Cyathostomins, or small strongyles, are the most pervasive internal parasites of horses and the primary targets of adult deworming programs. The larval stages of this species, which become encysted in the large intestine’s mucosa, are what cause disease and clinical signs in horses. In large numbers they can cause a rare, life-threatening disease called larval cyathostominosis—a mass eruption of encysted larvae that leads to a profound inflammatory response in the horse’s gastrointestinal tract.

“Concerns have been raised that killing larvae while encysted within the mucosal walls could lead to adverse (e.g., inflammatory) reactions,” said Ashley E. Steuer, DVM, PhD, assistant professor of parasitology at Texas Tech University’s School of Veterinary Medicine, in Amarillo.

She and a team from the University of Kentucky recently performed a study to assess horses’ inflammatory response to deworming. “We were especially keen on evaluating the difference between a product that kills those pesky encysted larvae (moxidectin) and a product that does not (ivermectin),” Steuer said. She presented their findings at the 2020 American Association of Equine Practitioners’ virtual convention.

“It’s important that we discuss this inflammatory response, because this can cause disease—not necessarily the parasites themselves, but the reaction to their existence within the host,” Steuer said, adding that very few studies have looked at post-deworming inflammation in horses