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Approach to Parasitic Infections


Chelsea Marie

, PhD, University of Virginia;

William A. Petri, Jr

, MD, PhD, University of Virginia School of Medicine

Reviewed/Revised Jun 2021 | Modified Sep 2022
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Human parasites are organisms that live on or in a person and derive nutrients from that person (its host). There are 3 types of parasites:

  • Single-cell organisms (protozoa, microsporidia)

  • Multicellular helminths (worms)

  • Ectoparasites such as scabies and lice

Parasitic infections due to protozoa and helminths are responsible for substantial morbidity and mortality worldwide. They are prevalent in Central and South America, Africa, and Asia. They are much less common in Australia, Canada, Europe, Japan, New Zealand, and the US. By far, the greatest impact is on residents of impoverished tropical areas with poor sanitation, but parasitic infections are encountered in developed countries among immigrants and travelers returning from endemic regions and, on occasion, even among residents who have not traveled, particularly those with AIDS or other conditions that cause immunodeficiency.

Some parasites have adapted to living in the lumen of the intestine or vagina where conditions are anaerobic; others reside in blood or tissues in aerobic conditions.

Many intestinal parasitic infections are spread through fecal contamination of food or water. They are most frequent in areas where sanitation and hygiene are poor. Some parasites, such as hookworms, can enter the skin during contact with contaminated dirt or, in the case of schistosomes, with freshwater. Others, such as malaria, are transmitted by arthropod vectors. Rarely, parasites are transmitted via blood transfusions or shared needles or congenitally from mother to fetus.

Some parasites are endemic in the US and other developed countries. Examples are the pinworm Enterobius vermicularis, Trichomonas vaginalis, Toxoplasma gondii, and enteric parasites such as Giardia intestinalis (also known as G. duodenalis or G. lamblia) and Cryptosporidium species.

The characteristics of protozoan and helminthic infections vary in important ways.



Microsporidia Microsporidiosis Microsporidiosis is infection with microsporidia. Symptomatic disease develops predominantly in patients with AIDS and includes chronic diarrhea, disseminated infection, and corneal disease... read more are intracellular spore-forming organisms that used to be classified as protozoa, but genetic analysis indicates that they are fungi or closely related to them. Human disease is mainly limited to people who have AIDS or another severe immunocompromising condition. The clinical manifestations depend on the infecting species and include gastroenteritis, involvement of the eyes, or disseminated infection.


Helminths are multicellular and have complex organ systems. Helminths can be further divided into

In contrast to protozoa, helminths do not multiply in humans but can elicit eosinophilic responses when they migrate through tissue. Most helminths have complex life cycles that involve substantial time outside their human hosts. A few, including Strongyloides stercoralis, Capillaria philippinensis, and Hymenolepis nana, can increase in number because of autoinfection (offspring reinfect the same host rather than being shed to infect another host). In strongyloidiasis Strongyloidiasis Strongyloidiasis is infection with Strongyloides stercoralis. Findings include abdominal pain and diarrhea, rash, pulmonary symptoms (including cough and wheezing), and eosinophilia.... read more Strongyloidiasis , autoinfection can result in life-threatening, disseminated hyperinfection in immunosuppressed people, particularly those taking corticosteroids.

The severity of helminthic infections usually correlates with the worm burden, but there are exceptions as when a single ascaris Ascariasis Ascariasis is infection with Ascaris lumbricoides or occasionally Ascaris suum (a closely related parasite of pigs). Light infections may be asymptomatic. Early symptoms are pulmonary... read more Ascariasis causes life-threatening acute pancreatitis Acute Pancreatitis Acute pancreatitis is acute inflammation of the pancreas (and, sometimes, adjacent tissues). The most common triggers are gallstones and alcohol intake. The severity of acute pancreatitis is... read more Acute Pancreatitis by migrating into and obstructing the pancreatic duct. The worm burden depends on the degree of environmental exposure, parasite factors, and the host’s genetically determined immune responses. If a person moves from an endemic area, the number of adult worms diminishes over time. Although a few parasites (eg, Clonorchis sinensis) can survive for decades in humans, many species have life spans of only a few years or less.

Trematodes (flukes Introduction to Trematodes (Flukes) Flukes are parasitic flatworms that infect the blood vessels, gastrointestinal tract, lungs, or liver. They are often categorized according to the principle organ system they invade: Clonorchis... read more ) are nonsegmented flatworms that infect the blood vessels, liver, lungs, or gastrointestinal tract. They are usually no more than a few centimeters in length; however, some are only 1 mm, and some are as large as 7 cm. In humans, most fluke infections are caused by Schistosoma species (schistosomiasis Schistosomiasis Schistosomiasis is infection with blood flukes of the genus Schistosoma, which are acquired transcutaneously by swimming or wading in contaminated freshwater. The organisms infect the... read more ), liver flukes including Fasciola hepatica (fascioliasis Fascioliasis Fascioliasis is infection with the liver fluke Fasciola hepatica, which is acquired by eating contaminated watercress or other water plants. Clinical manifestations include abdominal... read more Fascioliasis ) and Clonorchis sinensis (clonorchiasis Clonorchiasis Clonorchiasis is infection with the liver fluke Clonorchis sinensis. Infection is usually acquired by eating undercooked freshwater fish. Most infections are asymptomatic, but when present... read more Clonorchiasis ), and lung flukes including certain Paragonimus species (paragonimiasis Paragonimiasis Paragonimiasis is infection with the lung fluke Paragonimus westermani and related species. Humans are infected by eating raw, pickled, or poorly cooked freshwater crustaceans. Most infections... read more Paragonimiasis ).

