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Approach to Parasitic Infections

ByChelsea Marie, PhD, University of Virginia;
William A. Petri, Jr, MD, PhD, University of Virginia School of Medicine
Reviewed/Revised Apr 2025
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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 and microsporidia)

  • Multicellular helminths (worms, including nematodes, cestodes, and trematodes)

  • Ectoparasites (eg, scabies, lice)

Parasitic infections due to protozoa and helminths are responsible for substantial morbidity and mortality worldwide (1). Parasitic infections are prevalent in Central and South America, Africa, and Asia. They are much less common in Australia, Canada, Europe, Japan, New Zealand, and the United States. By far, the greatest impact is on residents of resource-limited tropical areas with poor sanitation. However, parasitic infections do develop in high-resource countries with adequate sanitation in immigrants and travelers returning from endemic regions and, on occasion, in residents who have not traveled, particularly those with HIV infection 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 of various organs where conditions are aerobic.

Many intestinal parasitic infections are spread through a fecal-oral route (ingestion of food or water contaminated by feces). 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 contaminated freshwater. Some parasitic infections, such as malaria, are transmitted by arthropod vectors, and others, such as trichomoniasis, are sexually transmitted. Rarely, parasites are transmitted via blood transfusions or shared needles or congenitally from pregnant patient to fetus.

Some parasites are endemic in the United States and other high-resource countries. Examples include Trichomonas vaginalis, Toxoplasma gondii, and enteric parasites such as Giardia intestinalis (also known as G. duodenalis or G. lamblia), Cryptosporidium species, and Enterobius vermicularis (pinworm) (2).

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

Protozoa

Protozoa are single-celled organisms that multiply by simple binary division (see Overview of Free-Living Amebas and Overview of Intestinal Protozoan and Microsporidia Infections). Protozoa can multiply in their human hosts, increasing in number to cause overwhelming infection.

With rare exceptions, such as Isospora species, protozoan infections do not cause eosinophilia.

Microsporidia

Microsporidia 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 HIV infection or another immunocompromising condition.

The clinical manifestations depend on the infecting species and include gastroenteritis, involvement of the eyes, or disseminated infection.

Helminths

Helminths are multicellular and have complex organ systems.

Helminths can be further divided into

In contrast to protozoa, helminths usually 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, autoinfection can result in life-threatening, disseminated hyperinfection in immunosuppressed people, particularly those taking corticosteroids (because they kill eosinophils, which are directly involved in anti-helminthic immune responses) or other immunosuppressants, or in hematopoietic stem cell transplant recipients.

The severity of helminthic infections usually correlates with the worm burden, but there are exceptions, for example, when a single adult Ascaris worm causes life-threatening 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 immune responses. If an infected person moves away from an endemic area, the number of adult worms diminishes over time because of the lack of frequent re-exposure. Although a few helminths (eg, Clonorchis sinensis) can survive for decades in humans, many species have life spans of only a few years or less.

Nematodes (roundworms) are nonsegmented cylindric worms ranging from 1 mm to 1 m in length. Nematodes have a body cavity, distinguishing them from tapeworms and flukes. Depending on the species, different stages in the life cycle are infectious to humans. Hundreds of millions of humans are infected with nematodes that live in the intestines and are transmitted by ova or larvae in feces; the most common are Ascaris (ascariasis), Enterobius vermicularis (pinworms), hookworms, Trichuris (trichuriasis), and Strongyloides (strongyloidiasis).

Cestodes (tapeworms) as adults are multisegmented flatworms that lack a digestive tract and absorb nutrients directly from the host’s small bowel. In the host’s digestive tract, adult tapeworms can become large, up to 40 m for one species. Tapeworms that infect humans include Diphyllobothrium latum (fish tapeworm), Taenia saginata (beef tapeworm), and Taenia solium (pork tapeworm).

Trematodes (flukes) 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 long as 7 cm. In humans, most fluke infections are caused by Schistosoma species (schistosomiasis); liver flukes, including Fasciola hepatica (fascioliasis) and Clonorchis sinensis (clonorchiasis); and lung flukes, including certain Paragonimus species (paragonimiasis).

Ectoparasites

Ectoparasites are organisms that live on the exterior surface of a host and feed on its tissues or bodily fluids. They produce eggs and lay them on or in the skin of the host. Ectoparasites include lice, ticks, mites, bedbugs, and fleas.

Lice, including head lice (Pediculus humanus var. capitis), body lice (P. humanus var. corporis), and pubic lice (Phthirus pubis), feed on blood. They are usually asymptomatic but can cause itching; they are highly contagious.

