Overview of Parasitic Infections: Parasitic Infections: Merck Manual Home Edition

Infections

Parasitic Infections

Overview of Parasitic Infections

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Overview of Parasitic Infections

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Parasitic infections are more common in rural or developing areas than in developed areas.
In developed areas, these infections may occur in immigrants or in people with a weakened immune system.
Parasites usually enter the body through the mouth or skin.
Doctors diagnose the infection by taking samples of blood, stool, urine, phlegm or other infected tissue and examining or sending them to a laboratory for analysis.
Travelers to areas where food, drink, and water may be contaminated are advised to cook it, boil it, peel it, or forget it.

A parasite is an organism that lives on or inside another organism (the host) and harms the host.

Parasitic infections are common in rural or developing areas of Africa, Asia, and Latin America and less common in developed areas. A person who visits such an area can unknowingly acquire a parasitic infection, and a doctor may not readily diagnose the infection when the person returns home. In developed areas, parasitic infections may also affect immigrants and people with a weakened immune system (such as those who have AIDS or who take drugs that suppress the immune system). The infections may occur in places with poor sanitation and unhygienic practices (as occurs in some mental institutions and day care centers).

Parasites usually enter the body through the mouth or skin. Parasites that enter through the mouth are swallowed and can remain in the intestine or burrow through the intestinal wall and invade other organs. Parasites that enter through the skin bore directly through the skin or are introduced through the bites of infected insects (called vectors because they convey or transmit organisms that cause disease). Some parasites enter through the soles of the feet when a person walks barefoot or through the skin when a person swims or bathes in water containing the parasites. Rarely, parasites are spread through blood transfusions, through injections with a needle previously used by an infected person, or from a pregnant woman to her fetus.

Parasites that infect people include protozoa (such as amebas), which consist of only one cell, and worms (helminths, such as hookworms and tapeworms), which are larger and consist of many cells and have internal organs. Protozoa, which reproduce by cell division, can reproduce inside people. Helminths, in contrast, produce eggs or larvae that develop in the environment before they become capable of infecting people. Development in the environment may involve another animal (an intermediate host). Some protozoa (such as those that cause malaria) and some helminths (such as those that cause river blindness) have complex life cycles and are transmitted by insect vectors.

