CEPS/Veterinary Extension
2938 Vet. Med. Basic Sciences Bldg.
2001 S. Lincoln Ave.
Urbana, Illinois 61802
Phone: 217/333-2907

By Theresa A. Fuess, Ph.D.
Information Specialist
University of Illinois
College of Veterinary Medicine

"To maintain optimum health of your horse, you should follow a deworming program tailored to the number, age, and population density of the horses on your property, whether the horses are stabled or on pasture, how often they are exposed to new horses, pasture management practices, and the local climate," says Dr. Doug Hutchens, a veterinary parasitologist at the University of Illinois College of Veterinary Medicine at Urbana. "The needs of individual horses must also be considered. For example, foals may require a different program than older horses."

Deworming programs are designed to effectively remove parasites from the horse and to reduce the transmission of parasite eggs. Dewormers are usually rotated by class to limit the chance that the parasites will become resistant to the drugs. Most often, different classes of broad spectrum drugssuch as ivermectin, pyrantel pamoate, and oxibendazoleare rotated every two months throughout the year. Another approach is the seasonal program, based on the local climate and parasite populations. For example, a seasonal program may call for a deworming at the start of grazing season, with dewormers rotated every two months while the horse is on pasture. At the end of the grazing season, a dewormer effective against bots should be used.

There is also a daily dewormer that is incorporated into an alfalfa pellet, pyrantel tartrate, that can be used to help prevent nematode infections. A purge dewormer should be given before the daily dewormer program is started.

"Whichever way you choose to go, it is important to keep in mind which parasites a particular dewormer is effective against," says Dr. Hutchens. "The deworming program should include drugs effective against the common equine parasites, including roundworms (Paracaris equorum), large and small strongyles, and pinworms (Oxuris equi). However, also of concern in adult horses are bots, tapeworms, and encysted small strongyles. (Encysted small strongyles cause a syndrome known as cyathostomosis, the symptoms of which range from chronic weight loss and diarrhea to colic and death.) Treatment for these parasites can be part of the two-month interval rotation program or can be given in addition to a seasonal or daily program."

Ivermectin alone is highly effective against bots infections. It should be given in the spring, after bot eggs are produced, and again in the late fall, to remove any bots picked up during the grazing season.

Tapeworms can cause intermittent colic. A horse may have tapeworms even if no eggs are found in a fecal test. Pyrantel pamoate (Strongid paste) is effective against tapeworms, and many equine veterinarians recommend giving it once a year at double the label dose for effective removal of tapeworms. However, Dr. Hutchens advises always consulting your equine veterinarian before using doses higher than the label recommendation.

Moxidectin, a new drug on the market, or high doses of fenbendazole are effective against encysted small strongyles. Moxidectin is also reported to be effective against roundworms, small and large strongyles, pinworms, bots, hairworms, and stomach worms. It has been shown to be effective against strongyles for up to three months in adult horses, a month longer than the standard two-month dosing interval. However, owners should be cautioned that moxidectin suppresses roundworms for only six to eight weeks. Because foals are highly sensitive to roundworm infections, horses under 2 years of age that are given moxidectin should be dewormed at 2-month intervals.

"The method of administration should not affect efficacy of the dewormer," says Dr. Hutchens. "So long as the horse consumes the paste, it will work as well as using a stomach tube. The advantage of a stomach tube over an oral paste is that the horse can't spit out the drug, so delivery of the entire dose is assured.

"Deworming programs should be carefully planned in consultation with your equine veterinarian," says Dr. Hutchens. "All programs should be supported by management practices to decrease the number of infective parasites in the environment." Such practices include treating all horses pastured together at the same time with the same dewormer, promptly deworming new arrivals, and removing feces from stalls and paddocks regularly. And, finally, yearly fecal exams on 10 percent of the horses on the property will tell you how well your program working.

For more information on horse health care, consult your local equine veterinarian.