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A new study from Wales suggests that the anthelmintic fenbendazole kills internal parasites without disrupting the delicate balance of other microorganisms in a horse’s hindgut.

Researchers at Aberystwyth University investigated the potential secondary effects of fenbendazole on the microbial population of the equine intestinal tract, in part because changes in the hindgut microbiome have been linked to other digestive problems. They first collected fecal samples from 10 horses kept on the same pasture and determined their fecal egg count. Five of the horses had high counts and required deworming with fenbendazole and five with low counts were not treated at all and served as controls. The researchers collected more fecal samples from each horse two and 14 days later.

Using polymerase chain reaction (PCR) technology to analyze the samples, the researchers studied the population of microbes present in each horse’s hindgut both before and, if applicable, after deworming. The data showed no significant difference between the two.

These results, say the researchers, show that “the deworming agent has relatively little impact on the rest of the microbial population of the hindgut. This is true for both bacteria … and protozoa.” They conclude that “the only effect to be had from treatment with this worming agent is its intended purpose of killing intestinal worms.”

Reference: “The effect of administration of fenbendazole on the microbial hindgut population of the horse,” Journal of Equine Science, July 2018