Disease Organisms Information Sheets
For Disease Organism Lab



Virus Group

General Traits found in all Viruses -

Viruses are so strange and specialized that they may not even qualify as real living things - they don't eat or move under their own power, and have virtually no metabolism unless they are inside a host cell. Viruses have coating molecules that stick to potential host cells, which the virus must then take over and use to reproduce. Cells that are made into virus factories are harmed, often killed, in the process. Symptoms of viral diseases tend to be a combination of 3 factors: effects from invaded cells, effects from the person's own immune response, and symptoms that serve to spread the viruses to new hosts.

Virtually all viruses are too small to be seen with a light microscope, so you are NOT expected to see or sketch any for this lab. Answer the other questions for the families of viruses below.

Herpes Viruses -

THE "ORGANISM": First, read the "General Traits found in all Viruses" at the beginning of the virus section. Herpes viruses are constructed with an outer envelope made from the host cells' own membranes, with viral proteins added. These viruses contain DNA. There are many different types of herpes viruses that can affect many types of body cells

DISEASE CAUSED: There is a list of human diseases caused by the various herpes viruses, including chicken pox and shingles, cold sores and genital herpes. Cytomegalovirus, only rarely a cause of serious disease, and Epstein-Barr, the cause of mononucleosis, are herpes-type viruses. In most of the common diseases, cells near the surface are attacked; this, combined with a surface-immunity reaction, causes blister-like lesions on the surface. Many herpes viruses also invade nerve cells, where the virus may remain dormant for long periods of time before re-emerging; the recurrent nature of cold sores, genital sores, and shingles comes from this ability. There are drug treatments available that in some people shorten outbreaks and/or lessen symptoms. There is no treatment to get rid of the virus.

TRANSMISSION: These viruses usually are shed from the lesions they produce, although some types can be passed in respiratory droplets as well. A person can be contagious without any obvious lesions, but this is unusual.

PREVENTION: Avoiding contact with infected individuals who are experiencing an outbreak helps, although someone can be infective and not know it. Use of condoms can prevent the spread of genital herpes.

Orthomyxoviruses -

THE "ORGANISM": First, read the "General Traits found in all Viruses" at the beginning of the virus section. These viruses infect many different types of animals - new variations of these viruses are thought to emerge from pig and duck populations into human populations. The viruses tend to infect ciliated cells of the upper respiratory tract.

DISEASE CAUSED: Influenza. Death of infected cells tends to provoke a reaction in the respiratory lining, leading to coughing and increased mucus production. As more cells are affected, a general reaction, including fever, weakness, and aching may follow. The viruses themselves seem to become inactive after about a week. Influenza by itself tends to not be life-threatening, but the damage it causes can lead to bacterial infections which are much more dangerous. There have been episodes in the past to indicate that some strains of flu, such as the one that spread around the world during World War I and killed large numbers, that are extremely dangerous.  Some diseases commonly called "flu" are not caused by viruses from this group.

TRANSMISSION: Viruses shed into the fluids of the respiratory system are spread by coughing and sneezing when breathed in by the next host. Antibodies to a specific strain are produced, preventing reinfection, but there are a great many related strains that would not be affected by that antibody response. Vaccines tend to be strain-specific as well. 

PREVENTION: Vaccination can prevent infect by certain strains, but strains may arise and spread to rapidly for a vaccine to be developed. Among infected people, avoiding infection may require a surgical mask, but the less spraying of droplets from coughing and sneezing, the better.  There are some drug treatments available.

Picornaviruses -

THE "ORGANISM": First, read the "General Traits found in all Viruses" at the beginning of the virus section. This group includes a variety of different types, such as rhinoviruses and polioviruses.

DISEASE CAUSED: Rhinoviruses cause the common cold; polioviruses cause polio. Hepatitis A belongs to this group, as does the Coxsackievirus, first described just south of Albany. Cold and polio viruses invade and destroy ciliated respiratory cells and cells in the central nervous system, respectively; the body's response to cell destruction produces most of the symptoms, from the cough and runny nose of a cold to the paralysis sometimes associated with polio.

TRANSMISSION: Cold viruses spread through respiratory droplets, and viruses that have settled on environmental surfaces remain infective for an indeterminate amount of time; polio viruses pass mostly in feces, so fecal contamination is a common source of infection.

PREVENTION: Cold-infected people should try not to spray droplets from coughing and sneezing; contaminated hands should be cleaned, as they are a major source of surface contamination. Polio, being harder to transmit, is easier to avoid, usually through sanitary practices.

Retroviruses -

THE "ORGANISM": First, read the "General Traits found in all Viruses" at the beginning of the virus section. Retroviruses are unusual in that they carry RNA (by itself, that is not rare in viruses) that, once inside the host cell, must be read "backward" into the creation of DNA. Most RNA viruses use their RNA to directly make proteins, but retroviruses start as RNA, make DNA from it, then use the cell's abilities to make RNA from the DNA as a step toward making viral proteins.

DISEASE CAUSED: The most common human retrovirus is Human Immunodeficiency Virus (HIV), which causes AIDS (Acquired Immunodeficiency Disorder)The main host cell for HIV is called the T4 cell, a type of white blood cell and a major player in fighting off potential threats by the immune system. As the disease progresses, at first the immune system fights it with antibodies and by stepping up T4 production. However, part of the nature of HIV's RNA to DNA conversion is a tendency to produce mutations, and often antibodies cannot attach to mutated viruses. This allows HIV to "get ahead" of the immune response, and getting ahead means killing a large fraction of the T4 cells and actually shutting down the response against it. As the immune system gets less and less able to control HIV, the system's ability to fight off other invaders is also compromised. AIDS is a set of symptoms arising from common sources that rarely cause disease in people, such as bacteria that everyone carries in their lungs (these produce a common form of pneumonia in AIDS), and viruses that produce a rare skin cancer, Kaposi's sarcoma. Eventually, the body's inability to fight off all of these other diseases causes weakening and wasting away, and eventually death. Current drugs tend to be given in combination to disrupt several aspects of HIV chemistry at once, but they do not wipe out the virus, only slow it down, and in some people have less and less effect as time goes on.

