The Past, Present, and Future of Equine Deworming

Forty years ago, Don McLean was singing "Bye, Bye Miss American Pie" on the radio, gas was 55 cents a gallon, just 52% of American households had color television sets, and rotational deworming was considered the best and most efficient treatment for equine parasites. Not anymore.

"Horse owners should be saying 'bye, bye' to the outdated practice of calendar-based rotational deworming," says Hoyt Cheramie, DVM, MS, equine specialist for Merial's Large Animal Veterinary Services. "We have new science that tells us there is a better, more effective way to control parasites in horses."

Where We've Been
In 1966, Gene Lyons, PhD, and Harold Drudge, DVM, introduced an equine parasite control program designed to suppress large strongyles, the most prevalent and threatening equine parasite at the time. Using information then available, they proposed a simple formula: treat every horse on the property the same--deworming every other month, year-round. They also suggested alternating between products to target all parasites.

Horse owners began adopting their suggestions. Then, as new drug classes came on the market, pyrimidines (pyrantel) in the 1970s and avermectin/milbemycins (ivermectin and moxidectin) in the 1980s/90s, rotating between products became even easier, making the practice widely suggested by veterinarians and a standard way of deworming among horse owners.

Where We Are
Since then, we've learned a lot about equine parasites, their life cycles, and how best to control them. Large strongyles have essentially been eradicated on well-managed farms, and parasitologists have shifted their focus to controlling small strongyles. Additionally, another important discovery is that a very small percentage of horses in a herd, just about 20%, harbor about 80% of the parasites.

Instead of treating all horses the same on a calendar-based schedule, parasitologists now recommend treatment plans based upon the each horse's needs. To develop effective plans, owners can work with their veterinarians to collect manure and conduct fecal egg counts on each horse. This test identifies which parasites are present and which of the horses are high, medium, or low shedders. The results help determine whether a horse needs treatment or not.

Another important step in effective parasite control is identifying which products still work against the farm's parasites. There is evidence of small strongyle resistance to two of the three major dewormer classes, benzimidazoles and pyrantels. Because of this resistance, the products horse owners "rotate" throughout the year might or might not be working. Another test, called the fecal egg count reduction test, can help horse owners figure out which products should remain a part of their deworming program.

Where We're Going
Despite new science suggesting calendar-based rotational practices don't effectively control parasites, a number of horse owners continue to use these strategies. Results of a survey showed 52% of horses were on interval schedules and a majority of the participants had never done fecal egg testing.

"We've got to help horse owners and veterinarians understand they might be compromising their horses' health and wasting resources by using outdated deworming practices," says Cheramie. "Using products, without knowing if they work, is a bad practice. We have the tools to determine which horses need which products and how often. The science of parasite management will continue to evolve, but first horse owners and veterinarians have to catch up with what we already know and become more strategic in their deworming practices."

Another important factor for horse owners to consider in an overall deworming plan is whether products help protect their horses against all parasites, including tapeworms. Because tapeworms have been proven to be a threat to horses' health, they are important to control. Selecting a broad-spectrum product will help horse owners manage parasite challenges.

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