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Deworming An Easy-to-Understand Guide

by Stephanie Sanders

When it comes to internal parasites and your horse, there’s good news and bad news. The bad news: There’s evolving resistance by parasites to chemical dewormers. The good news: Though the problem of resistance is serious, with a savvy schedule developed with your veterinarian, coupled with good stable management practices, and fecal testing, you not only can stay a stride ahead of an infection but can potentially save money by decreasing how often you deworm and lessen your horse’s chances of a case of colic as well.

Dewormers were introduced to the horse community more than 50 years ago. Now, the easy-to-use paste dewormer can be found in every barn, local tack shop, and vet’s truck. Effectiveness, safety, and ease of use in controlling internal parasite populations and minimizing parasiterelated diseases, including the dreaded colic, were the benefits to horse owners in the 1960s with the introduction of the first benzimidazole-type drugs (Panacur, Safe-Guard) that were highly effective at eradicating large strongyles.

When tubes of paste dewormers were first introduced and available to the consumer without involving a veterinarian, the most common and concerning internal parasite was large strongyles. The goal of those initial tubes of dewormer was to eliminate adult large strongyles before they could lay eggs and infect pastures and paddocks. Because strongyle eggs reappeared in manure about two months after deworming, most parasite control schedules involved deworming every two months.

In 1966, J. H. Drudge and E. T. Lyons were the first to describe a modern equine anthelmintic program based on suppressive treatments. With the availability of Panacur and Safe-Guard, Drudge and Lyons used the best scientific evidence available at the time to recommend a treatment protocol involving yearround, bi-monthly treatment of all horses.

This protocol has since then been

denoted the standard interval dose regimen. If you’re still following this 52-yearold program, this article is for you!

Today, dewormers are used in parasite-control programs but concerns about effectiveness have emerged due to parasites’ increasing resistance against various dewormers.

Now, we’re seeing that small strongyles, roundworms, and tapeworms are the leading causes of parasite infection and parasite-related diseases. The life cyles of these worms differ from that of the large strongyle. This is one reason that frequent deworming on a two-month schedule is not effective in controlling small strongyles, roundworms, and tapeworms.

“In terms of management priorities, an effective parasitecontrol program is second only to supplying clean, plentiful water and high-quality feed. It’s that important.”

American Association of Equine Practitioners (AAEP)

Large strongyles are now rare, and small strongyles are the parasites of concern in adult horses, while roundworms remain the parasite most frequently seen in foals and weanlings.

While different worms have different life cycles, all types progress through similar stages: eggs hatch and develop into larvae, migrate through a host body and mature into adults that, in turn, lay from hundreds of thousands to millions of eggs daily. Horses are infected with eggs or larvae by eating grass, grain, or hay contaminated with manure or by physical invasion, via the mouth or skin. (Think of that mouthy foal or young horse that tastes everything.)

According to the American Association of Equine Practitioners (AAEP), “In terms of management priorities, an effective parasite-control program is second only to supplying clean, plentiful water and high-quality feed. It’s that important.”

Left untreated, the damaging consequences of parasite infection can lead to obstruction of blood vessels or the gastrointestinal tract, tissue invasion/ destruction, toxic reactions, anemia, depletion of nutrients, and increased susceptibility to disease such as colic.

In short, stopping a parasite problem before it starts is your best defense.

Growing Resistance

Stopping parasite infection early is more important than ever because parasites are continuously becoming resistant to dewormers. Nature has a way of adapting its creatures to a changing environment and that resistance is happening now.

Resistance develops with repeated use of dewormers to populations of worms that have resistant genes. Over time, those organisms with the greatest resistance to a drug pass on their genes, allowing evergreater numbers of their species to remain unscathed. Ultimately, enough of a proportion of the parasite population will possess resistance genes, resulting in dewormer treatment failure.

Dewormer resistance is a major threat to the current and future control of worm parasites in horses. It will be years before a new class of dewormer is developed and tested for use in the horse.

According to the AAEP Parasite Control Guidelines, “Resistance is real and researchers have identified it in the United States against three classes of dewormers.” A large majority of the studies have been performed in the southeastern states, but there’s little information for the Northeast. However, here are the current levels of resistance seen in the southeastern states for the four dewormer drug classes:

Benzimidazoles – generic names: fenbendazole and oxibendazole; brand names: Panacur, Safe-Guard, Anthelicide EQ – widespread resistance in small strongyles in the Southeast, no resistance