The first North American cases of small strongyle resistance to macrocyclic lactones, the chemical class of dewormers most widely used in horses, has been reported by researchers at the University of Kentucky. Parasites are considered resistant when a significant proportion of the population can withstand the effects of chemical agents administered to eliminate them.

While resistance to the other two chemical classes of equine dewormers—benzimidazole and tetrahydropyrimidine—has been widespread for years, cases of resistance to macrocyclic lactones, which includes ivermectin and moxidectin, have been rare and isolated. Nonetheless, parasitologists have been warning that macrocyclic lactone resistance was likely or even inevitable, and now it appears to be occurring.

The resistant parasites were discovered on a Kentucky breeding farm that cooperates with UK researchers in monitoring the efficacy of deworming products through fecal egg count (FEC) testing. In February of this year, all of the farm’s yearlings—a group of homebreds and three groups of youngsters imported from Ireland—were dewormed with ivermectin and then underwent FEC testing.

The FEC tests showed that the homebred yearlings had a 100-percent reduction in small strongyle eggs after the treatment, but the groups of imported youngsters had reductions of only 93.5, 70.5 and 74.5 percent—indicating the parasites were resistant to the dewormer. To verify these results, the two groups of imported yearlings with poorest response were retreated with ivermectin and retested, but the reduction in the number of parasite eggs was lower and the efficacy was even lower.

To investigate further, the researchers divided the horses with the resistant parasites into two treatment groups: One was given moxidectin and the other received a triple combination of moxidectin, oxibendazole, and pyrantel pamoate. The groups treated with moxidectin alone had fecal egg count reductions low enough to indicate resistance, while the group given the triple combination had a 100 percent reduction. This, the researchers say, confirms that resistance to macrocyclic lactones, rather than some other external factor, caused the initial poor response.

Four months later, the researchers again retested the efficacy of ivermectin in all three groups of imported yearlings. Those tests showed poor egg reduction rates, even as the fecal egg count reductions of all the homebred yearlings remained in the 99 to 100-percent range.

The fact the resistance was found in the Irish horses does not mean it will be limited to them, says Martin Nielsen, PhD. “They carry these parasites with them wherever they go, and the parasites produce eggs that are released into the environment when the horses graze. Thus, other horses will quickly be exposed to and infected by these resistant parasites. This study demonstrated how drug resistant parasites can quickly traverse continents, and it has probably happened several times already.”

Nielsen adds that if horse owners were to meticulously test as this breeding farm did, they may find resistance to ivermectin and moxidectin on their own properties. “We know from several published surveys that very few people routinely test for resistance on their farms. This study demonstrates how important resistance testing is, and horse owners, farm managers, and their veterinarians should be encouraged to follow this example.”

CLICK HERE to read an in-depth article on how to implement deworming practices that prevent the development of resistance. 

Reference: “Importation of macrocyclic lactone resistant cyathostomins on a US thoroughbred farm,” International Journal for Parasitology: Drugs and Drug Resistance, December 2020

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