickettsial diseases. Transmission is by ticks or direct contact with infected animals. Broad spectrum antibiotics control it.
b. Viral infections of the lower respiratory tract
(1) Viral pneumonia - This sometimes occurs as a complication of influenza or chicken pox. In infants respiratory syncytial virus is the most common form of viral pneumonia. Symptoms include coughing and wheezing which continues for more than one week. The drug ribavirin can be given as an aerosol. It seems to diminish the severity of the disease.
(2) Influenza - This is a lower tract disease that differs from pneumonia in that the bronchiole tubes and not the air sacs are principally involved. The cause is a group of viruses known as theInfluenzavirus. The disease is unpleasant, but rarely fatal in itself. Fatalities from flu usually are due to secondary bacterial infections.
(a) Control - Vaccines exist for many of the flu varieties. One difficulty is that the flu virus mutates rapidly, presenting new antigenic characters to an immune system primed for another strain.
(b) Epidemics and pandemics - Influenza seems to sweep the world periodically. Such world wide epidemics are termed pandemics. One of the worst of all time was the l9l8 pandemic which involved an estimated 500 million people in a six to eight week period, and 20 million died. Interestingly, contrary to most flu strains, the fatalities were highest in the young adults.. Major flu epidemics occur about every l0 years, probably due to mutation of existing viruses into forms not previously seen by the immune systems of the
(3) Hantavirus pulmonary syndrome - This is a disease which has had several outbreaks primarily in the western states. It is a frequently fatal pulmonary infection, in which the lungs fill with fluid. Transmitted by inhalation of hantaviruses in dred urine from infected small rodents. Prevention is by sanitation and rodent control.
c. Fungal infections
(1) Coccidioidomycosis - This is termed valley fever or desert rheumatism. It is a disease of certain hot, dry, and dusty regions of the Americas. The causative organism is known as Coccidioides immitis which lives as a mould in the soil, and as a yeast in the body. Fever, cough, chest pain, and loss of appetite are symptoms. Diagnosis is by skin test, microscopic examination of sputum, and laboratory culturing. Transmission is by the inhalation of spores along with dust from the ground. The disease resembles TB but is milder and usually subsides in a month. Serious cases are treated with amphotericin B. Prevention is largely by dust control.
(2) Histoplasmosis - Similar to coccidioidomycosis except for the distribution which is spotty throughout the temperate and tropical regions. The disease is caused by Histoplasma capsulatum a dimorphic fungus which prefers soil contaminated with bird droppings. Caves and chicken coops are major sources. Identification is by means of skin tests. For most people the disease is not serious (although it can be).
(3) Pneumocystis pneumonia - This is caused by the organism known as Pneumocystis jiroveci. The taxonomic position of this organism is obscure. Some think it is a protozoa, others a fungus. It has characteristics of both. While many people are infected, few show any symptoms. The disease is most prevalent in immunosupressed individuals, especially aid patients. Its presence is considered to be a primary indicator of full blown AIDS. Treatment is by means of antibiotics
(4) Other Fungi can cause respiratory disease on an opportunistic basis. Aspergillus, Rhizopus, and Mucor can all cause infections if spores are inhaled by persons with impaired immune systems, diabetes, or cancer.
K. Infections of the digestive system -
For purposes of this discussion the digestive system will be divided into the mouth (oral region) and the gastrointestinal tract.
1. Oral disease
a. Normal flora - A number of different microorganisms inhabit the mouth. The warm moist environment coupled with abundant nutrients and many nooks and crannies makes the mouth one of the more hospitable environments for microbes. Numbers can range from several millions in a ml of saliva to about 40 billions in a gram of plaque (tooth deposit). This normal flora is significant because it is usually an overgrowth of these that causes oral disease.
b. Natural defenses of the mouth - Overgrowth of normal flora is prevented by the following activities.
(1) Shedding of epithelium and the organisms attached to it.
