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Risk and Reality: Horse Parasite Control and Anthelmintic Resistance

Risk and Reality: Horse Parasite Control and Anthelmintic Resistance

By Jill Griffiths

How can you reduce internal parasite infection and manage anthelmintic resistance on your farm?

Do you know which of your horses has parasites? Chances are they all do, to one extent or another. What’s more important is knowing which horses have the highest fecal egg counts and which anthelmintic (deworming) treatments are effective on your farm. These are the first steps in developing a strategic horse parasite control plan for your farm.

When adopting such a management strategy, the primary goal is to prevent horses from amassing extremely high worm burdens, which, although uncommon, can cause signs of colic, diarrhea, and weight loss. Two secondary goals help achieve this:

Reducing the worm burden on pastures (which reduces horses’ rate of infection and reinfection); and
Maintaining the long-term ­efficacy of available dewormers.
Equine parasitologist Martin Nielsen, DVM, PhD, Dipl. ACVM, an associate professor and the Schlaikjer professor of equine infectious disease at the University of Kentucky’s Gluck Equine Research Center, in Lexington, says that in managing your horses’ own parasite population, you are managing that of the entire farm.

“The worm population includes the worms in each of the horses and also in manure and on the pasture,” he says. “We need to understand that we are treating the parasites, not the horses. Each horse carries a biological sample of the parasite population present on the farm, and we need a coordinated effort for the entire population before we can claim that we have a parasite control program.”

Contrary to the approach taken in the early days of modern dewormers, worm control isn’t about eliminating parasites completely, Nielsen says. That’s proven to be ­impossible.

“Horse worms such as cyathostomins (small strongyles, which primarily affect the equine large intestine) are ubiquitous and affect all grazing horses,” he says. “But they only cause disease extremely rarely and only when infections reach extremely high levels.”

Frequent anthelmintic treatments are not needed to keep adult horses healthy. “What is needed,” says Nielsen, “are properly timed treatments with effective anthelmintics administered at the appropriate time of the year, which correspond to the parasite life cycles and the levels of parasite egg shedding in individual horses.”
 

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