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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 industrialized 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 industrialized areas, parasitic infections also may 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 (the vector). 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 humans include protozoa (such as amebas), which consist of only one cell, and worms (helminths, such as the 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
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Geographic Area
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Source
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Common Symptoms
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Diagnosis
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Treatment
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Dog heartworm (Dirofilaria species)
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Worldwide (but rare in people)
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Larvae are transmitted to people through the bite of infected mosquitoes
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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
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Biopsy of lung tissue
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Dwarf tapeworm (Hymenolepis nana)
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Worldwide
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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)
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Nausea, vomiting, diarrhea, abdominal discomfort, and weight loss in children with a severe infection
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Stool tests
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Praziquantel
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Echinococcus species (a tapeworm)
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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
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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
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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)
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CT, ultrasonography, or MRI of the liver; sometimes withdrawal of fluid from a cyst in the liver; and chest x-ray or CT of the lungs
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Albendazole alone or with surgical removal of cysts
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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)
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Intestinal flukes
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Most common in the Far East
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Flukes on aquatic plants (such as water chestnuts) or in raw or undercooked freshwater fish are ingested
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Usually no symptoms, but with severe infections, abdominal pain, diarrhea, and fever
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Stool tests
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Praziquantel
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Liver flukes (Clonorchis sinensis)
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The Far East
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Fluke cysts are ingested in raw, dried, salted, or pickled freshwater fish
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Abdominal pain, jaundice, diarrhea, and years later, cancer of the biliary tract
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Stool tests and sometimes colonoscopy
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Praziquantel
If the biliary tract is blocked, surgery
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Loa loa (a filarial worm)
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Rain forest belt of western and central Africa
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Larvae are transmitted to people through the bite of infected tabanid flies (such as horseflies and deerflies)
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Itchy, red areas of swelling (most commonly on the wrists and ankles), and awareness of worms passing across the eye but no eye damage
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Blood tests
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Diethylcarbamazine (DEC)
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Lung flukes (Paragonimus westermani)
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Most common in the Far East
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Cysts in raw, pickled, or undercooked freshwater crabs and crayfish are ingested
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Difficulty breathing, cough, chest pain, and coughing up blood
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Sputum or stool tests, chest x-ray or chest CT, and blood tests
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Praziquantel
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Sheep liver fluke (Fasciola hepatica)
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Areas of the world where sheep or cattle are raised, including Bolivia, Peru, Portugal, France, Iran, Egypt, and Asia
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Flukes on watercress or other water plants contaminated by sheep or cattle feces are ingested
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Abdominal pain, fever, fatigue, vague discomfort (malaise), and weight loss due to liver damage
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Stool tests and CT, ultrasonography, or MRI of the liver
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Bithionol or triclabendazole
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Strongyloides stercoralis, or threadworm (a roundworm)
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Moist subtropics and tropics and in the southeastern United States
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Larvae in stool (feces) contaminate the soil and enter through the skin, usually the feet
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Abdominal pain,
diarrhea, nausea, vomiting, hives or a rash that changes
location, wheezing, and asthma
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Stool tests, blood tests, and sometimes colonoscopy
If infection is severe and widespread, sputum testing
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Ivermectin or albendazole
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Trypanosoma brucei gambiense (West African) and Trypanosoma brucei rhodesiense (East African) (protozoa that cause African sleeping sickness)
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Parts of equatorial Africa
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Protozoa are injected through the skin when tsetse flies bite
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Painful bump at 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. The course in West African disease is over many months; East African disease occurs in weeks
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Blood tests and spinal tap
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West African sleeping sickness:
Eflornithine or the same drugs used to treat East African sleeping sickness
East African sleeping sickness:
Suramin, pentamidine , or
melarsoprol (if the brain and spinal fluid are infected)
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Trypanosoma cruzi (protozoa that cause Chagas' disease)
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North, Central, and South America
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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
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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
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Blood tests
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Nifurtimox or
benznidazole
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Wuchereria bancrofti, Brugia malayi, and Brugia timori (worms that cause lymphatic filariasis)
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Tropical and subtropical areas worldwide
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Larvae are transmitted to people through the bite of infected mosquitoes
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Fever, swollen lymph nodes in the groin and armpits, swelling and pain in the groin and limbs, and bacterial infections
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Blood tests, including for Wuchereria bancrofti only, antigen testing
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Diethylcarbamazine (DEC) alone or with doxycycline
Antibiotics to treat coexisting bacterial skin infections
Local measures (such as elevation and an elastic bandage) to reduce swelling (edema)
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CT = computed tomography; MRI = magnetic resonance imaging.
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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
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