(2) Rinsing action of saliva.
(3) Neutralization of microbial acids by buffers in saliva.
(4) Microbial inhibitors of saliva such as lysozyme and dextranase (breaks down dextran, an important component of plaque).
(5) IgA which is secreted in saliva.
c. Plaque - This is a coating formed by microorganisms plus an organic matrix. It forms on tooth surfaces which are not cleaned and it is the major cause of dental disease.
(1) Formation of plaque
(a) Initially a thin pellicle (membrane) made up of positively charged salivary proteins attaches to the tooth enamel. This is due to the electrical attraction of the negative enamel.
(b) Aerobic cocci and filamentous bacterial attach to the pellicle and begin to grow.
(c) Plaque mass increases due to microbial growth and addition of organic molecules. An important contributor to plaque is dextran, a polymer formed by various bacteria, especially Streptococcus mutans, when sucrose (table sugar) is metabolized.
(d) Accumulating plaque results in anaerobic conditions immediately next to the tooth surface. This permits the growth of anaerobic species.
(2) Microbial activities in plaque
(a) Acid production - When supplied with sugar, plaque bacteria produce lactic acid. If sugar is frequently in the mouth acid production may be continuous.
(b) Toxin production - Microbes in plaque below the gum line produce endotoxins which stimulate inflammation. This contributes to periodontal (gum) disease.
(c) Calculus formation - Certain microorganism in plaque below the gum line will causes precipitation of calcium and phosphate thereby causing the plaque to harden into calculus. Calculus can damage the epithelium by cutting and tearing.
d. Dental caries - The acid produced by bacteria in plaque dissolves away tooth enamel creating cavities or caries. Once the cavity breaks through into the living pulp of the tooth, tissue destruction will occur and abscesses will form.
e. Periodontal disease - This is an inflammatory disease of the supporting tissues of the tooth and its roots. If left untreated it will ultimately result in tooth loss and is the leading cause of tooth loss in middle to old age Americans. There are three clinical forms.
(1) Gingivitis - Inflammation of gingiva caused by plaque and calculus in the gingival crevice between the tooth and gum. It does not cause separation of the tooth from its supports.
(2) Periodontitis - This follows untreated gingivitis. The gingival crevice enlarges due to an epithelial retreat which is a response to the chronic inflammatory condition. This creates a bigger pocket which permits rapid overgrowth. The pocket becomes anaerobic and the organisms growing there produce a number of substances which dissolve away the connective tissue including bone in which the teeth are embedded. As a result the teeth loosen and fall out.
(1) Acute necrotizing ulcerative gingivitis (ANUG, "trenchmouth") - This occurs in individuals between ages l3 and 30, apparently precipitated by stress. There is acute necrosis of the inter-dental epithelium of the gum. Pain, bleeding, and whitish membranes on the gums occur. Antibiotics and local cleaning are the treatments of choice. The name trenchmouth came from the wide incidence of this disease among soldiers in the trenches during World War I.
f. Bone infections
(1) Osteomyelitis - This occurs whenever microbes enter spongy bone or red bone marrow. In the mouth is may result from advanced caries, extractions, or severe periodontitis. Intense pain, fever, chills, and necrosis of bones are characteristic. Strep species along with Staphylococcus aureus are frequent causes.
(2) Dry socket - This sometimes occurs following an extraction. The clot which normally protects the hole left in the bone by the extraction becomes dislodged and infection follows. Tissue destruction and osteomyelitis are possible outcomes. It is treated by cleaning and antibiotics.
g. Focal infections - These are localized infections that shed organisms to other parts of the body. Tooth abscesses as well as many dental procedures (even brushing) can result in the entry of microbes into blood. One of the more severe consequences can be endocarditis, and infection of the heart lining and valves that can be fatal. It is especially common in people with abnormal hearts such as rheumatic hearts. Individuals of high risk usually must take an antibiotic treatment prior to dental procedures.
