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Approach to Parasitic Infections

by Richard D. Pearson, MD

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: protozoa, helminths (worms), and 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, 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.

Many 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 the hookworm, 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 (lamblia) and Cryptosporidium spp.

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


Protozoa are single-celled organisms that multiply by simple binary division (see Intestinal Protozoa and in Extraintestinal Protozoa). Protozoa can multiply in their human hosts, increasing in number to cause overwhelming infection. With rare exceptions, protozoan infections do not cause eosinophilia.


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

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

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, autoinfection can result in life-threatening, disseminated hyperinfections 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 causes life-threatening 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, many species have life spans of only a few years or less. More information about parasitic infections is available at the CDC’s Division of Parasitic Diseases .


Methods for the diagnosis of specific parasitic infections are discussed in the chapters to follow and are summarized in Collecting and Handling Specimens for Microscopic Diagnosis of Parasitic Infections.

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 the returning traveler suggests the possibility of malaria. Experience indicates that immigrants from developing areas to developed countries who return home to visit friends and relatives are at particular risk. They frequently do not seek or cannot afford pretravel 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 an endemic or imported parasitic infection must also be considered in residents of developed countries who present with suggestive clinical syndromes, even if they have not traveled.

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.

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

“Laboratory Identification of Parasites of Public Health Concern” provides detailed descriptions of diagnostic methods and is available from the Centers for Disease Control and Prevention (CDC) at .

Collecting and Handling Specimens for Microscopic Diagnosis of Parasitic Infections


Optimal Specimen

Collection Details



Plasmodium sp

Thick and thin smears of capillary blood (ie, finger or earlobe, using a disposable lancet) or 5–10 mL of fresh anticoagulated blood

Collect multiple samples during acute illness.

Prepare smears from capillary or anticoagulated blood within 3 h after collection.


Use Wright or Giemsa stain.

Ensure that glass slides are clean.

Babesia sp

Thick and thin smears as for Plasmodium sp

Collect as for Plasmodium sp.

Use Wright or Giemsa stain.

Morphology is similar to Plasmodium sp ring forms but without pigment and gametocytes. Tetrads are diagnostic of Babesia sp but are infrequent.

Trypanosoma sp

Thin smears of capillary blood or 5–6 mL of anticoagulated blood

Collect capillary or anticoagulated blood. Smear on glass slides.

Use Wright or Giemsa stain.

Various concentration techniques are used to enhance sensitivity for African trypanosomiasis.

Filarial worms

1 mL of anticoagulated blood; if first specimen is negative, 5–10 mL, concentrated by centrifugation or filtration

Wuchereria bancrofti and Brugia malayi: Draw blood between 10 pm and 2 am .

Loa loa, Dipetalonema perstans, and Mansonella ozzardi: Draw blood between 10 am and 6 pm .

Use Wright or Giemsa stain directly or, for greater sensitivity, after concentration.

Bone marrow, other reticuloendothelial tissue, or CSF

Leishmania sp

Aspirates of bone marrow, spleen, liver, or lymph nodes or buffy coat smears

Smear on glass slides.

Use Wright or Giemsa stain.




Fresh spinal fluid

Use aseptic collection technique.

Examine specimen as soon as possible.

Examine using light or phase-contrast microscopy.

Parasites may be detected by their movements; they can be cultured or fixed and stained with Giemsa.

Trypanosoma brucei gambiense and rhodesiense

Aspirates of lymph nodes or chancre

Fresh spinal fluid

Use aseptic collection technique.

Use wet mount and Giemsa stain directly or after concentration by centrifugation.

Duodenal aspirate or jejunal biopsy

Giardia sp

Cryptosporidium sp

Cystoisospora sp

Cyclospora sp


Strongyloides sp

Duodenal aspirate or jejunal biopsy specimen placed in a sterile jar or tube with a little saline or on a glass slide with a coverslip

Examine immediately or fix for histopathologic examination.

Multiple stains may be required for optimal diagnosis (see below for details).

Rectal biopsy

Schistosoma mansoni

Schistosoma japonicum

Rectal biopsy specimen from level of dorsal fold (Houston valve), about 9 cm from anus

Fix for histopathologic examination, and crush a segment between slides for increased sensitivity.

Speciation is based on the morphology of ova.

Sigmoidoscopy (proctoscopy)

Entamoeba histolytica

Fresh scrapings with at curet or Volkmann spoon, a piece of mucosa snipped off with a surgical instrument, or aspirate from a lesion via a 1-mL serologic pipette with a rubber bulb (cotton-tipped swabs are not satisfactory)

Examine specimen immediately, or preserve it for later examination.

Stool should be assayed for the E. histolytica antigen to differentiate E. histolytica from nonpathogenic E. dispar and E. moshkovski.