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Other Parasites
Geographic Area	Source	Common Symptoms	Diagnosis	Treatment
Dog heartworm (Dirofilaria species)
Worldwide (but rare in people)	Larvae are transmitted to people through the bite of infected mosquitoes.	Usually, no symptoms Occasionally, chest pain, cough, and blood in phlegm (sputum) Rarely, nodules under the skin, swelling of the face or eyelid, and a change in vision	Biopsy of lung tissue	None needed
Dwarf tapeworm (Hymenolepis nana)
Worldwide	Eggs may be ingested in food or water contaminated by human feces or may be transferred to the mouth after contact with infected people. Or infected insects, such as fleas and beetles, may be ingested accidentally (for example, in insect-infested grains).	Nausea, vomiting, diarrhea, abdominal discomfort, and weight loss in children with a severe infection	Stool tests	PraziquantelSome Trade Names BILTRICIDE
Echinococcus species (a tapeworm)
Areas of the world where sheep or cattle are raised, as in the Mediterranean, Middle East, Australia, South Africa, and South America, and in areas of Canada, Alaska, California, and Midwestern United States	Eggs excreted in the feces of infected dogs or wild carnivores may be transferred from the hands to the mouth after touching the animal's fur or be ingested in contaminated food.	Abdominal pain and jaundice if the liver is involved Chest pain and coughing up blood or the contents of cysts if the lungs are involved Hives or a severe life-threatening allergic reaction (anaphylaxis)	CT, ultrasonography, or MRI of the liver Sometimes withdrawal of fluid from a cyst in the liver Chest x-ray or CT of the lungs	AlbendazoleSome Trade Names ALBENZA alone or with surgical removal of cysts or Drainage of the cyst with a needle guided by ultrasonography, followed by injection, then removal of a salt solution to kill the parasites in the cyst (percutaneous aspiration-injection-reaspiration)
Intestinal flukes
Most common in the Far East	Flukes on aquatic plants (such as water chestnuts) or in raw or undercooked freshwater fish are ingested.	Usually no symptoms, but with severe infections, abdominal pain, diarrhea, and fever	Stool tests	PraziquantelSome Trade Names BILTRICIDE
Liver flukes (Clonorchis sinensis)
The Far East	Fluke cysts are ingested in raw, dried, salted, or pickled freshwater fish.	Abdominal pain, jaundice, diarrhea, and, years later, cancer of the biliary tract	Stool tests and sometimes colonoscopy	PraziquantelSome Trade Names BILTRICIDE If the biliary tract is blocked, surgery
Loa loa (a filarial worm)
Rain forest belt of western and central Africa	Larvae are transmitted to people through the bite of infected tabanid flies (such as horseflies and deerflies).	Itchy, red areas of swelling (most commonly on the wrists and ankles) and awareness of worms passing across the eye but no eye damage	Blood tests	Diethylcarbamazine (DEC)
Lung flukes (Paragonimus westermani)
Most common in the Far East	Cysts in raw, pickled, or undercooked freshwater crabs and crayfish are ingested.	Difficulty breathing, cough, chest pain, and coughing up blood	Sputum or stool tests, chest x-ray or chest CT, and blood tests	PraziquantelSome Trade Names BILTRICIDE
Sheep liver fluke (Fasciola hepatica)
Areas of the world where sheep or cattle are raised, including Bolivia, Peru, Portugal, France, Iran, Egypt, and Asia	Flukes on watercress or other water plants contaminated by sheep or cattle feces are ingested.	Abdominal pain, fever, fatigue, vague discomfort (malaise), and weight loss due to liver damage	Stool tests and CT, ultrasonography, or MRI of the liver	BithionolSome Trade Names BITIN LOROTHIDOL or triclabendazole
Strongyloides stercoralis, or threadworm (a roundworm)
Moist subtropics and tropics and the southeastern United States	Larvae in stool (feces) contaminate the soil and enter through the skin, usually the feet.	Abdominal pain, diarrhea, nausea, vomiting, hives or a rash that changes location, wheezing, and asthma	Stool tests, blood tests, and sometimes colonoscopy If infection is severe and widespread, sputum testing	IvermectinSome Trade Names STROMECTOL or albendazoleSome Trade Names ALBENZA
Trypanosoma brucei gambiense, mainly in West Africa, and Trypanosoma brucei rhodesiense, mainly in East Africa (protozoa that cause African sleeping sickness)
Parts of equatorial Africa	Protozoa are injected through the skin when tsetse flies bite.	Painful bump at the bite site, followed by fever, headache, rash, enlarged lymph nodes, and, eventually (when the brain and spinal fluid are infected), sleepiness, difficulty walking, coma, and, if untreated, ultimately death (infection progresses over many months in West African disease but within weeks in East African disease)	Blood tests and spinal tap	West African sleeping sickness: EflornithineSome Trade Names VANIQA or the same drugs used to treat East African sleeping sickness East African sleeping sickness: Suramin, pentamidineSome Trade Names NEBUPENT , or melarsoprol (if the brain and spinal fluid are infected)
Trypanosoma cruzi (protozoa that cause Chagas' disease)
North, Central, and South America	While biting a person, Triatomine bugs (kissing or assassin bugs) defecate, depositing the protozoa in their feces. The protozoa enter through the bug's bite wound, penetrate mucous membranes, or are rubbed into the eyes. The protozoa are sometimes transmitted through blood transfusions or organ transplants or from a pregnant woman to her fetus.	Initially, rash or swelling at the point of entry, swelling around one eye, generalized weakness, and rare but potentially fatal heart or brain infection Years later, long-term heart and gastrointestinal problems	Blood tests	Nifurtimox (United States) or benznidazole (Latin America)
Wuchereria bancrofti, Brugia malayi, and Brugia timori (worms that cause lymphatic filariasis)
Tropical and subtropical areas worldwide	Larvae are transmitted to people through the bite of infected mosquitoes.	Fever, swollen lymph nodes in the groin and armpits, swelling and pain in the groin and limbs, and bacterial infections	Blood tests, including for Wuchereria bancrofti only, and antigen testing	Diethylcarbamazine (DEC) alone or with doxycyclineSome Trade Names VIBRAMYCIN Antibiotics to treat coexisting bacterial skin infections Local measures (such as elevation and an elastic bandage) to reduce swelling (edema)
CT = computed tomography; MRI = magnetic resonance imaging.

