Methods for Controlling Equine Parasites in the Environment

One researcher describes research on controlling ascarids, or roundworms, before they enter a horse’s body.
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Most horse owners are familiar with deworming as a part of their parasite control programs. But did you know some strategies can help control parasites in horses’ environments? At the 2013 University of Kentucky (UK) Equine Showcase, held Jan. 18 in Lexington, Ky., Mary Rossano, PhD, an assistant professor in the UK Department of Animal and Food Science, discussed recent research about controlling equine ascarids before they enter the horse’s body.

Rossano explained that ascarids (Parascaris equorum) are the largest worms that infect horses and most commonly affect horses up to 18 months of age. Once a foal develops natural immunity to the parasite, they enjoy life-long protection against it, Rossano said. Foals with heavy ascarid infections often present with a pot-bellied appearance, anorexia, and coughing (from worms migrating to their lungs); in serious cases, pneumonia can develop. She also noted that large worm masses can obstruct horses’ bile ducts and intestines, leading to colic or death.

To make effective decisions on environmental parasite control, it’s important to understand the ascarid lifecycle. Rossano explained that affected horses shed one-celled eggs in their feces; however, these eggs can’t infect horses just yet. After one to two weeks, the one-celled eggs develope into infective eggs, which the horse can ingest from a variety of sources, including feces, soil or pasture, stall floors, feed buckets, and nearly any other surface the eggs and the horse might come in contact with. Once inside the horse, larvae in the eggs hatch and migrate from the small intestine to the host’s liver. Later, they migrate again, this time to the lungs. Once the larvae are in the lungs, the horse coughs them up and subsequently swallows them, returning them to the digestive tract. There, they develop into adult worms that reside in the horse’s small intestine; these adults will lay eggs, which are then expelled into the environment in the horse’s feces. The entire process takes about three months to complete, she said.

Rossano explained that once the eggs have been released into the environment their protective coating enables them to survive for years in soil in most climates, which can add to the challenges of environmental parasite control. Another ascarid challenge is the resistance to several common dewormers, she said. Multiple studies conducted around the world have shown that ivermectin and moxidectin are no longer effective against ascarids, she said. Further, researchers have reported some resistance against pyrantel pamoate and double doses of fenbendazole. At this point, she noted, resistance to the latter two products could be farm-dependant

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Erica Larson, former news editor for The Horse, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

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