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Success In Deworming Horses

A light parasite burden in a horse is preferable to a total kill.


The most successful deworming programs for equine intestinal parasites begin with a fecal egg count for the individual horse and an anthelmintic schedule based on those results, says Frank Andrews, DVM, MS and ACVIM diplomate. A light parasite burden in a horse is preferable to a total kill, because it perpetuates the horse’s immunity, he said.

A fecal egg analysis classifies a horse as a low, intermediate or high shedder of parasite eggs, said Andrews, director of the Equine Health Studies program at Louisiana State University’s School of Veterinary Medicine. Then a strategic deworming schedule can be conceived, with products and frequency based on types of worms and the client’s geographical location.

For example, in Louisiana and other southern states, summer heat generally kills off life stages of worms on pastures, Andrews said. He recommends deworming horses with a double dose (quantity, not repetition) of fenbendazole in the spring, and ivermectin in September or October. Depending on the patient, a third deworming might be warranted in January.

Experts discourage giving anthelmintics to horses routinely every eight weeks because it is probably unnecessary and breeds resistance, Andrews said.

“Ivermectin and moxidectin are the two most effective dewormers we have,” he said.

Small strongyles, or cyathostomins, are a common equine parasite picked up by grazing horses which bore into the colonic mucosa and form cysts that are impenetrable by some deworming products, Andrews said. The encysted stage inflames and thickens the gut wall, reducing volatile fatty acid absorption and thus affecting a horse’s energy level. Moxidectin and the fenbendazole power pack are usually effective on the encysted stage of small strongyles, he said.

Parascaris equorum, the largest nematode horse parasite, affects foals and yearlings, after which horses usually develop acquired immunity. The life cycle involves the small intestine, liver and lungs, where the horse coughs up larvae and swallows them to infect the small intestine yet again.
Andrews noted that an anthelmintic treatment will affect only the adult stage in the small intestine, so a second treatment in two weeks often is used.

This story is a supplement to Veterinary Practice News magazine's July 2012 article, Feed Routines Play Role in Equine GI Problems.


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