Other Parasites

Geographic Area

Source

Common Symptoms

Diagnosis

Treatment

Dog heartworm (Dirofilaria species)
Worldwide (but rare in people)	Larvae are transmitted to people through the bite of infected mosquitoes.	Usually, no symptoms Occasionally, chest pain, cough, and blood in phlegm (sputum) Rarely, nodules under the skin, swelling of the face or eyelid, and a change in vision	Biopsy of lung tissue	None needed
Dwarf tapeworm (Hymenolepis nana)
Worldwide	Eggs may be ingested in food or water contaminated by human feces or may be transferred to the mouth after contact with infected people. Or infected insects, such as fleas and beetles, may be ingested accidentally (for example, in insect-infested grains).	Nausea, vomiting, diarrhea, abdominal discomfort, and weight loss in children with a severe infection	Stool tests	PraziquantelSome Trade Names BILTRICIDE
Echinococcus species (a tapeworm)
Areas of the world where sheep or cattle are raised, as in the Mediterranean, Middle East, Australia, South Africa, and South America, and in areas of Canada, Alaska, California, and Midwestern United States	Eggs excreted in the feces of infected dogs or wild carnivores may be transferred from the hands to the mouth after touching the animal's fur or be ingested in contaminated food.	Abdominal pain and jaundice if the liver is involved Chest pain and coughing up blood or the contents of cysts if the lungs are involved Hives or a severe life-threatening allergic reaction (anaphylaxis)	CT, ultrasonography, or MRI of the liver Sometimes withdrawal of fluid from a cyst in the liver Chest x-ray or CT of the lungs	AlbendazoleSome Trade Names ALBENZA alone or with surgical removal of cysts or Drainage of the cyst with a needle guided by ultrasonography, followed by injection, then removal of a salt solution to kill the parasites in the cyst (percutaneous aspiration-injection-reaspiration)
Intestinal flukes
Most common in the Far East	Flukes on aquatic plants (such as water chestnuts) or in raw or undercooked freshwater fish are ingested.	Usually no symptoms, but with severe infections, abdominal pain, diarrhea, and fever	Stool tests	PraziquantelSome Trade Names BILTRICIDE
Liver flukes (Clonorchis sinensis)
The Far East	Fluke cysts are ingested in raw, dried, salted, or pickled freshwater fish.	Abdominal pain, jaundice, diarrhea, and, years later, cancer of the biliary tract	Stool tests and sometimes colonoscopy	PraziquantelSome Trade Names BILTRICIDE If the biliary tract is blocked, surgery
Loa loa (a filarial worm)
Rain forest belt of western and central Africa	Larvae are transmitted to people through the bite of infected tabanid flies (such as horseflies and deerflies).	Itchy, red areas of swelling (most commonly on the wrists and ankles) and awareness of worms passing across the eye but no eye damage	Blood tests	Diethylcarbamazine (DEC)
Lung flukes (Paragonimus westermani)
Most common in the Far East	Cysts in raw, pickled, or undercooked freshwater crabs and crayfish are ingested.	Difficulty breathing, cough, chest pain, and coughing up blood	Sputum or stool tests, chest x-ray or chest CT, and blood tests	PraziquantelSome Trade Names BILTRICIDE
Sheep liver fluke (Fasciola hepatica)
Areas of the world where sheep or cattle are raised, including Bolivia, Peru, Portugal, France, Iran, Egypt, and Asia	Flukes on watercress or other water plants contaminated by sheep or cattle feces are ingested.	Abdominal pain, fever, fatigue, vague discomfort (malaise), and weight loss due to liver damage	Stool tests and CT, ultrasonography, or MRI of the liver	BithionolSome Trade Names BITIN LOROTHIDOL or triclabendazole
Strongyloides stercoralis, or threadworm (a roundworm)
Moist subtropics and tropics and the southeastern United States	Larvae in stool (feces) contaminate the soil and enter through the skin, usually the feet.	Abdominal pain, diarrhea, nausea, vomiting, hives or a rash that changes location, wheezing, and asthma	Stool tests, blood tests, and sometimes colonoscopy If infection is severe and widespread, sputum testing	IvermectinSome Trade Names STROMECTOL or albendazoleSome Trade Names ALBENZA
Trypanosoma brucei gambiense, mainly in West Africa, and Trypanosoma brucei rhodesiense, mainly in East Africa (protozoa that cause African sleeping sickness)
Parts of equatorial Africa	Protozoa are injected through the skin when tsetse flies bite.	Painful bump at the bite site, followed by fever, headache, rash, enlarged lymph nodes, and, eventually (when the brain and spinal fluid are infected), sleepiness, difficulty walking, coma, and, if untreated, ultimately death (infection progresses over many months in West African disease but within weeks in East African disease)	Blood tests and spinal tap	West African sleeping sickness: EflornithineSome Trade Names VANIQA or the same drugs used to treat East African sleeping sickness East African sleeping sickness: Suramin, pentamidineSome Trade Names NEBUPENT , or melarsoprol (if the brain and spinal fluid are infected)
Trypanosoma cruzi (protozoa that cause Chagas' disease)
North, Central, and South America	While biting a person, Triatomine bugs (kissing or assassin bugs) defecate, depositing the protozoa in their feces. The protozoa enter through the bug's bite wound, penetrate mucous membranes, or are rubbed into the eyes. The protozoa are sometimes transmitted through blood transfusions or organ transplants or from a pregnant woman to her fetus.	Initially, rash or swelling at the point of entry, swelling around one eye, generalized weakness, and rare but potentially fatal heart or brain infection Years later, long-term heart and gastrointestinal problems	Blood tests	Nifurtimox (United States) or benznidazole (Latin America)
Wuchereria bancrofti, Brugia malayi, and Brugia timori (worms that cause lymphatic filariasis)
Tropical and subtropical areas worldwide	Larvae are transmitted to people through the bite of infected mosquitoes.	Fever, swollen lymph nodes in the groin and armpits, swelling and pain in the groin and limbs, and bacterial infections	Blood tests, including for Wuchereria bancrofti only, and antigen testing	Diethylcarbamazine (DEC) alone or with doxycyclineSome Trade Names VIBRAMYCIN Antibiotics to treat coexisting bacterial skin infections Local measures (such as elevation and an elastic bandage) to reduce swelling (edema)
CT = computed tomography; MRI = magnetic resonance imaging.