TRANSMISSION: HIV is found mostly in the blood, but can also get into other body fluids - it is particularly common in semen. The easiest transmission occurs when blood or infected fluid gets direct access to someone else's blood, though a cut or break in lining tissue or passed from one intravenous needle user to the next (some blood gets trapped in the needle itself); however, HIV can move through rectal lining without needing a break, which makes anal sex particularly dangerous, and there is some evidence that it can move through the lining of the vagina as well. There are several different strains of HIV, one of which is much easier to spread through heterosexual intercourse, especially from the man to the woman. Presence of other sexually-transmitted diseases generally raises the susceptibility to HIV infection.

PREVENTION: Any precautions that reduce exposure to others' body fluids reduces exposure to HIV. Condoms are a particularly useful barrier to the virus, and latex gloves should be used wherever exposure could be possible. Sharing IV needles should be absolutely avoided. Many laboratories around the world are trying to develop vaccines to prevent infection, but none have yet been effective.

Bacteria Group -

Bacillus anthraxes

THE ORGANISM: On the slide, look for pale purple color and small streaks or clusters made up of many bacteria. These bacteria can survive low or no-oxygen conditions. Infective strains create a capsule that resists consumption by white blood cells and does not appear to stimulate an antibody reaction.

DISEASE CAUSED: Anthrax, usually a disease of cows, horses, and sheep, but which can infect humans, usually as cutaneous (skin) anthrax; this can also cause woolsorter's disease if inhaled. In some parts of the world anthrax affects the digestive system. Variant strains have reportedly been developed as a biological weapon by some governments, including the U.S. In the human body, the bacteria produce an exotoxin, a poison that prevents white blood cells from eating it; this poison, built up to certain levels, can kill the host. Even before that point, anthrax in the blood can cause septicemia, blood poisoning, and/or meningitis (inflammation of brain lining membranes) if it gets to the brain and spinal cord. Antibiotics can be effective early, but once exotoxin builds up and the disease has taken hold, mortality rates are very high. There is little that can be done to counteract the exotoxin.

TRANSMISSION: Bacteria are shed from lesions in the skin of infected animals or people in resistant spore forms which can be found on plants or soil around infected livestock or in animal products such as meat and hides. Spores may be eaten, inhaled, or picked up through a break in the skin. Anthrax weapons release an airborne form meant to be inhaled.

PREVENTION: Contact with infected animals should be avoided. Animals and people can be vaccinated, although the safety and effectiveness of the human vaccine has been questioned.

Clostridium botulinum

THE ORGANISM: On the slide, look for pale purple staining - the bacteria are the smallest distinct shape that can be seen, although they may also be visible in short or long strings. Active bacteria and spores are different shapes, and both may be visible. Clostridium requires a virtually complete lack of oxygen to reproduce. This is a common bacterium, often found on foods that we eat. It is fairly heat resistant, able to resist boiling for up to 15 minutes.

DISEASE CAUSED: Botulism. There is the possibility that it causes other problems as well. This disease is considered an intoxication rather than an infection, because it is produced by absorption of bacterial-produced toxins rather than by invasion by the bacteria themselves. Different strains produce different toxins in different combinations, with mortality rates varying from around 20% to about 75% from exposure to very small amounts. The toxins typically get into the nervous system and block the passage of messages along the nerves. Within 24-72 hours, the first symptoms of vision problems and dizziness occur, followed by difficulty with swallowing and breathing. Nausea and vomiting are common symptoms as well, perhaps as a side-effect to the dizziness. Treatment includes an antitoxin and possibly respiratory intensive care. Antibiotics are not required, since the bacteria themselves are killed by stomach secretions. Only in infants do they rarely reach the intestines, where they may reproduce and release toxins later. Infant botulism may play a role in some cases of Sudden Infant Death Syndrome (SIDS).

TRANSMISSION: The disease is a product of the toxins, which can only be produced under low oxygen conditions. Low-acid fruits and vegetables or fish or game with inner airless spaces that have been left out too long or improperly canned may be a site of bacterial buildup, leading to high levels of toxin. Since the bacteria are heat-resistant, canning procedures must be carefully followed - live bacteria sealed into cans find themselves in the perfect no-oxygen environment.

PREVENTION: Potentially dangerous foods should not be left out at room temperature. Canning should be carefully done, and tiny cracks in dented cans may allow bacteria to enter. The toxin can be broken down by boiling for at least ten minutes.

Clostridium tetanuses

THE ORGANISM: On the slide, these are barely stained purple and are very difficult to see, but may be in large masses - look for the very smallest purple objects to see what single bacteria look like. The spore forms may be present - these may look like tennis rackets or kitchen matches. These are common in the environment and are even found in about a quarter of human intestinal tracts. These bacteria can only be active and dangerous under very low oxygen conditions, which may occur in deep wounds, especially puncture wounds.

DISEASE CAUSED: Tetanus. Bacterial toxins released in the wound work their way up nerves and block messages to muscles, causing them to contract continually - this produces the most well-known symptom of this disease, which is also called lockjaw. A buildup of the toxin can cause respiratory failure; if not treated early with an antitoxin, mortality rates are high. Treatment often will not reverse effects already experienced. Antibiotics can kill the bacteria to prevent more toxin from being produced.

