|Themes > Science > Life Sciences > General Biology > Microbiology > Parasitic Flukes and Worms|
Schistosomiasis is caused by an organism that is an obligate parasite of humans and snails. This means that is requires both humans and snails to survive. Infections generally occur in South America, Caribbean Islands, Middle East, Africa, China, the Phillipines and Indonesia.
Survival of this parasite depends on human insistence on polluting water with organic wastes. Adequate sewage treatment is sufficient to eliminate this disease in humans.
Schistosomiasis infection is contracted when a human comes in contact with the parasite in contaminated water. The parasite penetrates the skin and then enters the blood stream. If it reaches the liver, it can continue to grow in the human host. After a period of development, it migrates to the gut or bladder and begins producing eggs.
Symptoms of schistosomiasis appear in three distinct phases. In the initial phase, symptoms include: fever, skin rash, abdominal pain, bronchitis, enlargement of the liver and spleen, and diarrhea. In the intermediate phase, symptoms include pathological changes in the intestinal and urinary tracts, and eggs in the urine and feces. The final phase results in complications involving major bodily systems.
Diagnosis is accomplished by finding eggs in the urine or feces. The most effective drugs for treating schistosomoiasis are organic trivalent antimonials. However, these drugs are toxic to humans, and of these drugs, only pentavalent stibogluconate is still in use.
The current drug of choice is praziquantel. Other drugs used to treat schistosomiasis include metrifonate, oxamniquine, bithionol, albendazole, or mebendazole. The mechanism of these modern drugs is not known. However, it is known that one of the drugs (albendazole) starves the parasite and its offspring.
Control of schistosomiasis is extremely difficult. Some attempts to control the parasite in water sources have been somewhat successful. In particular, chemical molluscicides, predatory snails, and snail-eating fish have all been somewhat successful in controlling the parasite.
Fluke infections are usually acquired after
you ingest one of the infective forms. Most fluke infections are
Human infections are most common in the Orient, Africa, South America, or the Middle East. However, flukes can be found anywhere that human waste is used as fertilizer.
Flukes normally inhabit the digestive system and/or liver. Some flukes seek out the lungs, or may wander to the heart, brain or skin.
In the digestive system or liver, they disrupt digestion and nutrient absorption. Symptoms include chronic diarrhea and abdominal pain. Other symptoms occur from long-standing infections. These symptoms include ulcers, hemorrage and abscessess of the intestional wall, and liver damage. Sometimes severe toxemia results when the host's body absorbs the worm's metabolites.
You can kill flukes by thoroughly cooking meats and vegetable taken from suspect waters. Salting, pickling, drying, and smoking does not always kill the parasites in meat.
The most common medications used to kill
flukes are praziquantel, niclosamide, or tetrachloroethylene.
However, currently, only limited information about treating these
infections is available.
Tapeworms are creatures that live as mature organisms in the intestines of the definitive host. Definitive hosts include cats, dogs, cows, whales, and humans. Human infections occur when uncooked, infected meat is eaten, or by ingesting the eggs. Human infection with the common dog tapeworm is transmitted by the ingestion of infected fleas, and is most commonly seen in children.
Immature larval tapeworms may be found in various tissues throughout the body, where they may cause numerous symptoms. However, most infections are asymptomatic. Clinical symptoms include diarrhea, nausea, and weakness. Also, the nutritional status of the host may be severely compromised. In particular, the worm may inhibit the body's ability to absorb vitamin B12, and may cause anemias.
Praziquantel, niclosamide or albendazole may be used to treat the majority of tapeworm infections.
Hydatidosis is a serious disease in many parts of the world, particularly in areas where sheep farming is prevalent. Humans become infected when they accidentally ingest eggs, usually as a result of petting infected dogs and then touching the mouth or nose.
Hydatids are slow-growing and infections may not be symptomatic for years. As the hydatid grows, it forms a cyst. This cyst crowds adjacent normal tissue and impairs the tissue's ability to function. If the cyst ruptures, the fluid within the cyst can cause anaphylatic shock.
Surgery is the only effective treatment for
Whipworm exists in most parts of the world. Whipworm most often infects small children, who either drink contaminated water or place egg-contaminated fingers in their mouths.
The adult worms inhabit the large intestine, where they feed on blood cells by burrowing into the surface. Chronic hemorrhage and anemia may result. Secondary bacterial infections are common. Other symptoms include insomnia, nervousness, loss of appetite, vomiting, itching, diarrhea, constipation, and flatulence.
Diagnosis depends on finding the worms or eggs in the feces. Mebendazole is effective in treating whipworm.
Trichinosis is acquired by eating undercooked or underfrozen pork, bear, wild pig, or walrus. Theoretically, any wild mammal may be a source of infection. For example, arctic explorers have been killed by trichonosis acquired from eating uncooked polar bear meat.
When infected meat is ingested, digestion frees the adult worms from the meat. They promptly embed in the host tissue and begin to reproduce. The females penetrate the intestinal lining. The juveniles penetrate muscle cells and are turned into cysts by the host's immune response.
The juveniles can migrate to the liver, heart, lung, or virtually any other tissue in the body. When they reach skeletal muscle, they penetrate individual fibers and begin to grow. They are encysted by host tissues defenses. The most susceptible muscles are the eye, tongue muscles, and chewing muscles; then the diaphragm and intercostals; and finally the large muscles of the arms and legs.
During the migratory phase, juveniles damage tissues as they pass through. Symptoms depend on the type of tissue damaged. When the organisms invade the muscle, symptoms include muscular pain, difficulty in breathing or swallowing, heart damage, and various nervous disorders. Death is usually caused by heart failure, toxemia, or kidney malfunction.
The parasite can be killed by both freezing and cooking. Freezing at -15 C (5 F) for 20 days destroys all parasites (of the temperate zone strain). Fully cooking all meat is the best way to avoid infection.
