Diarrhea is a common complaint that can be a manifestation of an inconsequential and self-limited problem or the hallmark of a serious chronic illness or infection, occasionally with life-threatening potential. Although a variety of systemic illnesses, toxins, drugs, and malabsorption syndromes can cause diarrhea, the infectious causes are of most immediate concern. Although viruses and bacteria are generally the most common causes of infectious diarrhea, parasites constitute a significant cause as well.

The majority of diarrhea-inducing infections are noninflammatory, usually arising in the upper small bowel from the action of an enterotoxin (e.g., V. cholerae or enterotoxigenic Escherichia coli) or other processes that alter the absorptive function of the villous tip (e.g., Cryptosporidium, Giardia, rotavirus, and Norwalk-like virus). In contrast, inflammatory diarrhea, which often presents as dysentery (bloody stools), arises in the colon from an invasive process, sometimes mediated by a cytotoxin (e.g., Salmonella, Shigella, Campylobacter, Clostridium difficile, and Amoeba).

If diarrhea has persisted for more than a few days, is bloody in nature, or is accompanied by substantial fever, dehydration, or weight loss, the patient should be evaluated more closely. Diarrheal illness lasting 10 days or longer and diarrhea in those with risk factors (see T§b!e..J43:l) should prompt a vigorous search for parasites.

Examination of a stool sample for fecal leukocytes has been advocated in these cases, but this procedure is controversial. The presence of fecal leukocytes has been considered a sign of inflammatory diarrhea and has been used to exclude toxigenic bacteria, viruses, and most parasites. However, when studied, the presence of fecal leukocytes has been shown to be neither sensitive nor specific for inflammatory causes.7

Examination of a stool specimen for ova and parasites is the best method of detecting intestinal parasites, including the cysts and trophozoites of protozoa and the larvae, eggs, and adults of helminths. Three specimens collected on different days should be examined, and the stool must be free of substances such as bismuth, barium, nonabsorbable antidiarrheal agents, and mineral oil. 8 Antimicrobial agents should be stopped at least 1 week prior to the stool collection. Fresh specimens are best, and specimens over an hour old should be preserved with formalin or polyvinyl alcohol. Multiple stool examinations are particularly important when dealing with formed stool, which usually contains fewer parasites than do diarrheal specimens. In patients with suspected giardiasis, 7 or 8 specimens may be necessary.

Occasionally Giardia, Cryptosporidium, and the larvae of Strongyloides may be detected by examining a duodenal aspirate or by having the patient swallow a string with a gelatin capsule (Entero-test). Recent studies suggest that detection of Giardia by examination of seven stool specimens is just as effective as examination of duodenal contents.

Special procedures for removing parasites include the following: warm water concentration through a filter for Strongyloides (Baermann test), sticky tape swab of the perianal area for Enterobius (Swube test), passage of urine through a nucleopore filter for Schistosoma hematobium, and use of an acid-fast stain to detect Cryptosporidium, Isospora, and Cyclospora.

The enzyme-linked immunosorbent assay (ELISA) technique can be used to make a serologic diagnosis of a variety of parasitic infections. In addition, the ELISA technique has been used to detect antigens of Giardia and Cryptosporidia in stool. Malaria-causing plasmodia, Babesia, microfilaria of Wuchereria and Brugia, and the trypanosomes that cause Chagas disease can be detected by Giemsa-stained thick and thin films of peripheral blood.

Finally, organisms that affect the central nervous system (CNS; e.g., P. falciparum which causes cerebral malaria, and Acanthamoeba or Naegleria, which cause amoebic meningoencephalitis) can be detected by culture or microscopic examination of centrifuged cerebrospinal fluid. Pneumocystis is detected by characteristic findings on chest x-ray, elevated lactate dehydrogenase (LDH) levels, and evidence of hypoxemia, and is confirmed by lung biopsy with special stains, while Toxoplasma is detected by characteristic findings on computed tomography (CT) scan in association with elevated serum antibody levels and is rarely confirmed by brain biopsy.

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