Grazing horses are at risk of parasitic infection and resulting serious illness, so owners and veterinarians often rely on the dewormer ivermectin, among other anthelmintics, to treat and control internal parasites, including large and small strongyles. However, scientists know little about how equine endocrine disorders might impact ivermectin’s efficacy, leading a team at the University of Kentucky’s Gluck Equine Research Center to investigate.

Endocrine disorders such as insulin dysregulation (ID) and pituitary pars intermedia dysfunction (PPID, formerly known as equine Cushing’s disease) are more common in horses ages 16 and older. Researchers have reported that about one-fifth of horses in this age group have been diagnosed with PPID. They have also suggested PPID might negatively impact immune function, potentially making middle-aged or senior horses with PPID more susceptible to parasite infection.

This might explain why strongyle fecal egg counts were significantly higher in the senior horse population in one study, said Martin Nielsen, DVM, PhD, Dipl. EVPC, ACVM, professor at the Gluck Center, in Lexington. “Therefore, it is important to know if treatment responses might be different compared to what we see in senior horses without these conditions.

Studying the Efficacy of Ivermectin in PPID Horses

Nielsen and his team of researchers divided 20 horses into four groups: six with ID, three with PPID, seven with PPID and ID, and four healthy control horses. They measured each horse’s fecal egg count on the day they administered ivermectin, two weeks after deworming, and once weekly until eight weeks after ivermectin administration.

The team found the average egg count reduction in all groups two weeks post-treatment was at least 99.7%. Worm eggs reappeared at six and seven weeks in the ID and PPID/ID horse groups, respectively, while eggs reappeared at eight weeks in the PPID horse group and the healthy control horses. The expected egg reappearance period (ERP) in horses treated with ivermectin is 8-10 weeks, meaning that two of the groups displayed shortened ERPs

“Ivermectin efficacy was high in all groups,” said Nielsen. “Egg reappearance periods (ERPs) were also similar between groups, although horses diagnosed with ID and ID/PPID showed a tendency towards shorter ERPs than PPID horses and healthy controls.” Due to the small number of horses in each group, veterinarians and horse owners need to interpret the results with caution, he added.

Take-Home Message

Nielsen said he and his team believe ivermectin might be an effective dewormer in horses with PPID; however, more research with a larger sample size is needed to confirm these findings. “The main conclusion is that more studies are needed to evaluate if strongyle parasites harbored by ID horses might display a different response to ivermectin treatment,” he added.

The study, “Ivermectin performance in horses diagnosed with equine endocrine disorders” appeared in Veterinary Parasitology in June 2024.