Got Worms? Blind Routine and Rotational Deworming Falls Short


Got Worms? Blind Routine and Rotational Deworming Falls Short

We thought we’re on the safe side practicing our regular rotational deworming routines. Well, it sounds like we aren’t.  There’s new research that shows that rotational programs fall short and worms only become more resistant! To find out more read below. Courtesy of
Instead, horse owners and veterinarians should collaborate to apply the dwindling supply of effective deworming products more strategically – and more effectively.Bluntly challenging more than four decades of conventional wisdom, Merial today announced a new equine educational campaign urging horse owners to stop arbitrarily utilizing year-round or rotational deworming programs.

“Multiple studies from across the country have shown entire classes of dewormers are no longer working against small strongyles,” said Frank Hurtig, DVM, MBA, director for Merial’s Large Animal Veterinary Services. “Most horse owners think they have been doing the right thing to fight resistance by using a daily product or rotating dewormers. But indications are strong that these practices may contribute to resistance.

“After four decades of little or no progress, it’s time for the industry to rethink daily deworming and dewormer rotation,” says Dr. Hurtig.

Horse owners should stop routinely deworming based on the calendar, tradition, peer recommendations or dewormer sale price, advises Merial’s new “Greetings, Human” educational campaign. They should instead seek out a knowledgeable equine veterinarian to help them develop long-term, effective deworming protocols based on monitoring and management.

“Our educational campaign humorously recognizes that if worms could talk, they’d in fact be telling us to keep on doing exactly what we’ve been doing for decades,” says Dr. Hurtig. “In the long run, unfortunately, it’s actually better for the worms if horse owners continue to deworm the way most have been. The last thing a resistant small strongyle would want is for us to become more strategic in deworming.

Strategic guidance from knowledgeable veterinarians is the critical missing link preventing too many farms from effectively managing their parasite populations, he says. “There are about 50 species of small strongyles that can infect grazing horses. Eliminating all worms from all horses and pastures on a farm is a futile goal.  A much more worthy target is to maintain a low level of parasites on a farm with a large proportion of those parasites being highly susceptible to the dewormer(s) being used – an approach also known as refugia. It is important to remember most of the parasites are on the pasture and about 20 percent of the horses on a farm put out about 80 percent of the worm eggs to contaminate those pastures. Working with your veterinarian to identify these “high shedders” and then tailoring your deworming to target these horses with more effective products, while reducing treatments of the remaining horses, is the best approach to obtaining optimum worm control on your farm.”

This approach can be a significant departure from what drug companies, scientists and veterinarians have counseled in the past – urging horse owners to use daily dewormers or rotate deworming compounds from different classes based on a set calendar schedule. With new data, better understanding of products and how they work, and increasing evidence of parasite resistance, many in the equine industry are changing their approach to equine deworming.

He proposes that horse owners adopt refugia as a new goal for parasite control on their farms.  All the other reasons for which we use dewormers will become immaterial if this goal is not achieved. This means we must become better stewards to preserve the effectiveness of current products. Prudent use of deworming compounds requires us to be more targeted, more strategic and more judicious. Our job now is to keep the effective dewormers as effective as possible for as long as possible. Daily deworming and continual rotation will not do that.

“Think about it this way: You would never randomly use an antibiotic to blindly treat an undiagnosed and inapparent infection,” Dr.Hurtig says. “This practice wastes time and money, and it increases the risk for developing resistant pathogens. The same attitude must now be brought to bear against increasingly resistant parasites in dewormer selection.”

Dr. Hurtig suggests horse owners ask their equine veterinarian about a targeted testing, treatment and management program for their operation, or go to for more information about parasite resistance (see the parasite resistance tab on the home page) and Merial’s new “Greetings, Human” educational campaign.
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