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Enteric Pathogens

Most acute infectious diarrhea is self-limited, caused by bacteria or viruses, and has a duration of illness less than 7 days. When illness is severe or becomes prolonged, laboratory evaluation may be indicated.

Common bacterial causes of severe acute diarrhea include Campylobacter, Salmonella, Shigella, and enterotoxigenic or shiga toxin-producing E. coli. Less common bacterial pathogens include Yersinia enterocolitica, Aeromonas, Plesiomonas, and Vibrio species. Routine stool culture enzyme immunoassay (EIA) testing for Campylobacter and shiga toxin-producing E. coli (includes EC0157:H7), as well as culture for Salmonella and Shigella. If another bacterial pathogen, such as Yersinia is suspected, the laboratory should be notified so that appropriate selective media can be included in the culture set up.

Parasites are an uncommon cause of acute diarrhea, and many studies have shown that routine comprehensive fecal ova and parasite testing is not cost-effective. Most enteric parasite infections in the United States are caused by Giardia or Cryptosporidium. Enzyme immunoassay (EIA) is the most sensitive and specific test methodology for these pathogens. Routine O & P testing includes EIA for Giardia and Cryptosporidium. Comprehensive O&P testing will include Giardia & Cryptosporidium EIA, as well as a microscopic exam. Comprehensive O&P testing should be reserved for patients that have a history of travel outside the United States, are immunocompromised, or have a protracted diarrheal illness with no other etiology identified.

enteric pathogens

Stool specimens are grossly examined for adult worms. Microscopic examination of saline and iodine wet preps is performed for protozoan cysts, trophozoites, and helminth eggs.

Trichrome stained smears are used to detect trophozoites. The varying, cyclic production of diagnostic stages of parasites and the inherent heterogeneity of fecal content necessitate the need for multiple specimens. Therefore, a minimum of three stools should be collected during a week's time and submitted for examination. In hospitalized patients, single stools on three consecutive days can be submitted to avoid lengthening the hospital stay. All parasites seen will be identified.

Several substances will cause fecal specimens to be unsatisfactory for microscopic examination. Water and urine contaminations destroy fragile trophozoites. Barium, bismuth, anti-diarrheal compounds, and antibiotics may decrease the number of organisms present or interfere with their visualization. Dirt or lake water may sometimes contaminate outpatient specimens and introduce free-living soil and water parasites.

Delay in delivery of outpatient samples is also harmful. If delay is unavoidable, the patient should be given a vial of polyvinyl alcohol (PVA) fixative for each specimen to be collected. One part of feces should be mixed well with three parts of PVA fixative, capped tightly, and delivered to the laboratory, along with a portion of unpreserved feces in a separate, sealed container.

Reference values are:

  • Negative Giardia antigen
  • Negative Cryptosporidium antigen
  • Negative for E. coli shiga toxin
  • Negative C. difficile toxin
  • Negative bacterial cultures
  • No ova or parasites seen
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