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Deworming Young Horses: When Is Best?

Pet Column for the week of March 1, 2010

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Office of Public Engagement
2001 S. Lincoln Ave.
Urbana, Illinois 61802
Phone: 217/333-2907
Equine Medicine
Information Specialist

Source - Dennis D. French, DVM, DABVP-Equine Practice
The thought of your new foal becoming infected with worms is a bit too much to bear for many horse owners. All it takes is one face-to-face meeting with a squirmy white roundworm, the type of parasite most common in horses under two years of age, and you'll never again complain about deworming.

Although most equine dewormers are, by law, supposed to be prescribed by a veterinarian, everyone knows that these equine dewormers can be easily purchased in your local agricultural supply store for a relatively small price. The increased availability to horse owners may sound like a good thing, but using these drugs without a veterinarian's guidance may be asking for trouble. For example, their overuse and inappropriate use may lead to the development of a resistant population of worms, causing bigger problems for your animal.

Dr. Dennis French is an equine veterinarian at the University of Illinois Veterinary Teaching Hospital in Urbana who has studied various equine parasites extensively. He explains that, "most owners are aware of the importance of deworming and start doing so at 30 days of age, but this is not the best approach."

For one, it takes approximately six weeks for any parasite the foal would pick up at birth to mature into a stage that can be effectively killed with a dewormer. So if you deworm the animal before the worms are "mature" they probably will not be susceptible to the worm-killing drugs in the deworming paste.

"We try to get out to two months of age before we start deworming foals," explains Dr. French. As for what type of dewormer to use, most owners may be in for a surprise. One of the most commonly used dewormers, ivermectin, may not be a wise decision.

"The population of ascarids (roundworms) that are resistant to ivermectin is high," notes Dr. French. So a better approach to deworming the foal at 30 days, like is commonly practiced by many equine owners, may be to deworm the mare.

If the mare is greater than 30 days away from foaling, it is recommended to deworm her. If you miss that chance, deworming the mare on the day of foaling is also a good opportunity to prevent transmission to the foal.

The recommendation to put off deworming the foal until it is two months old should not be misconstrued to say that roundworms cause relatively little harm to young horses. Quite the contrary. The eggs that break open in the intestines of the foal can migrate through the body into the liver and lungs, causing serious damage. Everything from a rough hair coat and a potbelly to pneumonia, colic, and diarrhea can be linked to roundworms.

With such a harmful parasite, throwing a syringe full of a random dewormer (or whatever's on sale at your local agricultural store that month) at a foal each month is not a good idea. "The reason why veterinarians should be invited to develop a program for deworming is because with both ascarids and small strongyles (another type of intestinal parasite), there are over 43 different species that can infect foals," says Dr. French. For piece of mind's sake, it's better to know you are going after the right worm with the appropriate drug to prevent future problems.

For more information on deworming, contact your local equine veterinarian.