h. Oral manifestations of non-dental diseases - Many non-oral diseases may have oral manifestations or be transmitted via the saliva. Of particular interest to dental health personnel are hepatitis B and Herpes type I which are both saliva transmitted. In the past, dentists have had four times the incidence of hepatitis B as did the general population. A vaccine against hepatitis B is now available and is recommended for high risk people such as dental health care personnel and people who routinely come in contact with blood. The AIDS virus has also been detected in saliva, but there are no known cases of salivary transmission. Apparently the dose necessary for infection is much greater than that found in saliva.
i. Prevention of oral disease - Oral disease is almost l00% preventable. This is accomplished largely by controlling plaque. This can be done in the following ways.
(1) Regular brushing and flossing of teeth.
(2) Utilization of fluoride, preferably in the diet (drinking water) but as topical applications when fluoridated water is not available. Fluoride greatly toughens enamel and dentin. It is especially useful in pregnant women and young children where the teeth are still forming. It also has been shown to stabilize calcium in both teeth and bones and thus is useful to older people in the prevention of osteoporosis.
(3) Controlling diet, especially the frequency of carbohydrate ingestion in the form of sucrose.
(4) Current research is directed towards developing a vaccine against the microbes principally responsible for plaque formation.
2. Gastrointestinal infections
a. Normal flora - The flora of the mouth is absent from the stomach due to the high acidity and other antimicrobial agents found in stomach. The small intestine is also relatively free of microbes. The colon is a virtual fermentation chamber, containing millions of organisms per gram of material. These organisms break down material which has not been digested and absorbed in the small intestine. In doing so they produce a variety of products, including vitamins. This normal flora of the colon is harmless as long as it is confined to the colon. They can become pathogenic if they obtain entry into other parts of the body.
b. Enterobacterial infections - These are Gram - facultatively anaerobic rods which are almost always associated with the intestinal tract. Many are pathogenic. Transmission of enterobacterial infections almost always involves entry by the mouth and exit with the feces.
(1) Shigella dysentery - The most common species causing this disease is Shigella sonnei which causes a relatively mild disease. S.dysenteriae causes a much more severe form. The organisms invade and multiply within the cells of the intestines. This results in intense inflammation and ulceration of the of the tract. Symptoms include severe diarrhea, vomiting, and fever. Severe cases can be fatal. Proper treatment with antibiotics usually resolves most cases. There is a problem with resistant strains.
(2) Enteropathogenic Escherichia coli - Although this organism is a part of the normal flora, certain strains produce toxins that can cause a severe gastroenteritis (vomiting, diarrhea). Enterotoxin causes the small intestine to secrete an abnormally high amount of fluid, much more that can normally be reabsorbed by the intestinal tract. The result is a massive loss of fluid and electrolytes. This is the principal cause of travellers problems. It has been given many colorful names (Montezuma's revenge, Hong Kong dog, Delly belly, etc).
In recent years, enterohemorrhagic strains have become common. The best known serotype is O157;H7. This has the ability to adhere to the intestinal mucosa and produce shiga like toxins so the disease more closely resembles bacterial dysentery and can be fatal. This organism is found in the intestinal tract of many animals including cattle. Probably 90% of ground meats have some degree of contamination and therefore should always be cooked well.
(3) Salmonellosis - This is a disease that is on the increase world wide. It manifests itself as a gastroenteritis that can range from unpleasant to fatal. Most Salmonella are classified into serotypes or serovars. There are about 2000 known world-wide with about 50 occurring in the United States. The bacteria are found in the intestinal tract of many animals. Poultry and eggs are frequently contaminated. Raw milk has also been a major source.
(4) Typhoid fever - This is the most severe form of salmonellosis and is caused by S. typhi. It infects only human beings, and thus to contract the disease either food or water has to be consumed which has been sewage contaminated. The disease normally begins with a fever which increases over three days; this is followed by severe headache and abdominal pain. Sometimes intestinal rupture and hemorrhage occur.
