Recent research indicates that when selecting a dewormer, horse owners should consider the season, their horse’s access to grass, and his body condition. German scientists have shown that the bowel walls of small strongyle-infected ponies become inflamed when they are treated with fenbendazole, a dewormer sold as Panacur. The inflammation doesn’t appear to be caused directly by the fenbendazole, but by toxins excreted by dying and dead larvae.

This finding was a surprise to the researchers, as similarly infected horses treated with another dewormer, moxidectin (Quest), did not show an inflammatory reaction.

“We had thought a dead larva is a dead larva, but learned that there are drug-related differences–the reasons of which are presently not known to us,” remarked Horst Zahner, PhD, study co-author and head of the Institute of Parasitology of the Justus Liebig University Giessen in Giessen, Germany.

It’s important to note that the researchers did not observe clinical signs of illness in any of the study ponies. Craig Barnett, DVM, senior equine services veterinarian for Intervet, the manufacturer of Panacur, called reports of clinical signs following deworming “extremely rare