LabMed

Large Intestine, Infection, Worms

At a Glance

The presence of a worm or fragment of a worm in the stool may be the initial evidence of a helminthic intestinal infection. Perianal itching is characteristic of an Enterobius vermicularis (i.e., pinworm) infection. In many instances, the patient with an intestinal helminthic infection is asymptomatic. The patient may become symptomatic as the worm burden increases, which is most commonly seen in children in resource-limited settings. Anemia in these settings may be the result of hookworm infections.

What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

The first test to consider when an infection with a helminthic parasitic infection of the intestine is suspected is an ova and parasite (O&P) examination. This examination detects eggs from parasites and the larvae of Strongyloides.

Occasionally, the adult helminth will be passed in the stool. If this occurs, then the parasite should be submitted for examination and identification.

There are some parasites that reside in other parts of the body but may release eggs into the intestinal tract. Schistosoma mansoni and Clonorchis sinensis are such parasites, but there are others. Although these reside in the blood vessels and biliary tract, respectively, the eggs are passed in the stool. An O&P examination would be warranted if infection with these parasites is suspected.

Finally, Enterobius vermicularis is unique among the helminthic parasites in that the female migrates from the large intestine, usually the cecum, out the anus and lays eggs in the perianal region. Although occasionally the adult will be found and submitted for parasite identification, the diagnosis is usually achieve by performing a cellophane tape preparation from the anus and examining the tape for eggs.

The eggs of some parasites, such as Schistosoma species, may be few or distorted and missed in the O&P examination. Serologic studies may be useful for the diagnosis in such instances, particularly if the patient is a visitor to and not from a normally endemic area (i.e., individuals from endemic areas may that positive serologic studies from remote disease).

Tests Results indicative of the Disorder

Positive for a test that identifies the eggs or larvae of any parasite is indicative of disease.

Common examples of helminthic parasites (i.e., the eggs or larvae of which may be found in the stool) are nematodes, cestodes, and trematodes.

Nematodes include:

  • Ascaris lumbricoides

  • Trichuris trichiuria

  • Enterobium vermicularis

  • Hookworms (Ancylostoma or Necator)

  • Strongyloides stercoralis

Cestodes include:

  • Taenia spp.

  • Hymenolepis spp.

  • Diphyllobothrium spp.

  • Dipylidium caninum

Trematodes include:

  • Fasciola hepatica

  • Fasciolopsis buski

  • Paragonimus spp.

  • Some Schistosoma spp.

  • Clonorchis sinensis

Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications - OTC drugs or Herbals - that might affect the lab results?

Most medications do not affect the presence or morphology of the adult worms or the eggs of helminthes in the stool. Antihelminthic therapy, however, will cause morphologic distortion of the eggs, which may make identification challenging.

What Lab Results Are Absolutely Confirmatory?

The presence of eggs or adult parasites in the stool is confirmatory of disease.

Positive serologic findings in the appropriate patient population and setting may be considered confirmatory.

What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

A confirmatory test is not necessary, if helminth eggs or adult parasites are present in the stool.

An O&P examination may be useful if serologic studies suggest an active infection with a helminthic parasite, but this test has limited sensitivity.

A colonic mucosal biopsy may be needed to confirm an infection by S. mansoni or S. japonicum, since the O&P examination has limited sensitivity, as the eggs of these organisms become lodged in the intestinal mucosa.

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