Some individuals who appear to be perfectly healthy harbor the disease and are carriers. The most famous case is that of Typhoid Mary, a young Irish cook living in New York state in the early l900's. She is known to have been responsible for at least 53 cases over a l5 year period. At that time around 350,000 cases occurred in the U.S. each year. Today it is around 300, due largely to improved sanitation.
(5) Cholera - This disease is characterized by a huge outpouring of fluid from the intestine within a few hours. Sometimes as much as l5% of the body weight may be lost. This loss of fluid and electrolytes is frequently fatal unless promptly replaced. The causative organism is Vibrio cholerae It does not damage the intestinal tract as do some of the previous organisms. It produces an exotoxin which causes excess secretion of fluids by the small intestine. The colon is not affected, but its ability to absorb water is greatly exceeded.
c. Campylobacter gastroenteritis - This is a Gram negative, spirally curved rod, which is also a member of many different animal intestinal tracts. Almost all retail chieckens are contaminated with this organism. Cows frequently harbor this organism and may excrete it in their milk. It is now recognized that these organisms are second only to Salmonella in causing diarrhea in the U.S.
d. Helicobacter gastritis - This is caused by the organism known as Helicobacter pylori. This organism is unusual in that it can live in the highly acidic environment of the stomach. The organism causes inflammation of the mucosa of the stomach (gastritis). It is the cause of most cases of peptic ulcers. About 30-50% of the population is infected, but only about 15% of infected individuals develop ulcers indicating that environmental factors also play a role. Antibiotic treatment will cure most ulcer cases.
e. Control of intestinal bacterial infections - All of these infections can be treated with antibiotics, but the best treatment is prevention. The portal of entry and exit is the intestinal tract and thus the separation of food and drinking water from sewage is imperative. These diseases have been some of the greatest killers known. Their absence in developed countries today is a monument to proper sanitation. In fact a good argument can be made in support of the thesis that the high standards of health that we enjoy today is due more to the plumbers than to the physicians.
f. Viral infections
(1) Mumps - This illness is characterized by fever and painful swelling beneath one or both ears due to infection of the parotid salivary gland. It is caused by a paramyxovirus. Transmission is by saliva. A vaccine exists which provides partial protection, the MMR (mumps, measles, rubella) vaccine.
(2) Viral gastroenteritis - This is usually known as the 24 hour flu or intestinal flu. Diarrhea and vomiting are common but usually only last from l2 to 24 hours. In infants the rotavirus is the usual cause of diarrhea and vomiting which can last from two to seven days. It rarely affects adults probably due to immunity. In adults the principal cause of gastroenteritis of viral origin are the Norwalk-like viruses. Thess are members of the human enteric calciviruses which are single stranded RNA virsues. They have an incubation period of 24 to 36 hours. Nausea is the major symptom and lasts form 8 to 24 hours.
(3) Hepatitis - This means inflammation of the liver and is a disease that is often characterized by jaundice or yellowing of the skin and/or eyes. Although there are several infectious causes of hepatitis as well as some non-infectious causes, the most common cases are viral in origin. Currently hepatitis is the second most frequently reported infectious disease in the U.S. There are five different viruses that cause it.
(a) Hepatitis A - This is termed infectious hepatitis and spreads in epidemic fashion principally through fecal contamination of food and water. It is caused by the hepatitis type A (HAV), a RNA virus. The disease initially is marked by fever, nausea, loss of appetite, and abdominal tenderness. This is followed by jaundice. Symptoms clear rapidly and full recovery occurs by the end of three months in severe cases. Regeneration of damaged liver tissue may require a year. Mortality rate is about 1%.
There are vaccines that are recommended for high risk individuals.
