Description
Parasites causing intestinal infections in humans include protozoa and helminths. Examples of intestinal protozoa include flagellates (G lamblia), amoeba (E histolytica), sporozoans (Cryptosporidium spp), and ciliates (Balantidium coli). Helminths include nematodes (roundworms), cestodes (tapeworms), and trematodes (flukes).
The ova and parasite examination includes the following components:
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Macroscopic examination: This includes gross examination to determine fecal consistency, abnormalities (blood, excessive mucus), and the presence of larval or adult worms or proglottids. If the specimen is being transferred to preservative before delivery to lab, the collector can note consistency, as this will be difficult to ascertain once mixed with preservative solution.
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Microscopic examination of fresh specimen: This includes examination for protozoal trophozoites and cysts and helminth larvae and eggs. When performed on unfixed specimens, this allows for determination of trophozoite motility.
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Microscopic examination of concentrated specimen: Concentration by sedimentation or flotation aids detection of small numbers of organisms by removing background debris and concentrating parasites. Although trophozoites may be destroyed by the procedure, it increases sensitivity for cysts and helminth eggs and larvae.
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Microscopic examination of permanent stained smears: Wheatley’s trichrome or iron hematoxylin staining aid in identification of protozoan cysts and trophozoites, as well as providing a permanent and reviewable record of findings.
Indications/Applications
The etiology of most infectious diarrhea cases is viral or bacterial, rather than parasitic. The Infectious Disease Society of America guidelines suggest evaluating patients with persistent diarrhea (> 7 days duration) for parasites, especially if immunocompromised. [6] Although G lamblia and E histolytica are the most common intestinal parasites worldwide, pathogen prevalence varies regionally. [4] Patient travel history, exposures, and immune status should help direct test selection.
Considerations
The routine ova and parasite examination has very poor sensitivity for 2 of the most common parasites found in the United States, G lamblia (66-79% sensitivity) and Cryptosporidium spp (< 5%). [7, 8, 9] Another method, such as an immunoassay for parasite antigen, should be used for laboratory diagnosis of these organisms. [1] Many labs offer an “ova and parasite screen” that consists of an immunoassay to detect only these 2 organisms. Some laboratories offer an immunoassay for E histolytica, but this test must be performed on unpreserved sample.
The routine ova and parasite examination does not detect microsporidia or coccidia (Cyclospora cayetanensis, Cryptosporidium spp, Cystoisospora belli). Special stains (modified acid-fast for coccidian and modified trichrome for microsporidia) must be done to detect these organisms, so the physician must indicate to the lab that these pathogens are suspected. [1]
In a retrospective, case-control study of patients with diarrhea who were admitted to a multi-hospital US health-care system, Khan et al found that the yield of stool ova and parasite tests was 2.15%, with 37 out of 1723 exams being positive for potentially pathogenic organisms. Excluding data on Blastocystis spp, which were the most common parasites found in the exams, the investigators determined that at least one of the following risk factors existed in patients with positive tests: smoking history, prior parasitic disease, positive status for human immunodeficiency virus (HIV), travel to an endemic area, and institutionalization. The report also indicated that the number of inpatient stool ova and parasite tests conducted can be reduced by 51% by restricting them to patients with a history of smoking, prior parasitic disease, and HIV-positive status. [10]
A study from the University of Utah proposed a diagnostic algorithm for evaluating patients with persistent diarrhea or gastrointestinal illness for parasitic infection. [9] Their recommendations include the following:
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Nonimmunocompromised patients with no travel to parasite endemic areas: Initially evaluate these patients for Cryptosporidium spp and G lamblia by immunoassay. If negative and symptoms persist, repeat immunoassay and perform complete ova and parasite examination.
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Nonimmunocompromised patients with a travel history to parasite endemic areas: Initially evaluate these patients for Cryptosporidium spp, G lamblia, and E histolytica by immunoassay. If these evaluations are negative and symptoms persist, repeat immunoassay and perform complete ova and parasite examination. Consider serology testing based on exposure.
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Nonimmunocompromised patients hailing from parasite endemic areas: Initially evaluate these patients for Cryptosporidium spp, G lamblia, and E histolytica by immunoassay. Serology for S stercoralis and other parasites as geographically indicated. Complete ova and parasite examination 3 times. If negative and symptoms persist, repeat immunoassay and perform modified acid fast stain for coccidia.
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Immunocompromised patients: Initially evaluate these patients for Cryptosporidium spp, G lamblia, and E histolytica by immunoassay. Perform modified acid fast for coccidia, and special stains for microsporidia, as well as serology for S stercoralis and complete ova and parasite examination 3 times. If negative and symptoms persist, repeat immunoassay and perform modified acid fast stain for coccidia.
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If the cause is not found, consider referring to specialist for evaluation of infectious and noninfectious causes.
