Parasitology in 2018: Should we still be ordering O&P times three?

A terrific review article (S Mohapatra et al. Am J Gastroenterol 2018; 113: 805-18) provides a great deal of information about gastrointestinal parasites. Thanks to Ben Gold for this reference (& don’t forget to vote for NASPGHAN president).

Generally, the authors dispute the usefulness of testing for ova and parasites (O&P) with three separate specimens.  While classic training has noted the intermittent shedding of parasites and the suboptimal sensitivity of O&P, the authors note that a recent study showed a detection of 91% of parasites in the first stool sample.  In addition, newer PCR based assays are more appropriate in many clinical situations due to their improved sensitivity.

The authors first review the protozoa, which are single-celled, motile, free-living organisms, in depth & summarized in Table 1; these include the following:

  • Amoeba: Entamoeba histolytica (E histolytica),
  • Dientamoeba fragilis
  • Blastocystis hominis
  • Coccidia: Cryptosporidium, Cystiospora, Cyclospora
  • Ciliates: Balantidium coli
  • Flagellates: Giardia lamblia
  • Microsporidiosis
  • Trypanosoma cruzi

Next, they review the helminths in depth and in Table 2, which are large, multicellular organisms that can be seen with the naked eye and include the following:

  • Ascariasis: A lumbridcoides
  • Capillariasis
  • Diphyllobothriasis
  • Enterobiasis: E vermicularis
  • Hookworm disease: A dudenale, N amercanus
  • Hymenolepiasis
  • Strongyloides: S stercoralis
  • Schistosomiasis
  • Taeniasis
  • Trichinellosis
  • Trichuriasis
  • Groups of helminths: trematodes (eg. Schistosomes), cestodes (tapeworms eg. Taenia), and nematodes (roundworms eg. Ascariasis, hookworm, pinworms, and whipworms).

Key points:

  • For E histolytica, ELISA fecal antigen test is superior to O&P as is the PCR assay.  If the diagnosis of E histolytica is being considered in the setting of ulcerative colitis, the authors note that this infection must be excluded before the initiation of corticosteroid therapy since steroids can lead to hyperinfection and could be fatal.  Also, the so-called “flask shaped” ulcers seen with this infection refers to the microscopic appearance of the ulcer into the submucosa. Most infections (>90%) remain asymptomatic.
  • Blastocystis “is the most common parasite identified in stool samples in the US” though the pathogenicity remains controversial and is often self-limited.
  • D fragilis “as a pathogen is controversial…[but] recent studies on patients infected only with D fragilis have found an association with diarrhea, abdominal pain, nausea, weight loss, anorexia, and flatus which resolve after eradication.”
  • Giardiasis is “the most common intestinal parasitic disease affecting humans in the US.” PCR/molecular methods are highly sensitive (>90%) and specific (nearly 100%)
  • Enterobius vermicularis (pinworms). The “CDC does not recommend stool examination for O&P since the yield is low.” The diagnostic test is the “Scotch test” in which tape is left overnight in the perianal region and then examined for captured eggs.

Author Recommendations:

  • “Restrict stool examination [for parasites] to patients with persistent diarrheal illness with a duration greater than 7 days.”  Do not check O&P in hospitalized patients more than 3 days into their hospitalization.
  • The most  common parasitic infections, Giardia and Cryptosporidium, are best diagnosed with a stool immunoassay (EIA) rather than O&P.  For E histolytica EIA is recommended over O&P.
  • In those who are persistently symptomatic and with travel history with likely parasite exposure, stool O&P with wet mount/AFB stain/special stains for detection of rare parasites still is worthwhile.  In those without exposure history and with persistent diarrhea (after exclusion of Giardia and Cryptosporidium), consider non-infectious causes of diarrhea.
  • We discourage repeating the O&P due to the “very low incremental yield of second and third samples”

My take: This article makes a strong argument that “O&P times three” represents an outdated approach in the diagnosis of parasitic diseases in the US.

Related blog posts:

Near the top of Old Rag Mountain, Shenandoah Natl Park




2 thoughts on “Parasitology in 2018: Should we still be ordering O&P times three?

  1. Pingback: My Favorite Posts from the Past Year | gutsandgrowth

  2. Pingback: CDC Warns of Cryptosporidium in Pools | gutsandgrowth

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