(b) Hepatitis B - This is serum hepatitis which is spread mainly by contact with body fluids, notably blood, saliva, and semen. The virus is termed the hepatitis B virus (HBV) and is a DNA virus. This disease has a much longer incubation period that does the A type. The symptoms are similar to type A but vary widely in severity. Some individuals have subclinical cases and serve as carriers. Others have rapid and severe liver damage resulting in death. There is good evidence that the HBV can cause liver cancer. Mortality rates can reach 3%. A vaccine is available and is part of the recommended childhood regime. It is a must for health care workers. Employers whose employees come in contact with blood must offer the vaccine at not cost. It is a recombinant DNA vaccine.
(c) Hepatitis C (HCV) - This is hepatitis an single stranded RNA virus. A blood test developed in 1989 has proven useful in blood screening. Most cases are associated with IV drug use, exposure to blood, kidney dialysis, and sex. A majority of cases progress to chronic hepatitis. About 20% of these develop liver cirrhosis or liver cancer. HCV is probably the moar reason for liver transplantation.
(d) Delta hepatitis (HDV)- The delta agent is a defective virus that can only coinfect along with the HBV. However, the hepatitis that results is much more serious that other types and mortality rates are much higher.
(e) Hepatitis E (HEV) - This is much like type A and is spread in the same manner. It is found in parts of the world with poor sanitation.
(f) Treatment and control - There is no specific treatment for hepatitis. Prevention involves the usual sanitation practices common to all enteric disease prevention. Type B, C, and D prevention involves screening blood donors and avoiding injection with body fluids. A vaccine is available for HBV and is recommended for high risk personnel such as blood handlers and dental personnel as well as all children.
f. Protozoan infections
(1) Giardiasis - This is a disease characterized by diarrhea, cramps, bloating, and rarely, constipation. The causative organisms is a member of the flagellated protozoa known as Giardia lamblia Recovery occurs in a few days in some cases, but in others it may be prolonged into weeks. For complicated cases (which are rare) antimicrobial drugs may be used. The disease is contracted by means of ingestion of protozoan cysts. Diagnosis is by identification of cysts in the feces.
(2) Amebic dysentery - This unpleasant disease is caused by the protozoan Entameba histolytica It is a worldwide disease, especially in areas of poor sanitation. There are several thousand cases in the United States each year. The organisms are ingested in cyst form, hatch, and then actively feed upon the mucous and cells lining the intestine. Sometimes they burrow into the tissue, reach the blood stream, and then disperse to other organs such as the liver where they cause abscesses. The activity of the ameba on the colon lining results in severe diarrhea with blood. Drug therapy clears the organism but as with all enteric disease, proper sanitation is means of prevention.
(3) Cryptosporidiosis - Caused by Cryptosporidium parvum. The disease is cholera-like, with a duration 1- -14 days. It can be life threatening in immunodeficient patients. Other than rehydration, there is no good treatment. Transmitted by contaminated water.
g. Food poisoning - This is sometimes referred to as food intoxication because the causative organism does not establish an infection in the patient. Rather the organism grows in food, produces a toxin which is ingested, and it is the toxin that causes the symptoms. The major causes of food poisoning are bacterial and fungal.
(1) Bacterial causes
(a) Staphylococcus- One of the most common forms of food poisoning is causes by S. aureus Symptoms occur a few hours after food is ingested. Symptoms include nausea, vomiting, and diarrhea. Recovery occurs within the next few hours. The disease results from an exotoxin that is produced while the organism is growing in the food, especially foods high in carbohydrates (potato salad). Similar poisonings can be caused by Clostridium perfringens,and Bacillus cereus.
(2) Fungal poisons
(a) Ergot - This is a toxin produced by the fungus Claviceps purpurea which grows on improperly stored grain. It causes breakdown of capillaries, gangrene, hallucinations, and death. It is a disease mainly of an earlier age when grain storage methods were poor. One of the toxins is an hallucinogen similar to LSD.
