On many breeding farms, Parascaris equorum (roundworms) and other intestinal parasites in young foals are now controlled with one class of dewormer. This has become common because of the belief that certain drugs, like ivermectin, are highly effective and free from parasite resistance. However, a recent report from Ontario, Canada, describes foals from a breeding farm with fecal egg counts for P. equorum strongly suggestive of ivermectin resistance.

Sixteen of 51 foals from the farm had fecal samples collected to count parasite eggs for monitoring the effectiveness of the deworming program. While this practice is not very common, it is critical in the early identification of parasite resistance. The foals were then dewormed with ivermectin, and 12 days later, more fecal egg counts were done. Normally, the latter samples would be negative. Surprisingly, 10 of the 16 foals had a significant number of P. equorum eggs in their feces. Seven foals actually had more eggs than the original count. These findings led the farm veterinarian to contact researchers from the University of Guelph, including Andrew Peregrine, BVMS, PhD, DVM, an associate professor who set out to further investigate this suspected case of ivermectin resistance.

Another 21 foals had fecal egg counts performed 12-13 days after ivermectin treatment. Of these, 12 had P. equorum eggs identified, and five had egg counts greater than 100 eggs per gram (epg). “There is a poor relationship between egg numbers per gram of feces and the foal’s parasite burden,” Peregrine explains. Instead, he adds, these egg counts are “a good indication of the level of environmental contamination of parasites that is occurring on the farm and the efficacy of the deworming program.” After ruling out other causes of treatment failure (incorrect dose, poor storage condition, drug expiration, poor-quality drug, loss of drug during administration, etc.), these combined findings suggest that the population of P. equorum parasites on this farm was indeed resistant to ivermectin.

Subsequently deworming affected foals with fenbendazole was completely curative. However, Peregrine point outs that ivermectin-resistant P. equorum is now a reality. “It is highly likely,” he believes, “that ivermectin-resistant P. equorum occur elsewhere in North America.” Rotational deworming is a good way to protect against drug resistance, but, Peregrine adds, “Regular fecal examinations are just as important, as is monitoring the growth rate of foals and their health, in the early identification of a potential parasite problem