By Heather Thomas
Many breeders routinely deworm mares at foaling time, to prevent spread of threadworms (Strongyloides westeri) to newborn foals through the milk. Current thinking, however, is that this deworming is unnecessary and may lead to drug resistance in other types of worms.
Dr. Martin Nielsen (Gluck Equine Research Center, University of Kentucky) says there is some debate whether to treat mares before they foal (and when, and with what) or if it’s better to treat the foal within the first days of life, or even to treat them at all for threadworms.
“We don’t have much evidence that this parasite is very pathogenic. We sometimes see eggs in the feces during the first 2 weeks of life, and foals tend to have diarrhea (often called foal heat diarrhea) but this has turned out to be multi-factorial,” he says. We don’t know how much affect threadworms have, and no studies have evaluated whether it is efficacious to treat the mare.
“Whether or not you treat the mare just before foaling may depend on when she was last treated. If she’s on a good deworming program already, and was treated within the last 2 months, she probably doesn’t need another treatment just prior to labor. If you do treat the mare, you must use a drug with efficacy against migrating stages of threadworms. Don’t use one that only works on parasites within the GI tract. The stages we need to target are in the tissues hiding, waiting to migrate to the mammary gland and into the milk,” he says.
Dr. Craig Reinemeyer (East Tennessee Research, Inc.) says treatments for threadworms usually aren’t necessary, since this parasite is only occasionally pathogenic. “You won’t lose a foal from this parasite. It’s also very easy to treat if we diagnose a foal with a problem.”
He recommends just treating the occasional foal that shows symptoms–and not worry about treating every mare at foaling time, or every foal. “Even if fecal samples from the foal tell you the worm is present doesn’t mean we have to kill it. Foals develop complete immunity to threadworms by the time they are 5 to 6 months old. You never see eggs from these worms in feces of an adult horse; these worms are only passed through the mare’s milk. They are just sitting in the mammary tissue, waiting to be called into action by the hormones of pregnancy and lactation,” he explains.
“These early dewormings merely expose other worms, especially ascarids, to sub-optimal doses (not high enough levels to kill them) of whatever dewormer we were using for threadworms. As a result, the non-targeted parasites have been developing resistance to these drugs. Any time you treat a foal with ivermectin or moxidectin you are treating every worm inside that horse—wherever it might be in the body. That’s not the case when using Panacur at the single one-day dose, or Strongid (a pyrantel product which does not leave the gut). A worm migrating through the lungs or liver will not have much exposure to that drug,” he explains.
“What we’ve done, over time, with our deworming methods, has helped to rapidly select for resistant worms that are no longer susceptible to ivermectin and moxidectin—in these young horses,” says Reinemeyer.
Dr. Ray Kaplan, University of Georgia, says deworming mares at foaling time is totally unnecessary. “Threadworms are a non-issue today. I haven’t heard of a case of threadworm disease for a long time. Treatment at foaling was specifically to control this one particular parasite. Even in the rare situation where a foal might have a problem, threadworms can be treated—and also controlled through management. You only have this problem in a wet, dirty environment,” he says.
Threadworm infection causes temporary diarrhea in young foals and is easily treatable. Deworming mares at foaling time is unnecessary if they are on a good parasite control program, checking egg counts on the mares—to know that their parasites are under control. “If you are doing that, there is no need to give an extra treatment at foaling,” says Kaplan.
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