TRANSMISSION: This is a common soil contaminant that needs to be introduced into a deep wound to survive in the body. Infected people do not spread the bacteria.

PREVENTION: A vaccine is made containing a close copy (a toxoid) of the toxin that cannot hurt you but to which you will make antibodies. These antibodies will attach to and deactivate the actual toxin. Antitoxin contains vaccine as well as such antibodies, filtered from the blood of vaccinated animals. Vaccine-created immunity can fade, requiring follow-up treatments periodically.

Corynebacterium diphtheria

THE ORGANISM: On the slide, there are so many of these bacteria that you can only see individual bacteria in those areas where the spread is thin and some of the background white shows through. Look for the very smallest distinct stained shape. This bacterium requires high oxygen levels - it is commonly found in the upper respiratory tract. This bacterium contains a virus, and it is the virus that causes production of the toxin that causes disease.

DISEASE CAUSED: Diphtheria, which has a respiratory and a cutaneous (skin) version. Respiratory diphtheria occurs when bacteria invade lining cells in the upper respiratory tract - the toxin produced by the bacteria's own viruses kill the cells, producing sore throat, slight fever from the body's reaction to the infection, and weakness. As the infection progresses, local attempts by the body to fight it off can inflame the lining, producing swelling that can shut off airflow. If it spreads from the infection site, the viral toxin can cause the heart to stop. Cutaneous diphtheria can occur when bacteria enter through an uncleaned scratch or insect bite and produce lesions. Antitoxins and antibiotics can be used to treat these diseases.

TRANSMISSION: This passes between people in respiratory droplets, and many people carry it but show no ill effects; however, they can still pass it on.

PREVENTION: Vaccines are available; immunization at an early age is important to reduce the numbers of carriers. Immunization later in life is somewhat more risky, as some people will have developed an allergy to the vaccine from exposure to the bacterium.

Diplococcus pneumoniae (Streptococcus pneumoniae)

THE ORGANISM: On the slide, look for tiny, dark-staining dots among the "glop." As noted above, these are nowadays called Streptococcus, in a larger group called pneumococcus bacteria. These typically live in the upper respiratory system, and are common in people who show no ill effects. The bacteria are often seen in pairs.

DISEASE CAUSED: Pneumonia and related diseases, mostly in people whose systems are already weakened for some reason - it's a common complication of AIDS. In someone with such problems, the bacteria can reproduce at an uncontrolled rate and spread down into the lungs, where local tissue reactions produce inflammation and fluid accumulation. In high-risk groups, mortality can be high. Antibiotics, usually penicillin, are usually effective against these bacteria, although several antibiotic-resistant strains exist.

TRANSMISSION: These pass in respiratory droplets between unaffected people. Disease is usually caused by bacteria that have been in the system for a long time but controlled by the immune system.

PREVENTION: Since there are at least 80 distinct strains of these bacteria, a vaccine is very hard to develop. A few vaccine exist that target the more common or dangerous strains, but these are recommended only for people at high risk.


Haemophilus influenzae

THE ORGANISM: On the slide, there are so many of these bacteria that you can only see individual bacteria in those areas where the spread is thin and some of the background white shows through. Look for the very smallest distinct stained shape. This bacterium was first seen during the worldwide flu epidemic of 1890 and briefly thought to be the cause (that's how it got its name), but that outbreak's cause was really a virus. This is a common bacterium of the upper respiratory tract and doesn't usually cause disease in carriers.

DISEASE CAUSED: This bacterium can cause a wide range of infective diseases, mostly in children or those with compromised immune systems - the most dangerous illness is probably bacterial meningitis, but it also causes ear and throat infections. In adults, it often can cause pneumonia. Bacteria that are usually confined by immune response migrate out into tissues and begin to reproduce. It is primarily the body's response to this growing population that produces most of the symptoms of inflammation, especially fever and increased fluid pressure around the brain and spinal cord. This rise in pressure leads to headaches and a stiff neck (the swollen spinal cord does not bend as easily or without pain). Although there are resistant forms, there are antibiotic treatments. Sometimes, however, the treatments worsen the disease as dead bacteria break apart and set off an internal allergic reaction - this means treatments are best done early, before too many bacteria are present.

TRANSMISSION: Bacteria are spread in nasal and throat discharges of infected people, but the bacteria are not very hardy ands rarely spread through airborne droplets - close contact is required.

PREVENTION: A vaccine is widely available and commonly administered to children, and in fact most modern U.S. cases of bacterial meningitis are produced by two other types of bacteria unrelated to this one.

Haemophilus pertussis

THE ORGANISM: On the slide, the cells may be linked end-to-end; the smallest stained shape that you can see is a single bacterium. It is commonly found in people's upper respiratory tracts, where it often causes no problems. It is the only respiratory bacterium to use ciliated lining cells as attachment points.

DISEASE CAUSED: Whooping cough. As bacteria spread across breathing surfaces, the irritation tends to produce sneezing and coughing, sometimes with watery eyes. As it progresses, bronchial tubes tend to constrict and block up, and as the cough gets worse, inhaling through these narrowed tubes produces a whooping sound. The infection can spread into the lungs and can produce convulsions and sometimes death. People can commonly recover without treatment, but antibiotics may also be used, especially to stop the spread of the bacteria.

TRANSMISSION: Through sputum, especially during the whooping stage. The bacteria may often be found in older children and adults, but they do not appear to easily spread it.

PREVENTION: A vaccine is widely used, and whooping cough has become rare in the U.S. In other countries without immunization programs, however, there have been recent epidemics. There is some evidence that protection from the vaccine fades, so booster shots may be needed to prevent the spread through older hosts.