Entamoeba histolytica

Entamoeba dispar

Entamoeba moshkovski

Other amebas

3 freshly passed stools collected in am every other day

Examine unformed or diarrheal specimens within 15 min.

Keep formed stools refrigerated until examination.

Use wet mounts and permanent stained slides (eg, trichrome stain) and concentration techniques for cysts.

Stool should be assayed for the E. histolytica adherence lectin antigen, which is more sensitive and can differentiate E. histolytica from the nonpathologic E. dispar, E. moshkovski, and other Entamoeba sp.

Giardia sp

3 freshly passed stools collected in am every other day

If initial series of 3 specimens is negative, examine 3 more 1 wk later.

Obtain duodenal aspirates if necessary.

If immediate examination is not possible, preserve specimen in polyvinyl alcohol.

Examine direct mounts and concentrated specimen for cysts and trophozoites. Assays for fecal antigens are more sensitive.

Cryptosporidium sp

Multiple freshly passed stools collected daily or every other day

Refrigerate and examine fresh samples, or preserve in 10% buffered formalin or acetate-acetic acid-formalin or suspend in 2.5% aqueous K dichromate.

Handle with care; fresh and dichromate-preserved stools are infectious.

Duodenal aspirate or jejunal biopsy can be obtained if stool specimens are negative.

Examine wet mounts by conventional light, differential interference contrast, and immunofluorescence microscopy.

Stain specimens with modified acid-fast or modified safranin. Assays for fecal antigens are more sensitive.

Cystoisospora sp

Multiple freshly passed stools collected daily or every other day

Concentration techniques enhance sensitivity.

Oocysts can be visualized in wet mounts by bright-field differential interference contrast or epifluorescence microscopy. Stain fixed specimens with modified acid-fast stain.

Cyclospora sp

Multiple freshly passed stools collected daily or every other day

Specimens should be refrigerated or preserved in 10% formalin or 2.5% K dichromate. Concentration techniques increase sensitivity.

Examine wet mounts by conventional light, bright-field differential interference contrast, and UV fluorescence microscopy. Oocysts are autofluorescent under UV light. Fixed specimens can be stained with modified acid-fast stain or modified safranin.


Multiple stools collected daily or every other day

Small-bowel biopsies may be necessary if stools are negative.

Specimens stained by chromotropic methods are most widely used. Chemofluorescent agents such as calcofluor white can also be used for quick identification.

Electron microscopy is the most sensitive method and used for speciation.

Trichuris sp

Ascaris sp


Strongyloides sp



3 stools collected daily (up to 7 needed for Strongyloides)

Refrigerate specimen if necessary.

Immediate examination is not critical, but larvae from hatched hookworm eggs in old stools may be confused with those of Strongyloides.

Active larvae are seen with Strongyloides; ova are seen with the rest.

For Strongyloides, the agar plate assay is more sensitive than ova and parasite examination.

Enterobius sp

Taenia sp

Ova collected from area around the anus on cellophane tape

Collect from area around the anus in the am before a bowel movement or bath.

Enterobius ova are occasionally seen in a stool specimen or in a vaginal specimen obtained in a Papanicolaou test.

Sputum or aspirate from respiratory tract

Paragonimus sp

Fresh sputum

Examine specimen as soon as possible, or preserve for later examination.

Concentration techniques may be necessary.

Strongyloides sp (hyperinfection)

Sputum, any aspirated material, fluid obtained by BAL or drainage material

Examine specimen as soon as possible, or preserve it for later examination.

Active larvae may be seen in wet mounts or can be fixed and stained with Giemsa.

Lung biopsy

Paragonimus sp

Open lung biopsy

Percutaneous biopsy guided by fluoroscopy or CT

Collect and place in sterile container in sterile saline.

Biopsy may allow species identification if a fluke is recovered.


Onchocerca volvulus

For patients infected in Africa, skin snips from the thigh, buttocks, or iliac crest

For patients infected in Latin America, skin snips from the head, scapula, or buttocks

For skin snips, disinfect the skin with alcohol, insert a 25-gauge needle just under the epidermis, raise it, and slice off small piece of tissue with a scalpel or razor blade, or use a sclerocorneal punch biopsy tool.

Bleeding should not occur.

Examine the specimen suspended in saline for motile microfilaria.

Leishmania sp

Ulcer rim biopsy or slip smear scrapings

Look for amastigotes in touch preparations of the lesion or ulcer wall.

Use Wright- or Giemsa-stained touch preparations, histopathology, and culture.

Urogenital secretions or biopsy

Trichomonas sp

1 sterile swab of vaginal, urethral, or prostatic secretions placed in a tube with a small amount of sterile saline

Tell female patients not to douche for 3–4 days before collecting the specimen.