Ticks feed on blood and transmit various diseases, such as Lyme disease (Borrelia species, Ixodes species) and Rocky Mountain spotted fever (Rickettsia rickettsii).

Mites are tiny insects that feed on dead skin cells. They can cause skin infestations, such as scabies (Sarcoptes scabiei) or mange (Demodex mites). Because of their preference for humid and warm environments, they are often found indoors, especially in bedrooms and other rooms with carpeting. They are primarily implicated in exacerbating allergic rhinoconjunctivitis and asthma in sensitized people. Dust mites can cause pulmonary hypersensitivity.

Bedbugs (Cimex lectularis and C. hemipterus) are small ectoparasites that exclusively feed on blood and live on the external surfaces of human and animal hosts. They have become widespread human ectoparasites globally, largely attributed to increased international travel and the development of insecticide resistance. Bedbugs use various cues, including heat, carbon dioxide, and body odors, to locate their primary hosts, humans.

Fleas feed on blood and cause dermatitis and secondary bacterial infections. They also serve as vectors that transmit infections, including cat-scratch disease (Bartonella henselae), flea-borne typhus (Rickettsia typhi), and plague (Yersinia pestis). Flea bites can cause itching and allergic reactions.

Ectoparasites usually undergo a life cycle that includes egg, larval, and adult stages. Their life cycles can vary widely, with some ectoparasites reproducing quickly and affecting hosts in a short period. They are usually transmitted through direct contact with an infected host or through the environment (eg, fleas and lice tend to spread easily in crowded living conditions, ticks can attach to a host walking through grass or wooded areas).

General references

  1. 1. Torgerson PR, Devleesschauwer B, Praet N, et al. World Health Organization Estimates of the Global and Regional Disease Burden of 11 Foodborne Parasitic Diseases, 2010: A Data Synthesis. PLoS Med. 2015;12(12):e1001920. Published 2015 Dec 3. doi:10.1371/journal.pmed.1001920

  2. 2. Pyzocha N, Cuda A. Common Intestinal Parasites. Am Fam Physician. 2023;108(5):487-493.

Diagnosis of Parasitic Infections

  • Microscopic examination

  • Antigen and DNA tests

Methods used to diagnose specific parasitic diseases are summarized in the table Collecting and Handling Specimens for Microscopic Diagnosis of Parasitic Infections.

Table
Table

Parasitic infection should be especially considered among the differential diagnoses in some clinical situations, for example, fever in a traveler returning from an endemic area suggests the possibility of malaria. People who have immigrated from endemic areas to nonendemic areas and who return home to visit friends and relatives are at particularly increased risk. Immediate family members, such as spouses and children, are also at increased risk. Immigrants travelling to their country of origin may be newly susceptible to endemic infections because of waning immunity due to lack of continued exposure. Consequently, they may be less likely to seek pre-travel vaccinations, prophylactic medications, or preventive health counseling. Compared to tourists who stay in resort settings, immigrants are also more likely to visit areas with higher rates of disease transmission (1).

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

Exposure 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 on the presence of antibodies in serum, but diagnosis is being increasingly based on identification of parasite antigens or molecular tests for parasite DNA.

Clinicians 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

The various stages of protozoa and helminths that infect the gastrointestinal tract are typically shed in the stool. Routine detection requires the examination of stool specimens, preferably 3 collected on different days, because shedding can vary. The sensitivity of stool examination for ova and parasites is low enough that when clinical suspicion is strong, empirical treatment should be considered even if stool examination is negative.

Sensitive and specific molecular assays, such as multiplex polymerase chain reaction (PCR)–based assays, are available to detect antigens of Giardia, Cryptosporidium, Entamoeba histolytica, and Cyclospora in stool; however, these assays can be expensive (2) (see table Serologic and Molecular Tests for Parasitic Infections). Multiplex PCR assays can detect multiple enteric pathogens with greater sensitivity and speed than traditional diagnostic modalities such as cultures; however, these assays do not differentiate between viable and nonviable organisms. This lack of differentiation can present challenges for interpretation, in particular when multiple pathogens are detected in a single specimen. In these cases, the patient's history, the clinical and temporal manifestations of infectious symptoms, and the relative quantity of pathogen DNA can be helpful in guiding treatment decisions (3).

Freshly passed stool specimens uncontaminated with urine, water, dirt, or disinfectants (regardless of whether obtained in the clinic or in the patient's home) should be sent to the laboratory within 1 hour; unformed or watery stools are most likely to contain motile trophozoites. Once the specimen is in the laboratory, if not examined immediately, they should be refrigerated, but not frozen. Laboratory technicians should then emulsify portions of fresh stools in fixative to preserve gastrointestinal protozoa. Concentration techniques can also be used to improve sensitivity. If strongyloidiasis is suspected, one or more specialized stool tests should be done if larvae are not seen on direct examination of fresh stool. Antibiotics, radiographic contrast material, purgatives, and antacids can hinder detection of ova and parasites for several weeks. If nematode infections are suspected, anal cellophane tape or swabs may also be used to collect pinworm or tapeworm eggs.

