How to choose an equine parasitology test

Melanie Barham, Mary Lake


When choosing an equine fecal parasite detection method, it is important to determine the goal of the test. Are you:

  • Deworming after the winter?
  • Performing fecal Egg Count Reduction testing to look for anthelmintic resistance?
  • Trying to find tapeworm eggs?
  • Deciding to treat an individual animal or a herd?

There are 3 major tests used in the equine industry:

McMaster fecal egg count: This test uses a quantification technique that allows the veterinarian to know the number of eggs per gram (epg) of feces, and involves using a specific counting/quantification slide.  It is the recommended method for screening populations of horses to determine which require treatment.  The technique is sensitive to 50 epg (each egg detected on the slide is calculated to be equal to 50 epg) and all types of parasite eggs identified are reported. The sensitivity of this test is quite appropriate for most fecal testing prior to treatment or in situations where a high load of parasites are suspected, and is significantly less expensive than the Modified Wisconsin technique.

Modified Wisconsin technique: The Modified Wisconsin technique is another fecal quantification technique that is sensitive to 1 epg and is more sensitive for tapeworm eggs than a McMaster, although will still only pick up 15-20% of tapeworm eggs.  As with the McMaster technique, all types of parasite eggs identified are reported.  The cost per test is higher than the McMaster test as 2 centrifugations are required.  In cases where there is a high fecal egg load, or a high load is suspected, this technique is not ideal, as eggs can become too numerous to count, and may require McMaster testing first with a secondary scan for tapeworm species via the Wisconsin technique, increasing the cost.  It is recommended to use this type of testing in fecal egg count reduction tests (FECRT), or in situations where previous treatment has not alleviated signs.

Standard gravitational fecal flotation: The fecal flotation test is the traditional test for fecal parasite eggs, but does not quantify exact numbers of eggs per gram. Again, all parasite eggs identified are reported, but because exact quantification is not performed, results are reported as 1+, 2+, or 3+.

A note about tapeworms: if a tapeworm parasite load is suspected, sampling 10 or more horses on the farm is advised, and submitting for Wisconsin testing will yield the best results.  Experimental testing by fecal ELISA is being studied, but is not currently available for commercial use.

Sample submission tips:

  • All fecal tests give the best results when samples are collected and submitted within 24 hours of the horse passing the feces.
  • Immediate and continual refrigeration in an airtight container prior to submission significantly improved results in research trials, and samples collected need only be 1 fecal ball in size; 5 g is all that is required for samples.
  • Samples can be refrigerated for up to 7 days before analysis, although immediate submission is best.
  • Fecal samples should be submitted in fecal containers.
  • We are happy to send containers to you at no charge: containers typically arrive within 24-48 hours of your request.

A comprehensive document outlining treatment and test interpretation has been released by the American Association of Equine Practitioners and can be referenced here: