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