Sigmoidoscopy or colonoscopy should be considered when routine stool examinations are negative and amebiasis is suspected in patients with persistent gastrointestinal symptoms. Sigmoidoscopic specimens should be collected with a curet or spoon (cotton swabs are not suitable) and processed immediately for microscopy. Upper GI endoscopy may be performed to include duodenal aspirates or small-bowel biopsy specimens; upper GI endoscopic specimens may sometimes be necessary for diagnosis of such infections as cryptosporidiosis and microsporidiosis.

Serologic testing for parasitic infections

Some parasites can be detected by serologic tests (see table Serologic and Molecular Tests for Parasitic Infections).

Table
Table

Diagnosis references

  1. 1. Centers for Disease Control and Prevention (CDC): Visiting Friends & Relatives: VFR Travel. CDC Yellow Book 2024. Accessed January 31, 2025.

  2. 2. Shane AL, Mody RK, Crump JA, et al. 2017 Infectious Diseases Society of America Clinical Practice Guidelines for the Diagnosis and Management of Infectious Diarrhea. Clin Infect Dis. 2017;65(12):e45-e80. doi:10.1093/cid/cix669

  3. 3. Miller JM, Binnicker MJ, Campbell S, et al. A Guide to Utilization of the Microbiology Laboratory for Diagnosis of Infectious Diseases: 2018 Update by the Infectious Diseases Society of America and the American Society for Microbiology. Clin Infect Dis. 2018;67(6):e1-e94. doi:10.1093/cid/ciy381

Treatment of Parasitic Infections

  • Various treatments, depending on the specific infection

See under specific infections elsewhere in The Manual.

Advice for treating parasitic infections is available from experts at major medical and public health centers and travel clinics, from the CDC's A-Z Index of Parasitic Diseases, the CDC's Clinical Care of Soil-transmitted Helminths, and in textbooks of infectious diseases and tropical medicine.

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

Prevention of Parasitic Infections

The recently developed antimalarial vaccines, R21/Matrix-M and RTS,S, are the first licensed vaccines for the prevention of a parasitic infection (see WHO: Malaria vaccines (RTS,S and R21)). Vaccines for the prevention of other parasitic infections, including schistosomiasis, leishmaniasis, and hookworms, are in development. Otherwise, prevention is largely based on avoidance strategies.

Transmission of most intestinal parasites can be prevented by the following:

  • Sanitary disposal of stool and feces

  • Handwashing

  • Adequate cooking of food

  • Provision of purified water

Handwashing is very important after use of bathrooms and latrines and prior to food preparation. During travel to areas with potential exposure to parasitic infections with fecal-oral transmission, the best advice is “cook it, boil it, peel it, or forget it,” meaning that travelers should avoid the following: eating meat, fish, shellfish, and eggs that are not fully cooked; eating uncooked fruit or vegetables; and eating or drinking unpasteurized dairy products or fruit juices. An exception to this guidance is that fruit or vegetables with a thick peel (eg, bananas) are usually considered safe to eat raw if the peel is washed before it is removed. Care should be taken to verify that potable water is obtained from safe sources. Because some parasites survive freezing, ice cubes can sometimes transmit disease unless the cubes are made from purified water. 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.

Other safety measures to prevent parasitic infections 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 have contact of bare skin to surfaces in areas where hookworms are found.

Prevention of malaria and many other vector-borne diseases involves the following:

  • Wearing long-sleeved shirts and pants

  • Applying diethyltoluamide (DEET)-containing insect repellents to exposed skin and permethrin to clothing (however, treating clothing is less effective than using insect repellents on skin)Applying diethyltoluamide (DEET)-containing insect repellents to exposed skin and permethrin to clothing (however, treating clothing is less effective than using insect repellents on skin)

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

  • Using insecticide sprays, including permethrin or pyrethrum sprays in homes and outbuildings

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

Travelers to rural Latin America should not sleep in adobe dwellings where reduviid bugs can transmit Chagas disease. In Africa, travelers should avoid bright-colored clothing and wear long-sleeved shirts and pants to avoid tsetse flies in regions where African sleeping sickness occurs.

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

More Information

The following English-language resource may be useful. Please note that The Manual is not responsible for the content of this resource.

  1. Centers for Disease Control and Prevention (CDC): Laboratory Identification of Parasitic Diseases of Public Health Concern

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