Small Strongyles in Horses: Dewormer Resistance a Global Issue

Evidence that small strongyles are becoming resistant to common deworming medications in Australia, just as they are in many other parts of the world, has been discovered for the first time recently.

Of all the internal parasites that plague horses, experts identify small strongyles as the most prevalent and pathogenic. Because of this, control of small strongyles ranks high among priorities for parasitologists. Indiscriminate use of deworming medications, also called anthelmintics, has resulted in resistance in small strongyles throughout the world. To determine the efficacy of commonly used anthelmintics against small strongyles in Australia, researchers recruited two Thoroughbred farms, both of which house horses of various ages.^*

Horses on one farm were treated with the recommended dose of either a single anthelmintic or a common combination of anthelmintics, including oxfendazole, abamectin, abamectin and morantel, moxidectin, and oxfendazole and pyrantel. Horses on the second farm received only moxidectin at the recommended dose. Researchers conducted fecal egg count reduction tests to determine the efficacy and egg reappearance period.

In the end, researchers found that small strongyles showed resistance against oxfendazole, abamectin, and the combination of oxfendazole and pyrantel on one farm but high efficiencies for moxidectin and the combination of abamectin and morantel. On the second farm, researchers observed resistance to moxidectin.

“This study provides the first report of resistance to abamectin, moxidectin, and a combination of anthelmintics in small strongyles,” concluded the researchers. “Moxidectin is arguably the last effective anthelmintic to manage small strongyles in horses; however, resistance was detected in this study.” They also suggested the use of alternative parasite-control strategies to reduce overuse of anthelmintics.

Though large numbers of mature parasites may cause lethargy, weight loss, and diarrhea, immature larval stages can cause more serious problems as they develop in the mucosal wall of the intestine. Innumerable third-stage encysted larvae may infiltrate large portions of the intestinal wall, eventually damaging the tissue and reducing nutrient uptake. The mass emergence of encysted fourth-stage larvae from the intestinal wall, usually in the spring, may cause larval cyathostominosis, a syndrome characterized by profuse, watery diarrhea, ventral edema, and serious colic. Despite the best care, mortality may be as high as 50%.

“Most owners understand the importance of deworming as part of any comprehensive health plan for a horse. If a horse is persistently underweight despite adequate energy intake, I will ask for a deworming history,” explained Catherine Whitehouse, M.S., a nutritionist at Kentucky Equine Research. As part of her work, Whitehouse frequently fields nutrition-related questions from horse owners. “If severe enough, parasite load can affect a horse’s weight and general appearance, so it’s important that deworming needs are met.”

Effective deworming depends on fecal egg count, a diagnostic test performed on a manure sample to identify the type and number of parasite eggs. A veterinarian can perform the test and can then prescribe an appropriate anthelmintic.

“Fecal egg count tests are inexpensive, so veterinarians will often advocate for one or more tests each year. By doing this, they know exactly what treatment to recommend, an important step in cutting back on excessive or improperly used anthelmintics, which only adds to the worldwide resistance problem,” Whitehouse said.

^*Abbas, G., A. Ghafar, J. Hurley, J. Bauquier, A. Beasley, E.J.A. Wilkes, C. Jacobson, C. El-Hage, L. Cudmore, P. Carrigan, B. Tennent-Brown, C.G. Gauci, M.K. Nielsen, K.J. Hughes, I. Beveridge, and A. Jabbar. 2021. Cyathostomin resistance to moxidectin and combinations of anthelmintics in Australian horses. Parasites and Vectors 14:597.