(b) Aflatoxin - This is a toxin produced by Aspergillus flavus that grows on peanuts and stored animal feeds. Aflatoxin is the most potent liver carcinogen known. Consumption by animals can cause acute poisoning. A few milligrams can kill a dog in 72 hours. It finds its way into human foods by way of peanut butter and by grains that have been stored in damp areas. Rigid controls of food in the United States minimize the risk from this poison, but evidence suggests that it contributes to cirrhosis of the liver and liver cancer in both Africa and India.
L. Infections of the genitourinary tract
l. Normal flora - Like the other systems that open to the outside, the genitourinary tract has a flora that lives in the outer region on a permanent basis.
a. Urethra - Flora here include Enterococcus, Neisseria, Mycobacterium, and Bacteroides.
b. The female genital tract has a flora that varies with estrogen level. During active reproductive life when estrogen levels are high, the principal flora include the lactobacilli which produce lactic acid and therefore an acid pH in the vagina. Prior to puberty and after menopause, the pH is more neutral and the flora resembles that of the urethra.
2. Urinary tract infections - Cystitis - bladder infections - These are infections that commonly occur but do not reach the kidneys. There are a number of factors that predispose the urinary tract to infections.
a. Urine stasis - Any condition which prevents the urine from moving (paralysis, anaesthesia) can result in infection. Urine makes a good culture medium for many species and during stasis it accumulates increasing the risk of infection.
b. Females are more likely candidates that are males. This is due to the shortness of the female urethra, about l.5 inches as compared 8 inches for males, and the close proximity of the genital tract to the digestive tract.
c. Ninety percent of the infections originate in the normal flora. Most are bacterial. E. coli causes about 50% of the cases. Other Gram negative rods such as Proteus and Enterobacter are often involved. Pseudomonas aeruginosa is a very troublesome pathogen because is resistant to virtually every antibiotic. Enterococcus fecalis, a normal colon bacteria, is the most frequent Gram positive infectious organism.
d. If left untreated, cystitis may proceed into pyelonephritis, and infection of the kidneys. Treatment is by antibiotics,
3. Leptospirosis - Primarily a disease of animals, it can be passed to humans and sometimes cause severe kidney or liver disease. It is caused by the spirochete Leptospira interrogans. It is obtained by exposure to urine from animals, usually rats or dogs. It enters through skin abrasions in the skin or mucous membranes. Headaches, muscle aches, chills and fevers are typical symptoms. After several days the symptoms disappear only to return a second time. In a small number of cases the kidneys and liver become infected. Death is usually due to kidney failure.
4. Sexually transmitted diseases (STD) - This term is gradually replacing the older venereal term. Many diseases associated with other organ systems (Hepatitis B for example) can be transmitted sexually and thus STD is a more inclusive term than venereal which referred to sexual transmission only. Currently over 30 different diseases are known to be transmitted sexually and 15 million new cases appear each year in the U.S.
a. Bacterial infections
(l) Nongonococcal urethritis (NGU) - This disease is caused mainly by Chlamydia trachomatis. The most recent figures from the CDC indicate that this may be the most prevalent bacterial disease in the nation, surpassing even gonorrhea. In both males and females it causes an uncomplicated inflammation. It is treated with tetracyclines and sulfa drugs.
(2) Gonorrhea - This is the most abundant reportable bacterial disease in the country. It is caused by the Gram negative diplococcus, Neisseria gonorrheae.
(a) Manifestations - The course of the disease is different in the two sexes. In males it causes a severe inflammation of the urethra. There is pain during urination and a thick and copious pus discharge. It is usually self limiting in males and will clear up on its own accord. In females the organisms thrive in the cervix and fallopian tubes, causing pelvic inflammatory disease (PID). Symptoms of early cervix infections are mild and the victim may not be aware of them. Left untreated it will proceed into the tubes causing severe scarring and frequently resulting in sterility. Damage done also leads to ectopic pregnancy.
(b) Control - Identification and treatment of infected individuals with penicillin has been the usual control method. Penicillin resistant strains are appearing. Consequently more often now, fluorqunolone antibiotics such as ciprofloxacin are used.
