Published November, 2013

Owners and veterinarians must consider the early return of parasite eggs in fecal samples after ivermectin treatment as a sign of developing resistance, said Gene Lyons, PhD, professor in classical parasitology, and colleagues from the University of Kentucky’s (UK) Department of Veterinary Science.

Small strongyles (cyathostomes) are the most common parasites in horses, and they can cause severe damage when encysted stages merge in large numbers from the intestinal wall. Thus, it is important to treat infected horses with potent and effective drugs used for parasite control, particularly because small strongyles have the ability to develop resistance to dewormers. In addition, these parasites naturally have a relatively short life cycle.

Because no new drugs against small strongyles are currently being developed, an effective parasite control program should aim to prevent further resistance. Previous studies performed by Lyons and others have shown that strongyle eggs are currently returning sooner after ivermectin treatment than when the drug was introduced about 30 years ago. This has raised concerns about whether strongyles are developing ivermectin resistance.

Recent studies performed by researchers at UK have shown that ivermectin resistance is developing in the immature larvae of small strongyle parasites. The horse ingests infective larvae from the pasture that develop into adult egg-laying worms within the bowel. Small strongyles reside in the colon and cecum of the horse and undergo several developmental stages before they begin to shed eggs.

Lyons said they recently performed a follow-up study to obtain further information about ivermectin's activity against immature small strongyles in four Central Kentucky-born and -raised weanlings.

The weanlings in the study had not been given a dewormer prior to study, but other horses in the same herd had received ivermectin repeatedly over the course of several years, Lyons said. While the weanlings were housed in stalls, their body weights were measured and each horse received ivermectin according to a standard protocol. Worms were then recovered from fecal matter collected daily for six days following treatment. The researchers found 12 species of small strongyles parasites after counting and identifying worms.

Lyons said the study confirmed previous findings: “Ivermectin efficiency still was 100% against adult parasites, which is good news, but only ranged between 0 and 16% against the immature stages,” he said. “Overall, the efficiency both against adult and immature stages combined was in the range of 68%-83%.”

The reason for the early return of strongyle eggs after treatment, Lyons said, is most likely a lowered drug efficiency on immature small strongyles in the horse's large intestine. Interestingly, the drug efficiency against immature strongyles was lower in this study compared to a previous study, which might indicate ivermectin efficiency is declining.

Shaila Sigsgaard is an editorial assistant for the Bluegrass Equine Digest.