Published February, 2015

Q: Is it possible to detect pinworms in a fecal egg count test? One of my horses rubs his rump on the stall wall at every opportunity, and he begs me to scratch him under his tail whenever I am close by. I know Oxyuris equi worms are difficult to eliminate, but I feel bad that he itches so much!

We live in the Central Sacramento Valley of Northern California, where it can freeze in winter, but not for long, and where it rarely, if ever, snows. It can get pretty warm in summer—over 100°F—but generally it reaches the 90s. We have 15 acres of pasture, run cows in the same fields as the horses, and clean the loafing areas, stalls, and corrals one to two times daily when in use.

Generally, my horses are wormed two times per year, usually with an ivermectin-type product. (And I shave off all the bots eggs I find in the fall.) I administered the Panacur five-day double-dose fenbendazole regimen last winter, but did not conduct a fecal count before or after treatment. However, I wonder if that would be accurate or conclusive; in my opinion it’s easy for vets just to say, “Oh, yeah, worm them.” Any suggestions?

Ginny Paschke, via e-mail

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A: Pinworms (Oxyuris equi) are common in horses, but they usually do not cause much harm. Some horses scratch their tails, but bear in mind that this can be due to many other conditions as well. Occasionally, we find pinworm
eggs in the feces—the eggs are very characteristic and cannot be confused with any other egg type in the horse.

However, as a rule, pinworms lay their eggs outside the intestinal tract. In fact, the females stick out their posterior
ends and deposit their eggs around the anus. This is the reason for the itching we sometimes see. A more reliable method for detecting pinworm infection is the so-called Scotch-tape technique, where the tape is first applied to
the perianal skin, then it is viewed under the microscope. Often many pinworm eggs can be seen this way. Another method to collect pinworm eggs is to simply scrape off the perianal skin with a lubricated wooden tongue depressor.

Your parasite control program appears to be well thought-out, but I strongly recommend you run some fecals to get information on parasite levels and drug efficacy.

Martin Krarup Nielsen, DVM, PhD, Dipl. EVPC, ACVM, an assistant professor at the University of Kentucky Gluck Equine Research Center, provided the expert advice.