(3) Syphilis - This disease is caused by the spirochete, Treponema pallidum. The organisms are so thin that they normally cannot be seen in Gram stains and special techniques must be utilized to visualize them. Syphilis occurs in three stages, primary, secondary, and tertiary.
(a) Primary - Here the disease is transmitted and grows in local areas of the genitalia. Usually there is a chancre (hard ulcer) which appears at the site of infection after about three weeks. The chancre may disappear in 4 to 6 weeks and the patient may think that he or she is cured.
(b) Secondary - This stage appears about 2 to l0 weeks later. A rash, mild sore throat, or other general symptom may appear. The organism has by now spread throughout the body. This phase lasts for weeks to months and then subsides. Fifty percent of the infections go no further.
(c) Tertiary - This represents hypersensitivity reactions to small numbers of organisms that grow and persist in the tissues. It usually appears 5 to 20 years after the infection. Symptoms depend upon which part of the body the allergic reactions occur in. If they occur in the walls of the heart or major blood vessels then the disease becomes life threatening. If they occur in the
eye, blindness results. In the CNS paralysis and
insanity are the end products. History is filled with persons of note who degenerated into madness, mental incompetence, or some other severe side effect due to neurosyphilis. Among these were Henry VIII, Ivan the Terrible, Catherine the great, Benito Mussolini, Emil von Behring (microbiologist), Lord Randolph Churchill,
and Adolph Hitler.
(d) Diagnosis - These are primarily by means of serological tests.
/l/ Wassermann - This was the original test but is used only rarely today. It is a complement fixation test. It was found that beef hearts had an antigen identical to one found on the spirochete. Mixing some of this antigen with a patient's serum will result in complement fixation if antibody against the organism is present.
/2/ Flocculation tests - These tests form visible
aggregates which are easier to detect than complement fixation. These tests have a variety of names based upon the investigator who developed each modification. One of the most widely used is the VDRL.
/3/ Other tests include an ELISA test using the VDRL antigen and fluorescent antibody tests.
(e) Control - Penicillin is effective in all stages. Other antibiotics also are effective. The best control is prevention. Public health workers attempt to track down all sexual partners of known cases but this is difficult. The increasing use of oral contraceptives and the concurrent decline in the use of physical birth control devices such as condoms has also contributed to the spread of syphilis as well as other STD. The resurgence of condom use with the advent of AIDS is resulting in a decline in syphilis cases.
(4) Vaginitis - This inflammatory disease is usually caused either by the fungus Candida albicans, the protozoan Trichomonas vaginalis, or the bacterium known as Gardnerella vaginitis. Gardnerella infections probably account for about 40 to 50% of all cases. Usually a pH of above 5 is required, which is considerably higher than the normal pH of 4 or less. Symptoms include a frothy discharge and a distinctly fishy odor. Anaerobic bacteria are required for this type of infection to take place and control is by the use of drugs which take out the anaerobes.
b. Viral infections
(1) Genital Herpes - This is caused by the Herpes type II virus (HSV-2). It expresses itself as lesions around the ccreproductive organs. Active lesions are infective. The condition is painful but not especially harmful. This is one of the most common of STDs in the United States today. In certain people the disease reoccurs. In others it never appears again after the initial infection symptoms. There is no cure. The disease can be severe in pregnant women. The baby can contract a fatal encephalitis from the virus which it obtains while passing through the birth canal. Blindness of the fetus can be another problem. For this reason, infected mothers usually have their babies delivered by cesarean surgery. There is also a statistical association of genital herpes and cervical cancer.
(2) AIDS (Acquired Immunodeficiency Syndrome) - Persons suffering from this disease experience a collapse of the immune system. As a result they become subject to a wide variety of infections and cancers. At one time it was considered to be 100% fatal, but new drug therapies have relegated to a chronic but controllable disease. Although this disease is primarily one of the immune system, it is covered here because of the importance of sexual transmission in its spread.
