Deworming horses used to be a treatment that was performed primarily by veterinarians. It involved pumping a liquid through a tube that was passed through a horse’s nose into the stomach. Not surprisingly, your typical horse owner was more than happy to leave that chore to a professional. But then drug manufacturers made dewormers (also called anthelmintics) much easier to give. Nowadays, all that’s involved is squirting a small amount of gel or paste into the horse’s mouth or adding a daily dewormer to the feed.

The convenience of being able to deworm your own horse has been a boon to owners, but the world of equine parasites is rapidly changing. I’m afraid that since veterinarians aren’t much in the deworming loop anymore, information about best practices may not be making its way to owners. The American Association of Equine Practitioners (AAEP) recently put together new parasite control guidelines. They are written with an audience of veterinarians in mind but are an excellent resource for anyone who owns or works with horses. Here’s a sampling of what’s included.

The true goal of parasite control in horses (and other equids) is to limit parasite infections so animals remain healthy and clinical illness does not develop. The goal is NOT to eradicate all parasites from a particular individual. Not only is eradication impossible to achieve, the inevitable result is accelerated development of parasite drug resistance.

Cyathostomins [small strongyles] are truly ubiquitous, and all grazing horses are infected. But they are relatively mild pathogens and only produce disease when infections reach extremely high levels. Thus disease from strongyle parasites is much less of a concern in adult horses today than it was decades ago when S. vulgaris [a large strongyle] was highly prevalent. Frequent anthelmintic treatments are therefore not needed to keep adult horses healthy. What is needed are properly timed treatments with effective anthelmintics administered at the appropriate time of the year, which correspond to the epidemiological cycles of transmission and the relative parasite burdens in individual horses.

All adult horses should benefit from a basic foundation of one or two treatments per year. Low egg shedding horses with naturally strong immunity to cyathostomins will need no other treatments because the two treatments have covered the needs of the other parasites and these horses are protected naturally from cyathostomins by their immune state. In traditional deworming programs, repeated treatment of low shedder horses every 2-3 months accomplishes little to improve their health, but it does promote drug resistance. Moderate and high egg shedders will need a third or fourth treatment for cyathostomins. Daily [dewormers] can be considered for suppression of egg shedding in consistent high strongyle shedders.

Any additional treatments would be given on an “as needed” basis depending on whether a specific parasitic infection or disease is diagnosed.

Even though dewormers are available at every feed and tack store, owners should still talk to their veterinarians about their horses’ parasite control program. From a technical point of view, deworming has never been easier, but knowing when and what to give to a particular individual is still complicated.

Dr. Jennifer Coates

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