Diagnosis is difficult. There is no
satisfactory treatment for trichinosis. Albendazole and related drugs are
effective in the early intestinal form of the infection, but their
effectiveness against larvae that have migrated to muscle is questionable.
Hookworm is endemic to warmer areas of the world because frost kills eggs and juveniles. Transmission is increased in areas where human waste is used as a fertilizer, where there is poor sanitation, and/or in tropical areas where humans go barefoot.
When juvenile worms enter through the skin, they do little damage until they reach the blood vessels. There, a tissue reaction may kill the parasite.
The juveniles that survive travel to the lungs. In the lungs, the juveniles cause some coughing and sore throat. The worms are coughed up, then swallowed into the stomach. They pass through to the intestines.
In the intestines, the worms attach and begin to feed on blood. A moderate hookworm infection will gradually produce an iron-deficiency anemia. Abdominal pain, loss of appetite, and the desire to eat soil are common in moderate hookworm infection. In severe cases, malnutrition can be debilitating. Interestingly, in the 1920's, hookworm infection was much more common in the U.S. and one particular county was famed for the quality of its soil. A mail-order business was formed to ship clay soil to hookworm sufferers throughout the country.
Diagnosis is made by finding eggs or worms
in the feces. Bephenium hydroxynapthoate, pyrantel pamoate, and
mebendazole are used to treat hookworm.
Ascarids (roundworms) have been epidemic in the recent past in the southeastern U.S. Infection occurs when active eggs are swallowed from contaminated food and water. The eggs hatch and penetrate the gut, then migrate to the heart and lungs. Many juvenile worms get lost and migrate to other parts of the body.
In the lungs the juvenile worms continue to develop. They move up the respiratory tract to the pharynx, where they are swallowed into the stomach. They then pass through to the intestine, where they mature and feed on the intestine's liquid contents.
Occasionally an adult will migrate to the stomach and cause nausea, and sometimes vomiting. In these cases, the human host can expel the worm in the vomit. (Expelled worms as big as 1 ft long have been reported.) Worms that reach the esophagus while a person is asleep can exit the body through the nose or mouth.
Diagnosis is made by finding juvenile
worms, eggs in the stool, or by the appearance (in vomit or otherwise) of
the worm itself. Mebendazole is the most common drug used to treat
Pinworms are acquired by ingesting the eggs. These can be found on soiled fingers or objects, such as contaminated bedding or towels. Airborne eggs may be inhaled and swallowed.
Once in the host, the female pinworms move out of the anus to deposit eggs. This leads to itching around the anus, and sometimes to subsequent bacterial infection and intensified itching.
Diagnosis can be made only by finding eggs or worms on or in the patient. Infections can be discovered by examining the area around the anus at night with a flashlight. Wandering worms glisten and can be easily seen.
If infection is suspected, you can obtain eggs by using the cellophane-tape test. It is best for the physician to teach the parent how to perform this test.
The preferred drugs for treating pinworm
infections are pyrantel pamoate, mebendazole, or albendazole. Treatment
should be repeated after about 10 days to kill worms acquired after the
first dose. All family members should be treated simultaneously even
if the infection has not been detected in every family member. The
household and bedding should be cleaned to lower the prevalence of
Guinea worms (dracunculiasis) result when a person swallows a small crustacean (cyclopoid) that contains the parasite. The juvenile parasite is released in the gut. This parasite migrates to a position just under the skin, usually in the armpit or junction of the leg to the body.
Males die shortly after mating. The females then migrate to the skin of the arms or legs and cause a blister which contains juvenile parasites. The blister ruptures, and the young worms exit.
Cold water is a trigger that causes the female to release the juveniles. In the normal cycle, these juveniles are eaten by the crustacean that is then ingested by the human host.
The historical treatment for guinea worms is to pull them out slowly, centimeter by centimeter, by winding them on a stick. Cold water triggers the female worm to expel enough juveniles to allow about 5 cm of her body to be pulled out. This procedure is repeated once a day. Complete removal of the worm takes about 3 weeks.
Guinea worms can also be removed by surgery
or eradicated through drug therapy. Metronidazole is the drug of
choice, but thiabendazole or mebendazole can also be used.
Filarial worms are tissue-dwelling parasites. They are endemic in central Africa, Turkey, India, Southeast Asia, Australia, Phillipines, and South America. Filariasis was seen in the US as late as the 1920's but has since died out.
The parasite is transmitted by mosquitoes. After infection, the adult filarial worms migrate to and live in the lymphatic ducts of humans. There, they clog up the blood vessels and cause the enlargement of the various body parts. The worms then infiltrate connective tissue.
Diagnosis is by finding microfilaria in the blood. Diethlycarbamazine (the drug of choice) kills the microfilaria, and usually also kills the adult worms.
River blindness is a filariasis found in Africa, Arabia, Guatemala, Mexico, Venezuela, and Colombia. It is transmitted by the black fly. During infection, nodules (onchocercoma) that contain the parasites appear under the skin. This disease can be asymptomatic. In many other cases, it causes disfigurement and blindness.
The most dramatic type of infection caused by filarial worms is elephantiasis. This disease is characterized by horribly swollen body parts, and thickened and cracked skin. It is a common condition; dwarfism has also been seen when the pituitary is affected. Elephantiasis results in blindness when the cornea or retina is affected.
Diagnosis is by detection of the microfilia in skin nips. Treatment is by surgery or by the use of suramin. Suramin kills the adult worms and eradicates the microfilaria.
Loa Loa (Eye Worm)
This filarial worm is transmitted by the deer fly in the rain forest areas of west Africa and equatorial Sudan. Adult worms migrate through subcutaneous tissues, sometimes winding up in the eye.