Klebsiella pneumoniae

THE ORGANISM: On the slide, these are quite small and pink, spread often in clumps. Look for the tiniest stained shape to see an individual bacterium. These are found in the environment, especially in soil where they contribute extremely important nitrogen-containing molecules, and in a small proportion of human throats and intestines (the proportion is much higher in hospital populations).

DISEASE CAUSED: Pneumonia, especially that type picked up during hospital stays (hospital-caught infections are called nosocomial infections), and septicemia, a blood infection, caught in pediatric wards. Both infections are extremely dangerous and frequently drug resistant. The pneumonia often results in bleeding abscesses (bloody sputum is a typical symptom). More rarely, these bacteria are associated with urinary tract infections. Only a few antibiotics may be effective against these, and the treatments usually take longer than with other infections.

TRANSMISSION: Bacteria are spread through nasal secretions but since they are soil bacteria they are quite hardy, living outside the body for quite some time These commonly are picked up through breaks of the skin or inner linings, as may happen to people hooked up to respirators.

PREVENTION: Antibiotic given during hospital stays may prevent infection, although that is probably how the wide-range resistance was developed - whatever the common course of treatment is in a particular hospital, eventually a strain resistant to that treatment will develop there.

Mycobacterium leprae

THE ORGANISM: On the slide, there aren't many of these - search out the few stained areas by eye and look there. These bacteria invade body cells and live inside them, multiplying very slowly - the incubation period between infection and disease can be 4 - 8 years.

DISEASE CAUSED: Leprosy (Hansen's Disease). Usually cells of the skin are involved, producing various sorts of lesions. Advanced cases may lead to disfigurement and blindness. The nervous system is commonly also affected, especially the peripheral system. The exact activity by which the bacteria produce the symptoms isn't clear.

TRANSMISSION: Usually this is passed through nasal secretions, although transmission isn't easy and it isn't completely known how the bacteria get into the next host; infection also seems to depend a lot upon the susceptibility of the person. There are also increased risks connected to living in countries where the disease is common, where the strains of bacteria present may be more infective.

PREVENTION: Infection usually goes along with prolonged close contact with the victims, so victims should be isolated during treatment. A multi-antibiotic treatment is effective against the disease.

Neisseria gonorrhoeae

THE ORGANISM: On the slide, look for stained areas. The live bacteria are not very hardy, dying quickly if not in a warm, moist area. When passed, they stick to the lining cells of various human tracts, such as the reproductive tubes, the urethra, the mouth, or the rectum. They will accumulate on the surfaces in groups and invade the lining - the damage caused combines with a local immune response to produce the symptoms.

DISEASE CAUSED: Gonorrhea, probably the most common sexually-transmitted disease (std) in the world. Sores form at the invasion site and produce pain if there are any pain nerves in the area - symptoms from this may include sore throat or pain while urinating. The cell damage and local inflammation also combine to produce a nasty yellowish discharge. Both of these symptoms, if produced far enough inside the body, may be completely unnoticeable or nearly so. Damaged surfaces may scar over and close up affected tubes, affecting passage of sperm and/or urine in males or egg cells in females. In some cases, more commonly in women due to the arrangement of the female system, bacteria can spread into the abdominal cavity and cause pelvic inflammatory disease or through the body and cause system-wide complications. Infants can also pick up the bacteria while passing down the birth canal and get dangerous eye infections. Treatment is with antibiotics, but there are many antibiotic-resistant strains in the world.

TRANSMISSION: Almost exclusively by sexual contact, or to newborns during birth. The bacteria can be carried and spread by people who show no symptoms.

PREVENTION: Safe-sex practices such as condoms can block the spread of the bacteria. Antibiotics can stop the spread, although finding an effective drug for a particular strain can be difficult. There are enough different strains of this bacteria (which is probably why people don't become immune after exposure) that developing a vaccine isn't considered practical.

Salmonella schottmuelleri

THE ORGANISM: On the slide, look for stained areas, but in places where the bacteria have been spread out - look for the tiniest stained shaped to see an individual bacterium. There are many different types of Salmonella that can cause the problems described below. These bacteria commonly live in the intestines of animals, especially birds and reptiles but sometimes pigs, where they seem to cause few effects.

DISEASE CAUSED: Gastroenteritis, broadly a type of food poisoning. Nausea and vomiting are followed by abdominal pain, fever, and diarrhea. It all starts within a day of ingestion and progresses for maybe a week. No one is entirely sure how the symptoms are produced by the bacteria, but they do not appear to produce a toxin, so it's probably from a direct invasion of the intestinal lining that doesn't penetrate deeply enough to produce long-lasting effects. In rare cases, the bacteria may invade the bloodstream and spread through the blood, which can sometimes be fatal. Treatment is usually of the symptoms, with replacement of the water and salts loss through diarrhea; the symptoms will stop eventually without treatment.

TRANSMISSION: People don't generally pass Salmonella between themselves, but rather pick it up from food that has been contaminated with animals' intestinal contents - the bacteria can spread in contaminated meat, especially at room temperature, so food left out can be progressively more dangerous. There is evidence that Salmonella has begun to live in the reproductive tract of chickens - eggs before could have their shells contaminated from their passage out of the body, but now they may be packaged with the bacteria inside them as well.

PREVENTION: Proper cooking of foods kills the bacteria. When possibly contaminated meats are handled, care must be taken to not spread bacteria to foods that will not be cooked, which can happen on cutting boards, utensils, or hands.

