About 70 species of animal parasites commonly infect the human body (Table 7.4). More than half of these can be detected by examination of stool specimens because the parasites inhabit the GI tract and its environs. Of the parasites that can be diagnosed by stool examinations, about one third are single-celled protozoa, and two thirds are multicellular worms. Only six or seven types of intestinal protozoa are clinically important, but almost all of the worm classes are potentially pathogenic.
Diagnosis of parasites begins with ova and parasite examination. Other diagnostic options include sigmoidoscopy smears, biopsies, barium radiologic studies, and serologic tests. Collection of fecal specimens for parasites should be done before administration of barium sulfate, mineral oil, bismuth, antimalarial drugs, and some antibiotic drugs (e.g., tetracycline). For ova and parasite examination, ideally, one specimen should be collected every other day for a total of three specimens. At the most, these specimens should be gathered within 10 days.
For detection of Giardia, other diagnostic tests such as the Entero-Test (string test or duodenal parasites test) and duodenal aspiration or biopsy may be necessary. The Entero-Test consists of a gelatin capsule containing a coiled length of nylon yarn. The capsule is swallowed, the gelatin dissolves, and the weighted string is carried into the duodenum. After about 4 hours, the string is withdrawn, and the accompanying mucus is examined microscopically for Giardia. Duodenal fluid also can be submitted to be examined for Giardia and Strongyloides stercoralis. The specimen should contain no preservatives and should be examined for organisms within 1 hour after collection.
Cryptosporidium parvum has long been recognized as an animal parasite but is also capable of infecting humans, especially physiologically compromised patients. Organisms have been recovered from the gallbladder, lungs, and stool.
Clinical Alert
In the diagnosis of parasitic worms, the most important factor is the number of worms harbored.
Multiple specimens may be necessary to detect a parasitic infection.
Most parasites found in humans are identified in blood or feces but may also be evident in urine, sputum, tissue fluids, or biopsy tissues.
Fecal specimens should not be contaminated with water or urine. All specimens should be labeled with the patient’s name, healthcare provider’s name, identification number (if applicable), and date and time collected. Various commercial collection systems are available to allow collection of specimens at home, in a nursing institution, or in a hospital setting. Clear instructions should be communicated and given in writing to the patient to ensure proper collection. See Chapter 4, Stool Studies, for more information.
When sputum is collected for ova and parasites, it should be “deep sputum” from the lower respiratory tract. It should be collected early in the morning, before the patient eats or brushes the teeth, and it should be immediately delivered to the laboratory. See Appendix B, Guidelines for Specimen Transport and Storage, for more information.