Scientists Investigate Dewormers' Effects on Bowel Walls

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."

The German researchers compared fenbendazole and moxidectin because these were the only dewormers that historically have been shown as effective against small strongyle larvae encysted in the bowel wall. They studied 24 weanling ponies, four of which were stabled and dewormed as a control group. Twenty ponies that weren't dewormed grazed from September until December, then were stabled. Eight weeks later, they were treated either with the European dosage of fenbendazole  (25% lower than the U.S. dosage) or moxidectin. The ponies treated with fenbendazole showed a mild early reaction to the compound in their bowel walls, and marked inflammation or even ulceration two weeks after treatment. The ponies treated with moxidectin did not show inflammation or ulceration of the bowel wall, and the control group did not react to either dewormer (most likely because they were not significantly infected with larvae).

Zahner emphasized that the reaction only occurred when there was a heavy larval infestation, which generally accumulates during a full season of grazing.

"If this proportion of the parasite burden is absent or small, there may be no differences between moxidectin and fenbendazole," said Zahner. "At the end of the grazing season, the use of moxidectin for anthelmintic treatment may help to avoid/reduce tissue damage in heavily infected horses."

Craig Reinemeyer, DVM, PhD, president of East Tennessee Clinical Research in Knoxville, Tenn., noted that severe small strongyle infestation is more of a problem in Europe, where cooler, wetter weather favors worms.

As fenbendazole is sometimes used in "hard keepers" to help them gain or maintain adequate weight, Zahner advised horse owners to consult with their veterinarians prior to deworming to ensure the horse is not heavily infected with small strongyles.

Reinemeyer has conducted studies similar to the German study, and he observed that despite possible inflammation, bowel tissues of treated horses were healthier than in horses that have not been dewormed at all.

About the Author

Judith Lee

Judith Lee is a freelance health care writer who has written for a number of medical and health care journals and health care companies. As a long-time equestrian and horse owner, she has a particular interest in equine health care. She also operates an equestrian education program, Riding for Fun, geared toward adult beginners and returning riders.

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