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 priori-
ties, 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 Community Horse Spring/Summer 2021
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