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Common internal parasites, like roundworms and strongyles, are becoming resistant to a number of the deworming products currently available for horses, according to Martin Nielson, D.V.M., Ph.D., assistant professor at the University of Kentucky Gluck Equine Research Center.

“Several studies have proven that there is decreased efficacy of all three major drug classes of dewormers, also called anthelmintics, to common internal parasites like roundworms and small strongyles,” shares Nielsen.

Faced with no other available drug options for deworming horses, Nielsen is not sitting idly by, waiting for the resurgence in parasite-related diseases, including fatal colitis and colic.

“My laboratory is currently investigating a procedure called ‘combination deworming,’” explains Nielsen.  “Even though we know that some parasites are already resistant to pyrantel and fenbendazole, we wondered what would happen if we used both of those products together. Will there be an improvement in efficacy and if so, for how long? These are the questions we are currently trying to answer.”

The concept of combination deworming was studied in sheep in New Zealand and was found to be more effective than rotational deworming. This leaves Nielsen and his study colleagues hopeful that combination deworming might prove useful in horses as well.

“That said, we are only a few months into our one-year study, and we are not making any recommendations at this point. Only once we have data that I can rely on will I recommend instituting combination deworming,” Nielsen emphasizes.

In the meantime, discuss appropriate deworming with your veterinarian, including the use of fecal egg counts, and remember that pasture management and nutrition are natural ways of helping control parasite burdens in horses.

“Horse owners sometimes forget the physical toll that parasites place on a horse, including adding to its nutritional requirements and affecting its general well-being. Horses and ponies with significant parasite loads often look unthrifty and pot-bellied,” said Catherine Whitehouse, M.S., a nutrition advisor with Kentucky Equine Research (KER). “Once parasites are cleared, feed efficiency is likely to increase, which means horses and ponies may require less feed to sustain weight once moderate body condition is achieved.”

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