Send specimen to the laboratory as soon as possible.

Identification of motile organisms by wet mount is the most rapid. DFA for parasites is more sensitive; culture is most sensitive but takes 3–7 days.

Schistosoma haematobium, occasionally S. japonicum

Fresh urine or biopsy from the area around the trigone

Recommended time for urine collection is between noon and 3 pm .

Use wet mount, concentrated by centrifugation.

BAL = bronchoalveolar lavage; DFA = direct fluorescent antibody; UV = ultraviolet.

Based on the CDC’s Laboratory Identification of Parasites of Public Health Concern ( ).

GI tract parasites

Various stages of protozoa and helminths that infect the GI tract are typically shed in the stool. Routine detection requires examination of stool specimens, preferably 3 collected on different days, because shedding can be sporadic. With some parasites, relatively sensitive and specific assays are available to detect antigens in stool.

Freshly passed stools uncontaminated with urine, water, dirt, or disinfectants should be sent to the laboratory within 1 h; 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 GI protozoa. Concentration techniques can be used to improve sensitivity. Anal cellophane tape or swabs may collect pinworm or tapeworm eggs. If strongyloidiasis is suspected, fresh stool should be smeared on an agar plate and incubated to identify the tracks of migrating larvae. Antibiotics, x-ray contrast material, purgatives, and antacids can hinder detection of ova and parasites for several weeks. Serologic assays, fecal antigen detection tests (eg, for Giardia intestinalis, Cryptosporidium sp, or Entamoeba histolytica), or PCR testing may aid in diagnosis (see Table: Serologic and Molecular Tests for Parasitic Infections). Sensitivity of stool examinations for ova and parasites is low enough that when clinical suspicion is strong, empirical treatment may be given.

Sigmoidoscopy or colonoscopy should be considered when routine stool examinations are negative and amebiasis is suspected in patients with persistent GI symptoms. Sigmoidoscopic specimens should be collected with a curet or spoon (cotton swabs are not suitable) and processed immediately for microscopy. Duodenal aspirates or small-bowel biopsy specimens may 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).

Serologic and Molecular Tests for Parasitic Infections



Antigen or DNA/RNA


African trypanosomiasis (West)








Chagas disease










IFA, EIA (for visceral leishmaniasis)








IFA, EIA (IgG and IgM)






















CATT = card agglutination trypanosomiasis test for Trypanosoma brucei gambiense; CDC = Centers for Disease Control and Prevention; DFA = direct fluorescent antibody; EIA = enzyme immunoassay; FAST-ELISA = Falcon assay screening test–enzyme-linked immunosorbent assay; IB = immunoblot; ICG = immunochromatographic assay for Plasmodium sp and P. falciparum; IFA = indirect fluorescent antibody test; IHA = indirect hemagglutination assay; IIF = immunofluorescence assay; PCR = polymerase chain reaction, TEM = transmission electronic microscopy.

N ote : Some antigen and parasite detection kits are available commercially. Others are available at the CDC or other reference laboratories. Molecular tests (eg, PCR) for parasite DNA are available in reference or research laboratories for a number of parasites, but not in most general diagnostic laboratories.

Based on CDC’s Laboratory Identification of Parasites of Public Health Concern ( ).


Advice for treating parasitic infections is available from experts at major medical and public health centers and travel clinics, in textbooks of infectious diseases and tropical medicine, and in summary form from The Medical Letter on Drugs and Therapeutics . Drugs for unusual parasitic infections can be obtained from the manufacturer or from the CDC Drug Service .


Despite substantial investment and research, no vaccines are yet available for prevention of human parasitic infections; however, studies of a new malaria vaccine suggest that it may slightly decrease but not eliminate risk of infection. Prevention is based on avoidance strategies.

Transmission of most intestinal parasites can be prevented by

  • Sanitary disposal of feces

  • 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 risk of bacterial and viral gastroenteritis. Meat, particularly pork, and fish, particularly 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.

The risk of malaria and many other vector-borne diseases can be decreased by wearing long-sleeved shirts and pants and applying diethyltoluamide (DEET)-containing insect repellants to exposed skin and permethrin to clothing (see Amebic Keratitis : Prevention). Window screens, air-conditioning, and mosquito nets impregnated with permethrin or other insecticides provide further protection. In addition, prophylactic antimalarial drugs should be taken by those traveling in endemic regions.

Travelers to rural Latin America should not sleep in adobe dwellings where reduviid bugs can transmit Chagas disease (see Chagas Disease : Prevention). 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 (see African Trypanosomiasis : Prevention).

Country-specific recommendations for travel are available from the CDC web site ( ) and in CDC Health Information for International Travel 2010.

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