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