Streptococcus pyrogenes

THE ORGANISM: On the slide, look at the stained area - the bacteria are fairly easy to find. This common bacterium was once a common cause of death, before antibiotics were developed.

DISEASE CAUSED: When a wound "gets infected," this is usually the bacteria involved. The bacteria produce an enzyme which break down cells in the area.  Materials produced by the damaged cells produce a local immune response, with swelling, redness, and "pus." Strep throat is caused by these bacteria, as are most cases of peritonitis, commonly from penetrating belly wounds; uterine infections following childbirth; a type of blood poisoning; and diseases commonly called rheumatic fever and scarlet fever. In most cases, the symptoms arise from local attempts at the body to fight the bacteria, mixed with the cellular damage the bacteria themselves are causing. A variety called flesh-eating strep has a much more powerful effect in breaking down tissues and setting up massive immune reactions.

TRANSMISSION: People can pass this bacterium in respiratory droplets, but it is also common in the environment, where it can be picked up from wounding objects.

PREVENTION: Until recently, these bacteria were easily controlled by penicillin, but now several antibiotic-resistant strains exist. Soap and warm water can be very effective at cleaning these bacteria out of wounds. Rheumatic fever, which seems to be a follow-up allergic reaction, can be prevented by killing the bacteria early, but the disease itself is not affected by antibiotics.

Treponema pallidum

THE ORGANISM: On the slide, you have a yellow-stained tissue section - the bacteria, stained dark gray to black, are in the tissues - look for the smallest recognizable gray-stained shape to see a bacterium. These bacteria do not live well outside the body, and so must be passed directly from person to person.

DISEASE CAUSED: Syphilis. It is unknown exactly how the bacteria cause the disease, but it progresses through three stages. In the first stage, 10-90 days after infection, a small lesion appears on a surface, probably at the invasion point, which could be an external surface or an inner surface. The lesion forms an ulcerated sore which will eventually disappear. The second stage, following the first by about 6 weeks to several months, involves a rash of flat round spots and varying in extent. This may involve mucus membranes. The rash will eventually fade. After the second stage, the bacteria spread throughout the body, and the third stage may not be obvious for years. Symptoms in the third stage may be from many body systems, including the bones, heart, and brain, and it may result in aneurysms, paralysis, or insanity. Treatment with antibiotics and can be done at any point during the course of the disease and stop its progress - damage already done in the third stage is usually irreversible, however.

TRANSMISSION: Bacteria are passed by sexual contact. Infected person are usually infectious through the first two stages, which can last for months. Bacteria may cross the placental barrier and severely affect or kill a fetus.

PREVENTION: Safe sex practices can prevent transmission. People with symptoms should get checked and treated - people in high-risk groups should be periodically checked, symptoms or not.


Protozoan Group -

Balantidium coli

THE ORGANISM: These protozoans are fairly decent-sized, usually distinctly oval or round. They have a large, "dumbbell"-shaped nucleus, but it may not look like a dumbbell if it is turned certain ways. The cells are covered with cilia, but these may be difficult or impossible to see. Most of the slide are fecal smears, so there's lots of materials other than the Balantidium - the organisms themselves tend to spread to the edges of the smear, at the top and bottom of the preparation. This animal lives normally in pigs, where it isn't even considered a parasite.

DISEASE CAUSED: Balantidiasis, a fairly rare condition. Through some unknown interaction, the protozoans cause irritation of the intestinal wall, resulting in cramps, nausea, vomiting, and severe diarrhea, and sometimes even ulceration of the intestinal lining, causing bleeding and possible complications. Some people can carry these with little or no effects but be infective.

TRANSMISSION: Cysts of the protozoans, passed in feces from humans or pigs, get into water supplies and are drunk.

PREVENTION: Sanitation standards must be high enough to prevent contamination of drinking water. Infected people may be treated with tetracycline antibiotics, but these tend to not be very effective.

Giardia intestinalis (or Giardia lamblia)

THE ORGANISM: These protozoans, if you can see them well enough, are a bit like two fused cells - there are two nuclei, which on most specimens look lighter than the rest of the cell. The slides may be fecal smears, so there will be lots of debris to confuse you. Giardia are quite small, often tear-drop, oval or racket-shaped, and stain more smoothly than fecal debris, often with those lighter nuclei showing. They are quite common in the smear but may be hard to pick out at first. Once you can recognize them, look around for a good one to draw.

DISEASE CAUSED: Giardiasis, often called beaver fever. Parasites emerge from cysts and reproduce very quickly, building up numbers large enough to coat the entire lining of the small intestine, preventing absorption of water and nutrients, which can cause weakness and diarrhea, and causing irritation that produce cramps and nausea. Individual protozoans in resistant cysts pass in feces. Usually, before long the population shrinks and stabilizes, and symptoms go away, although the person still harbors protozoans and still passes cysts. People with resident populations do not get sick from drinking contaminated water, but will sustain their parasites. Without reinfection, the population will eventually dwindle away.  No immunity develops from exposure.

TRANSMISSION: Cysts pass in feces, but Giardia can live in and pass from many species of mammals, wild and domestic. In areas where Giardia is found, virtually any water source is likely to be contaminated from cysts drained from feces by rain.

PREVENTION: Water purification should keep this out of drinking water. While camping, water should be boiled or otherwise treated before drinking. There are drug treatments available for infected people.

Plasmodium, various species

THE ORGANISM: These tiny protozoans invade and reproduce inside our cells. They invade both liver cells and red blood cells - the slides are blood smears. In a blood smear, the vast majority of cells visible are red blood cells, which have no nuclei and little other than hemoglobin inside them. If there are dots or circles inside a red blood cell, that's malaria. Larger cells with obvious nuclei are white blood cells. Some slides may have tiny stained cells among the red blood cells - these may be Plasmodium parasites freed from host cells. Some smears are more heavily infected than others.