Diagnosis

Doctors suspect a parasitic infection in people who have typical symptoms and who live or have traveled to an area where sanitation is poor or where such an infection is known to occur. Laboratory analysis of specimens, including special tests to identify proteins released by the parasite (antigen testing), may be needed. Samples of blood, stool, urine, or phlegm (sputum) may be taken. The doctor may also take a sample of tissue that may contain the parasite. For example, a biopsy may be done to obtain a sample of lung or intestinal tissue. A sample of skin may be snipped. Several samples and repeated examinations may be necessary to find the parasite.

If parasites live in the intestinal tract, their eggs or cysts (a dormant form of the parasite) may be found in the person's stool when a sample is examined under a microscope. Or parasites may be identified by testing the stool for proteins or other materials that they release. Antibiotics, laxatives, and antacids should not be used until after the stool sample has been collected. These drugs can reduce the number of parasites enough to make seeing the parasites in a stool sample difficult or impossible.

Prevention

In areas of the world where food, drink, and water may be contaminated with parasites, wise advice for travelers is to avoid drinking tap water and to "cook it, boil it, peel it, or forget it." Because some parasites survive freezing, ice cubes can sometimes transmit disease unless the cubes are made from purified water. Information about precautions needed in specific areas is available from the Centers for Disease Control and Prevention.

Last full review/revision March 2007 by Richard D. Pearson, MD