(a) Causative organism - The HIV (Human Immunodeficiency Virus) is the causative agent. This virus apparently causes the shutdown of T helper T cells (T4 cells).
(b) Transmission - The transmission of AIDS closely parallels that of hepatitis B. It is sexually transmitted and also transmitted by blood. High risk groups include active homosexuals, drug addicts, and persons who must receive large quantities of blood products such as hemophiliacs. Recent screening tests developed for blood has reduced the hazard to hemophiliacs. Transmission from mother to fetus also occurs.
(c) Control - Currently the treatment is termed highly active antiretroviral therapy (HAART). This involves a combination of different drugs. Because of success in treatment, AIDS is now considered to be a chronic but controllable disease. Control is extremely expensive. Secondary infections are treated with drugs. Prevention is largely by properly screening and handling blood as well as by practicing safe sex (monogamy or condom use). Intensive research is currently directed toward the development of a vaccine. However the rapid mutation of the virus makes vaccine development and drug treatment difficult.
(3) Genital warts - These are caused by the human papillomavirus (HPV). Sexual transmission results in around a million new cases each year. There are a number of different viral serotypes, and some of these serotypes are associated with cancer, cervical cancer in women and penal cancer in men. Fortunately, 90% of genital warts are caused by serotypes not associated with cancer. Precancerous conditions are much more likely in females than in males. Treatment is by removal. Frequently genital warts are removed by laser. Sometimes interferon is used.
c. Fungal diseases - Candida albicans - This is an oval budding yeast found in the normal flora of the mucous membranes in about 35 to 40% of the population. Normally it is not pathogenic. If the ecological balance of the body is disturbed then it may become infective on the mucous membranes of the genitourinary tract and GI tract. It frequently manifests itself as vaginitis in females. Predisposing factors include pregnancy, therapy with
oral contraceptives, diabetes, antibiotic treatments as well as other factors. Although most cases are not sexually obtained, they can be. Treatment is by means of antifungal drugs.
d. Protozoan diseases - Trichomoniasis - Members of the genus Trichomonas normally live in the genitourinary tract of both males and females. Under certain circumstances it can multiply rapidly and set up an irritation that can range from itching to severe burning. Most cases are found in the vagina although prostate infections can occur in males, and bladder infections in both sexes. The treatment usually consists of drug therapy.
J. Emerging infectious diseases (EID) - These are either new diseases or older ones which are changing and showing an increase in incidence.
1. Factors that contribute to EID.
a. Evolution of new serovars (antigenic types). Influenzas.
b. Overuse of antibiotics and pesticides which select for resistant populations. MRSA, malaria
c. Climate changes such as global warming which increase the distribution and survival of reservoir and vectors. Malaria and Hanta virus.
d. Spread of old diseases to new geographic areas by modern transportation. Cholera and West Nile Virus.
e. Contact with new areas by human beings due to construction, migration, ecological changes. Ebola and Lassa fever.
f. Movement of animal diseases into human populations. AIDS
g. Breakdown in public health measures, such as failure to properly vaccinate populations or breakdowns in hygiene. Diphtheria in former Soviet republics.
2. Some of the diseases discussed qualify or have recently qualifed as EID. These include AIDS, the African Hemorrhagic fevers, Hanta virus, West Nile Virus, and others.
3. Current EID.
a. Severe Acute Respiratory Syndrome (SARS) - This is caused by a coronavirus. It was first reported in Asia in 2003, and a world wide outbreak followed. Only eight people in the U. S. were found to be infected. The disease begins with a high fever, headache, and body aches. Most patients develop pneumonia. Spread is by person to person contact, principally by respiratory droplets. In the original outbreak, 8,098 cases were reported and 774 deaths. Because of its potential seriousness, this disease is being carefully monitored by both WHO and the CDC.