DISEASE CAUSED: Malaria. When protozoans first enter the body, they quickly invade liver cells and reproduce, but cause no symptoms. They emerge after a time into the blood, invading red blood cells, where they reproduce and destroy the cells. Your immune system reacts to the free parasites, the damaged blood cells, and the destroyed cells, producing what is almost an allergic reaction involving chills, fever, and delirium. The symptoms may be extreme enough to kill, and sometimes other organs get involved as well. Many people suffer repeated bouts of illness and, if they survive, are symptom-free between.

TRANSMISSION: Contaminated blood must enter a mosquito from the victim. Plasmodium goes through a separate reproductive cycle inside the mosquito, with offspring invading the salivary glands from which they will be injected into the next host (saliva must be squirted down the mosquito's needlelike mouthparts as a first step to keep the sucked-up blood from quickly clotting in the tube).

PREVENTION: The most effective control method controls the mosquitoes that carry the protozoans. Some drugs can ward off infection, but few effectively treat a person once they are infected. Work continues on a preventive vaccine.

Trichomonas vaginalis

THE ORGANISM: The slides are almost nothing but these oval, flagellated cells obtained from a swab. If you look at the top or bottom edges where the stain is darker, you may see the characteristic flagella, like hairs on the cells, or the structural rod inside the cell.

DISEASE CAUSED: One type of vaginitis, a condition that can also be caused by many other types of organisms, including bacteria and yeast. Presence of Trichomonas may produce no symptoms at all, or itching and discharge, depending upon many factors. There is some evidence that nastier strains of the parasites produce toxins that affect the vaginal and cervical surfaces. Inflammations and secondary infections are possible. Males can harbor and pass the parasites but rarely show symptoms.

TRANSMISSION: Most commonly, this is person-to-person during intercourse. These protozoans can survive outside of the body for short periods, so occasional transmission on surfaces or towels has been known to happen.

PREVENTION: Safe sex practices can prevent transmission. Infected persons can be treated to kill the parasites, but partners need to be treated, too, to prevent reinfection.

Trypanosoma, various species except cruzi

THE ORGANISM: Most slides are blood smears, and swimming among the blood cells are the squiggly parasites, visible at high power. A single flagellum with attached membrane runs from one end of the cell to become a "tail" at the other. Usually a nucleus is visible. One interesting trait in these parasites is their ability to restructure their surface molecules, evading host antibody response - that's why these things can actually live right in the blood for years without our immune system wiping them out.

DISEASE CAUSED: African Sleeping Sickness. Right from their entry at a tsetse fly bite site, these parasites can cause itching and inflammation. As they move through the lymph system and blood, reaction to their presence can cause swelling of the lymph nodes and even enlargement of the spleen. No one seems to know just how exactly they cause damage, but it can be widespread, eventually moving into the central nervous system. Changes from this invasion can start as insomnia and personality changes, progressing to mental sluggishness, convulsions, prolonged sleepiness, and coma. Death usually is the result of malnutrition or other infections.

TRANSMISSION: The disease is carried in the biting tsetse fly, where it reproduces on its way to its next victim. Parasites move into the bite from the fly's saliva.

PREVENTION: Tsetse fly control can cut the disease off at its vector. Cases can be treated if caught early, but the cost is high.

Flatworm Group -

Clonorchis sinensis

THE ORGANISM: This is also known as the Chinese Liver Fluke. It is a member of the trematode family and is found throughout eastern Asia, including Japan and Taiwan. Like all trematodes, it has a complicated life cycle, going through different stages in a succession of different host animals. Eggs passed in human feces hatch only if they reach a body of water and are consumed by a common type of snail. A larva emerges and invades the snail's tissues, where it reproduces. Large numbers of the next larval form emerge from the snail and swim up and down in search of a fish (or sometimes a crustacean) to burrow into. This next form lies dormant in the muscles of the fish, activating only if the fish is eaten.

DISEASE CAUSED: Clonorchiasis. If infected fish (or crustaceans) are eaten raw or undercooked (typical Asian preparations of pickling, salting, or smoking will not kill the parasites), larvae emerge in the intestine and transform into the adult stage. Adults make their way up to the liver, probably by crawling up the bile duct. They attach to the lining of the bile ducts and begin to feed. The worms produce wastes to which some hosts react allergically. Erosion of the ducts leads to blockages which prevent the flow of bile from parts of the liver, but commonly little effect is seen unless there are a large number of worms affecting many ducts.

TRANSMISSION: How it gets from one person to the next is described above.

PREVENTION: Fecal contamination of water spreads the eggs to the needed snails, so good sanitation practices would stop the life cycle. Also, full cooking of the infected fish or crustacean would kill the larvae. However, there are many areas of Asia where fuel for cooking is very expensive, which makes this last prevention measure harder to implement than it sounds.

Echinococcus granulosum

THE ORGANISM: The adult of this worm species is a small tapeworm of dogs, which pass egg-filled segments in their feces. Eggs last a very long time in the environment, long after the feces have broken down, and can be picked up from the dirt and swallowed. If a human ingests an egg, it hatches in the intestine and releases a larva that can dig out through the intestinal lining and invade the blood. Since a human is not a dog, the larva does not become a tapeworm, but instead goes into a second larval stage, a hydatid cyst, a fluid-filled ball. This cyst will grow as large as the surrounding organs and structures will allow it to. If the person were to be eaten by a dog, the cyst would break apart and become several tapeworms in the dog.

