Published September, 2014

Q: I need advice about deworming our horse. He lives alone in a two-acre pasture, but he is hauled to a public barn for exercising. Our vet has advised us to cut back on the every-other-month deworming schedule after a manure sample showed he had no worms. How often should we deworm? Would you please recommend which medication we should use?

Marty Ward, Medford, Ore.

A: Thank you for asking this highly relevant question. I fully agree with your veterinarian. To deworm a horse every other month is a serious overtreatment. In the best case it is not needed and is a waste of medicine and money and, at worst, such treatment regimens inevitably lead to resistant parasites.

You do not mention your horse's age, but I assume he is an adult. Adult horses most often require only one or two treatments a year, and many may not need any treatments at all. If he is in good health and a fecal sample does not reveal any parasite eggs, there is simply no incentive to treat.

In your case there is one additional reason to assume that your horse will not require much deworming: The fact that he lives alone means the only source of infection is himself. If he does not shed many parasite eggs and the pasture is spacious, the infection pressure will be extremely low. Going to a local barn for exercising is unlikely to contribute to parasite infection, since these are acquired when grazing on pasture, not while active in a riding arena or when kept in a stall.

My recommendation for an adult horse is to have a veterinarian perform a fecal egg count twice a year--typically before and toward the end of the grazing season. As a general guideline, treatments can be performed if the egg count level is above 200 eggs per gram, but your veterinarian may set the threshold a bit lower or higher. Based on this, your horse is likely to not need any treatments at all, and I would have no worries with that as long as he is healthy and you continue to perform your two fecal egg counts every year.

When you do decide to deworm, I can give you no general recommendation regarding choice of drug for treatment. It fully depends on which potentially resistant parasites might be present. The only way to find out is to take another fecal sample 14 days after deworming and then see if the egg count was reduced satisfactorily. So my recommendation would be to start testing the drug that you would typically use. If it works fine, then continue with that. If not, you would need to change to a drug with a different mode of action.

Martin Krarup Nielsen, DVM, PhD, is an assistant professor appointment in parasitology at the University of Kentucky's (UK) Maxwell H. Gluck Equine Research Center in Lexington.