Diagnosis of Parasitic Infections

  • Microscopic examination

  • Antigen and DNA tests


Parasitic infections should be considered in the differential diagnosis of clinical syndromes in residents of or travelers to areas where sanitation and hygiene are poor or where vector-borne diseases are endemic. For example, fever in a traveler returning from an endemic area suggests the possibility of malaria. Experience indicates that people who have immigrated from endemic areas to developed countries and who return home to visit friends and relatives are at particular risk. They frequently do not seek or cannot afford pretravel vaccines, medications, and advice on disease prevention and are more likely to enter high-risk settings than tourists who stay at resort facilities.

Although less frequent, the possibility of locally acquired parasitic infection must also be considered in residents of developed countries who present with suggestive clinical syndromes, even if they have not traveled; some parasites are endemic in developed countries and others (mainly those transmissible by the fecal-oral route) may be acquired from infected travelers.

Historical information, physical findings, and laboratory data may also suggest specific parasitic infections. For example, eosinophilia is common when helminths migrate through tissue and suggests a parasitic infection in an immigrant or returning traveler.

The diagnosis of parasitic infections was once based on the identification of ova, larvae, or adult parasites in stool, blood, tissue or other samples or the presence of antibodies in serum, but diagnosis is being increasingly based on identification of parasite antigens or molecular tests for parasite DNA.

Physicians with expertise in parasitic infections and tropical medicine are available for consultation at many major medical centers, travel clinics, and public health facilities.

For detailed descriptions of diagnostic methods, see the Centers for Disease Control and Prevention (CDC) Laboratory Identification of Parasites of Public Health Concern.

Gastrointestinal tract parasites

Various stages of protozoa and helminths that infect the gastrointestinal tract are typically shed in the stool. Routine detection requires examination of stool specimens, preferably 3 collected on different days, because shedding can vary. Sensitivity of stool examination for ova and parasites is low enough that when clinical suspicion is strong, empirical treatment should be considered. Sensitive and specific assays are now available to detect antigens of Giardia, Cryptosporidium, and Entamoeba histolytica in stool. Although expensive, molecular tests also are available for Giardia, Cryptosporidium, E. histolytica, and Cyclospora. Tests for one or more of these organisms are typically included in multiplex polymerase chain reaction (PCR)-based screens for enteric bacterial, viral, and parasitic pathogens in stool samples (see table Serologic and Molecular Tests for Parasitic Infections Serologic and Molecular Tests for Parasitic Infections Serologic and Molecular Tests for Parasitic Infections ).

Freshly passed stools uncontaminated with urine, water, dirt, or disinfectants should be sent to the laboratory within 1 hour; unformed or watery stools are most likely to contain motile trophozoites. If not examined immediately, stools should be refrigerated, but not frozen. Portions of fresh stools should also be emulsified in fixative to preserve gastrointestinal protozoa. Concentration techniques can be used to improve sensitivity. Anal cellophane tape or swabs may collect pinworm or tapeworm eggs. If strongyloidiasis Strongyloidiasis Strongyloidiasis is infection with Strongyloides stercoralis. Findings include abdominal pain and diarrhea, rash, pulmonary symptoms (including cough and wheezing), and eosinophilia.... read more Strongyloidiasis is suspected, one or more specialized stool tests should be done if larvae are not seen on direct examination of fresh stool. Antibiotics, x-ray contrast material, purgatives, and antacids can hinder detection of ova and parasites for several weeks.

Serologic testing for parasitic infections


Treatment of Parasitic Infections

  • Various treatments, depending on the specific infection

See under specific infections in THE MANUAL.

Advice for treating parasitic infections also is available from experts at major medical and public health centers and travel clinics, at the Centers for Disease Control and Prevention (CDC) web site, in textbooks of infectious diseases and tropical medicine, and in summary form from The Medical Letter on Drugs and Therapeutics.

Some drugs that are not approved by the US Food and Drug Administration for parasitic infections can be obtained from the CDC Drug Service.

Prevention of Parasitic Infections

Despite substantial investment and research, no vaccines are yet available for prevention of human parasitic infections. Prevention is based on avoidance strategies.

Transmission of most intestinal parasites can be prevented by

  • Sanitary disposal of feces

  • Handwashing

  • Adequate cooking of food

  • Provision of purified water

For the international traveler, the best advice is “cook it, boil it, peel it, or forget it.” When followed, these measures reduce but do not eliminate the risk of intestinal parasitic infections as well as the risk of bacterial and viral gastroenteritis. Handwashing is very important after use of bathrooms and latrines and prior to food preparation. Meat, particularly pork, and fish, especially freshwater varieties, should be thoroughly cooked before ingestion. Other safety measures include removing cat litter boxes from areas where food is prepared to prevent toxoplasmosis. People should not swim in freshwater lakes, streams, or rivers in areas where schistosomiasis is endemic or walk barefoot or sit bare-bottom in areas where hookworms are found.

  • Wearing long-sleeved shirts and pants

  • Applying diethyltoluamide (DEET)-containing insect repellants to exposed skin and permethrin to clothing

  • Using window screens, air-conditioning, and bed nets impregnated with permethrin or other insecticides

  • For residents of nonendemic areas who travel in regions where malaria is transmitted, taking prophylactic antimalarial drugs

Country-specific recommendations for travel are available from the Centers for Disease Control and Prevention (CDC): Travelers' Health and from the CDC Yellow Book 2020.

More Information

The following are some English-language resources from the Centers for Disease Control and Prevention (CDC) that may be useful. Please note that THE MANUAL is not responsible for the content of these resources.

Drugs Mentioned In This Article

Drug Name Select Trade
Coleman Insect Repellent, OFF! Deeep Woods
Acticin, Elimite, Nix Lice Killing Creme Rinse
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