DISEASE CAUSED: Hydatidosis. As the cyst grows, it presses on surrounding areas and can interfere with their function. They may fill the inside of a bone, weakening it until it just breaks, or press on the brain inside the skull. In large body cavities, cysts can become quite large. The body can build up a resistance to new larval infections but usually can't affect the cysts. An allergy to the cyst fluid may build up, and if a cyst ruptures, the reaction could be life-threatening.

TRANSMISSION: People need to pick the eggs up and swallow them from areas where an infected dog once defecated.

PREVENTION: Contact with contaminated soil should be avoided, especially for children, who are more likely to put dirty hands into their mouths.

Fasciola hepatica

THE ORGANISM: Don't use a regular microscope on this worm, use a dissecting 'scope or view it by eye against a white background. Although this species is usually a parasite of sheep, it can infect humans, and it is very similar to a species more likely to infect humans. Eggs hatch if they reach water, and a swimming larva finds and penetrates certain species of snails (each species in this group has a particular snail species that it uses). A couple of reproductive phases occur in the snail, and eventually another swimming stage emerges. This stage swims to areas near the surface and shoreline, where it attaches to plants and other objects and encases itself in a cyst, a form that can resist short periods in the air and/or the trip through a stomach. When eaten, the larvae emerge in the intestine and migrate through the intestinal wall into the body cavity, where they move to and through the liver. In the bile duct system of the liver they become adults, reaching that stage about 3 months after ingestion. Adults lay eggs that pass down the bile ducts into the intestine and out with feces.

DISEASE CAUSED: Fascioliasis. During early infections, migration of larvae out and through tissues can cause abdominal pain, vomiting, diarrhea, and enlargement of the liver; immune response to the worms can cause fever and inflammation. Once in the adult form, symptoms are more subtle, but include back-up of bile in obstructed ducts and occasional liver inflammation. Misdirected larvae can cause damage throughout the body in some cases. There are some drugs available that work fairly well to kill the worms.

TRANSMISSION: Eggs passed with feces (from both infected humans, and from some animals, principally plant-eaters) must reach a body of water with the appropriate snail. Emerged larvae must attach to some item in the water that will be eaten by the next host. Watercress is commonly the source of infections in humans.

PREVENTION: It's difficult to be absolutely sure that the water body watercress or some other food plant comes out of is not contaminated, so either such plants must be very well cleaned or avoided.

Schistosoma, two species

THE ORGANISM: There are several Schistosoma species which affect humans; all have the same basic lifestyle. Eggs hatch if they reach water, and a swimming larva finds and penetrates certain species of snails (each species of Schistosome has a particular snail species that it uses). A couple of reproductive phases occur in the snail, and eventually another swimming stage emerges. This larva actively seeks out exposed skin and penetrates it, entering the bloodstream and migrating to the liver, where it matures into an adult flatworm. The adults move into veins on the linings of the intestines and bladder (different species have different preferences), where they mate and the females produce eggs. In most species, these eggs have spines so that they can tear out of the small blood vessels into the intestine or bladder, where they can be carried out of the body.

DISEASE CAUSED: Schistosomiasis. The presence of the worms can cause local problems, such a small ruptures of the intestinal wall and plugging of the blood drainage from the intestines into the liver, with following liver complications. Migration of the damaging eggs through the bloodstream can produce damage all over the body, especially in organs with lots of small blood vessels, such as the lungs, brain, or kidneys. There is a drug available, but the drug itself is dangerous and not recommended unless the infection is serious.

TRANSMISSION: Eggs passed with feces or urine (most often from infected humans, although possibly from some animals) must reach a body of water with the appropriate snail. Emerged larvae must have access to exposed skin to infect the host.

PREVENTION: Modern sanitation methods can prevent water contamination if people working outside actually use the facilities; boots and gloves can minimize contact with contaminated water.

Taenia pisiformis (Taenia solium)

THE ORGANISM: We are using a dog and cat tapeworm (T. pisiformis) partly as a stand-in for a closely-related human tapeworm (T. solium), also known as the pork tapeworm; the beef tapeworm (T. saginata) is also a close relative. Adult tapeworms live in intestines, where they produce segments, called proglottids, full of eggs that pass in feces. The eggs remain infective long after the feces they were passed in breaks down, for months and possibly years. If picked up and ingested by an animal (including humans), the eggs hatch in the intestines and release a larva that bores out and enters the blood. Somewhere in the body the larva will invade tissue and form an advanced larval stage sometimes called a bladder worm, a fluid-filled ball with inside-out tapeworm heads lining it. In order to finish its life cycle, the larva needs its current host to be eaten by an appropriate meat-eater; while the surrounding tissues are being digested, the larval heads will separate and become adult tapeworms, attaching with their heads to the inner lining of the intestine. An adult tapeworm is often close to as long as the intestine it's in, up to 20 feet in a human.

DISEASE CAUSED: This depends on the stage of the worm. If beef or pork tapeworm larvae are eaten in contaminated beef (the cow ingested eggs), it becomes an adult in your intestine, and the adult rarely causes much trouble - it's attachment method isn't particularly destructive to the lining and it doesn't eat much. The most striking symptom is the appearance of proglottids, which may be moving, in feces. Some people develop allergies to worm wastes, resulting in a type of intoxication. If eggs from a pork tapeworm are eaten, the formation of bladder worms in various parts of the body can cause problems as they take up space or as the body develops an allergy to the fluid within the bladder. There is a drug for the adult tapeworms, but if the patient is also carrying larvae, the drug will kill them and may produce a dangerous allergic reaction as they break down in the tissues.

