Ask the Vet: Internal Parasites

Answered by, Martin Nielsen, DVM, Ph.D., Gluck Equine Research Center, University of Kentucky
Courtesy of AAEP

Question: How often should I do fecal egg counts? I manage a boarding/lesson/show barn with 27-30 horses on the property.

Answer:For adult horses, a good starting point is twice a year. At least one of these two sample occasions should include a post-treatment follow-up to ensure that treatments worked as intended. Then, it is recommended to test, treat and monitor new arrivals as they have a tendency of staying elevated for a while upon arriving on a new farm.

Question: I have tried doing fecal egg counts and had cut back on use of dewormer, but my horse developed an eye infection (Uveitis) and I had to treat with Ivermectin. I feel this is a double edged sword… 

Answer:Yes, you are absolutely right that parasite control is a double-edged sword. On one hand we want to reduce further development of resistance as much as possible and on the other, our goal is to avoid parasitic disease. That said, I am not aware of any worm parasites capable of causing uveitis – certainly  not on this continent. There is an eye worm named Thelazia, but it does not invade the eye, so it does not cause uveitis. Rather, it stays in the conjunctival sac and can cause keratitis (inflammation of the cornea) in rare cases. The AAEP guidelines recommend treating all horses once or twice a year to maintain control over parasites that are not necessarily detected by fecal egg counts.

Question: How accurate are “mail order” test kits? How long is the fecal sample viable? If I receive a questionable result, what should I do next – re-test, or go ahead and deworm?

Answer:We tested this in a study, and we found that if samples are kept airtight and mailed in fresh, the parasite egg counts are fine if samples are done within a week upon collection. If they are shipped with icepacks to stabilize storage temperatures, it gets even better. So, there is no reason to believe that mail order test kits should be bad or less reliable than direct analysis if everything is done properly. I am not sure what you mean by a questionable result, but retesting is never a bad idea if you have reasons to doubt the result. Just keep in mind that two counts generated from the same sample are never going to be exactly alike. If you suspect parasitic disease, you shouldn’t waste time on running fecal egg counts, but just go ahead and deworm that horse.

Question: How effective is feeding diatomaceous earth at regular intervals? 

Anwer: Unfortunately, diatomaceous earth has not been found to possess any antiparasitic effects when evaluated in research studies.

Question: We have six horses at our barn. They have been given dewormer approximately every 6-7 weeks. We alter what type of dewormer is given. Is this the correct frequency? 

Answer:No, this is a serious overkill! You did not specify the age of your horses, but the large majority of adult horses do not require more than two annual treatments. Treating every 6-7 weeks year-round is a sure recipe for resistance, and you probably have a lot of it already. I recommend testing the efficacy of each dewormer on your farm and then giving your parasite control strategy a complete overhaul.