Scientists Investigate Dewormers' Effects

Recent research indicates that when selecting a dewormer, a horse owner should consider the season, the horse's access to grass, and the animal's body condition. German scientists have shown that the bowel walls of small strongyle-infected ponies become inflamed when they are treated with the European dosage of fenbendazole (25% lower than the U.S. dosage), 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. Similarly infected horses treated with moxidectin (Quest), didn't 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," said Horst Zahner, PhD, head of the Institute of Parasitology of Germany's Justus Liebig University Giessen.

It's important to note that the scientists didn't 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 signs following deworming "extremely rare."

Fenbendazole and moxidectin are the only dewormers shown to be effective against small strongyle larvae encysted in the bowel wall.

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," Zahner said.

Reinemeyer said similar studies have shown bowel tissues of treated horses were healthier than those of horses that have not been dewormed at all. For more information see

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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