TRANSMISSION: Larvae can be eaten in contaminated beef or pork. Eggs may be picked up from an area that was once contaminated with feces of an infected human or animal. A human carrying an adult beef tapeworm can infect themselves with eggs, and therefore bladder worms, by contact and ingestion of their own feces.

PREVENTION: Contaminated meat should be fully cooked. Hands that have come in contact with potentially contaminated areas should not be put in the mouth.

Roundworm Group -

Ancylostoma caninum

THE ORGANISM: The actual host of these hookworm-type roundworms (several different species can cause problems in humans) are dogs and cats - they are commonly seen in puppies and kittens, who tend to cough them up. Adult dogs and cats seem to be able to harbor them in their intestines with minimal symptoms. Humans do not become infected with adults, but larvae can penetrate the skin (there are a couple of related hookworms much more likely to invade humans). They get "stuck" in the underlayers of the skin, moving around just under the surface. On the slides, there are a pair of worms, a smaller male and a larger female, usually stained red.

DISEASE CAUSED: Creeping eruption. Worms wander under the skin. In the larger species, the outline of the moving worms can be seen. Local immune responses can inflame and swell the skin to the point that oozing sores result. If large enough to be seen, worms can sometimes be caught with a slot cut into the end of a small stick and wound out - this is the sort of treatment that became the medical symbol of the "snake on the pole," the caduceus. Eventually, the larvae die and break down.

TRANSMISSION: Eggs pass in the feces of hosts, dogs and cats, that harbor the adults. Larvae hatch and wander in the soil for a while, then migrate to the surface. They usually attach to the paws of the next host and are licked off and swallowed, but they can penetrate through thin skin exposed to the soil, like bare human feet or hands.

PREVENTION: Bare-skin contact with soil beneath or near cat or dog fecal contamination should be avoided. The most likely times of infection is when the soil is moist or wet.

Necator americanus

THE ORGANISM: There is a group of roundworms called hookworms - this is a human-infecting hookworm native to the Americas (there are other types of human hookworms), including the southern U.S. Eggs passed in feces hatch and the larvae migrate out into the soil, where they live for a while. When they are mature enough, the larvae move to the surface on a wet day, where they can congregate in large numbers. They "stand up," kind of like cobras, looking to make contact with exposed flesh , which they bore through, entering the bloodstream (cat and dog hookworms, which don't infect people, use a different approach, since dog and cat food pads are too thick and hard). The larval worms usually then travel through the blood to the lungs, emerge through the lung linings, crawl up the breathing tubes and get swallowed. In the intestine, now-adult worms attach to the walls, reproduce and lay eggs. On slides, there are usually two worms, one male (smaller) and one female. Note the hooked teeth in the mouths.

DISEASE CAUSED: Hookworm disease. Early symptoms, known as ground itch, are from the larvae first entering through the skin. As the larvae emerge in the lungs, there is a pulmonary phase, usually involving coughing and a sore throat, rarely resulting in large-scale hemorrhaging of the lungs. Then there is the intestinal stage, when adult worms bite into the intestinal lining and feed on blood. Even a moderate infection can cause enough bleeding to result in anemia. Abdominal pain and a loss of appetite are common symptoms, and somehow the worms can cause to host to develop a craving to eat dirt (in eras where hookworms are common, such as the U.S. in the 1920's, businesses have grown up to provide "tasty" dirt, such as southern clay). There are a few drugs available to treat an infection.

TRANSMISSION: Eggs pass in feces, hatch in soil, and larvae must contact exposed skin to invade.

PREVENTION: Both contamination of soil with human feces and contact with wet soil that might be contaminated should be avoided.

Trichinella spiralis

THE ORGANISM: This parasite is unusual in that it uses one host as a site for development, transmission, and reproduction. Worms taken in from contaminated meat emerge in the intestine and reproduce. Females bore out of the intestines, enter the blood, and produce larvae, which then migrate throughout the body. The larvae usually settle in muscle tissues, where the curl up and the host, unable to kill them with white blood cells, just walls them in with calcium similar to bone material. Preferred muscles are those around the eyes, the tongue and chewing muscles, and those involved in breathing. In the lab, there are two very different types of slides of these worms . One has just a pair of worms, somewhere within a blue circle - they're almost transparent, so close your iris diaphragm to see them. The other slides are slices of mouse tongue - the worms can be seen curled up in their walled-off spaces, usually several in each section. They tend to stain somewhat dark, and being a slice of a curled worm, appear as a cluster of dots or streaks.

DISEASE CAUSED: Trichinosis. Early symptoms, within one or two days, are intestinal, from the presence of the worms. Two to eight weeks later, the migration out of the intestine and throughout the body produces a wide range of symptoms, including such direct effects as aches and problems with coordination or breathing, and such indirect effects (from the body's immune reaction) as fever, chills, swelling, or itchy skin. Since wandering larvae made invade any tissue, a wide range of inflammations of internal organs is possible. Symptoms may persist for months as the worms find places to settle down and the body slowly seals them inside calcium nodules. There is no treatment that affects the worms - people are treated to lessen the symptoms. Rarely is an infection so heavy as to produce death, but it is possible.

TRANSMISSION: Infected meat must be eaten, and the infected animal must have eaten infected meat, so only carnivore or omnivore animals are a danger as a meat source. Pigs generally become infected by catching and eating rats; several hunted animals, such as bears, may be infected. Plant-eaters such as cows or deer are not carriers.

PREVENTION: Any meat that could be contaminated should be cooked thoroughly, which kills the worms. Lengthy hard-freezing usually kills them as well. Often, modern pig-farming techniques keep rats from becoming available as pig snacks, so pork in the U.S. is often free of the worms.


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Written, M. McDarby, 1992.  Last Update, 2006.  Uploaded, 2001.




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