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 might 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.

Dennis French, DVM, MS, Dipl. ABVP, 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