Is a horse ever too old to become infested by ascarids?

Horses normally outgrow susceptibility to certain internal parasites.
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Horses normally outgrow susceptibility to certain internal parasites.
Fotolia_horse in blanket

Question: Just over a year ago, we purchased an almost-6-year-old gelding. When I dewormed him with an ivermectin/praziquantel paste, I found a large quantity of ascarids in his manure over the next three to five days. Everything I read online says that ascarids affect only young horses, who develop a natural resistance once they reach about 2 years of age. This horse clearly has not developed any such resistance, and I am curious if this is happening more and more. 

I deworm my horse every two months, and I have tried both ivermectin and fenbendazole products. I continue to find ascarids in his manure after deworming, although the number has diminished from more than 100 adult worms the first time to 10 to 20. Is it likely that my horse will eventually outgrow this parasite? Am I doing the right thing to continue to deworm him every two months until there are no longer worms in his manure, and then start him on routine fecal tests?

Cathy Johnson
Roy, Washington

Answer: There is a very good reason why you have not seen this question addressed before. It is very unusual to see large ascarid burdens in adult horses. As you correctly describe, this species of roundworms usually infects foals and yearlings. Ascarids are only occasionally found in older horses, and then only in very low numbers. Over the years I have heard of a couple of cases similar to yours, but I have never actually seen one myself. So the answer to your first question---Is this something we see happening more and more?---is no.
Your next question is more difficult to answer. Because this happens so rarely, we cannot say anything about the probability of your horse growing out of this problem. It is fair to assume that his immune system is somehow deficient, but we do not know the nature of this deficiency and we are therefore not able to make any predictions of how this will develop over time.
As for whether you’re doing the right thing with your deworming regimen: One thing to be aware of is drug resistance. When you use a dewormer, you kill most of the parasites but leave behind those individual worms that are least susceptible to the chemical’s effects. Then, when they reproduce, they pass that ability to withstand the chemical to the next generation. Over time, the dewormers become increasingly ineffective.
Treating your horse every other month puts a very strong selection pressure on the worms for drug resistance, and we do not recommend such frequent treatments even in foals. Instead we advise using fecal egg counts to monitor the infection level and identify optimal times for treatment. Pre- and post-treatment egg counts can indicate whether the chemicals you are using are still effective against these parasites.
You may also want to think about possible sources of infection. Even if you could eliminate all of the ascarids from your horse’s system, if he remains susceptible, he will re-infect himself by picking up eggs from his grass, feed or water. So disinfecting stalls once or twice a year and maintaining pasture hygiene by regular removal of feces may be worth considering. The effectiveness of these strategies will depend on the layout of your barn, paddocks and pastures but could help reduce the infection pressure.

Martin K. Nielsen, DVM, PhD,
DEVPC, DACVM
M.H. Gluck Equine Research Center
University of Kentucky
Lexington, Kentucky

Martin K. Nielsen, DVM, PhD, DEVPC, DACVM, is an assistant professor of equine parasitology at the M.H. Gluck Equine Research Center, University of Kentucky.
His research is dedicated to providing solutions for equine parasite control and includes development and refinement of diagnostic techniques, objective evaluation of deworming regimens and anthelmintic resistance.

This article first appeared in EQUUS issue #460, January 2016.