Equine Dewormers: Use Care When Choosing Products

When confronted by shelves and shelves of dewormers, many horse owners can be confused about their choices--and the issue of equine parasite resistance adds even greater confusion.

"Multiple studies from across the country have shown entire classes of dewormers are no longer working against small strongyles, which are a serious equine parasite threat," said Frank Hurtig, DVM, MBA, director of Veterinary Services at Merial. "Now, the challenge to every person who cares about horse health is to help preserve the remaining dewormer classes--while keeping their horses healthy."

There are only three major chemical classes of dewormers. Small strongyles' resistance to benzimidazoles, one of the older classes of dewormer, is well-documented, Hurtig said. In addition 40.5% of farms surveyed in a study had small strongyles that were resistant to pyrantel, a second chemical class of dewormer.

Macrocylic lactones represent the third chemical class. These contain ingredients like ivermectin and moxidectin. Because ingredients like ivermectin and moxidectin work in similar ways, experts believe that if parasites were to develop resistance to either ingredient, it would leave virtually all dewormers ineffective against this serious parasite threat.

"It's time to choose and use deworming products with greater thought and care," Hurtig urged. "The first two concerns for all horse owners should be to identify which horses are shedding the most worm eggs and to determine the performance of the product most recently used at their barn or on their farm. Then, horse owners can selectively treat horses and this may help to slow the further development of parasite resistance."

Hurtig recommended talking to your veterinarian about doing fecal egg count tests to identify the horses on your property that are shedding the most worm eggs. Once identified, deworming treatments can be directed at these animals.

"About 20 to 30 percent of the horses on a farm, or in a barn, put out about 80 percent of the eggs," Hurtig said. "This apparently relates to individual animal variability with regard to immunity to some parasites but much remains unknown on the topic."

"The goal of today's deworming program should no longer be to eliminate all worms," Hurtig explained "One approach is to reduce transmission and limit worm egg output. If owners or veterinarians can identify and selectively treat horses with high worm egg output, they can help control parasites on the farm and help reduce the possibility of those parasites developing resistance. Of course, one's first instinct often is to try to eliminate all parasites from every horse, but it may not be the best option--now or in the future."

Finally, Hurtig encouraged horse owners to choose a product from a reputable manufacturer so any concerns can be addressed fully.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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