With newfound knowledge of the importance of the intestinal microbiome to animal health, more efforts are being made to investigate the effect of oral medications on the intestinal microbiome and metabolome. One such commonly used non-antibiotic medication in horses is treatment with anthelmintics. Because deworming products are often administered multiple times a year, this study is relevant to describing how some deworming protocols may affect overall horse health.

A recent study looked specifically at the effect of fenbendazole and metabolites on intestinal health, specifically bacterial and protozoal populations [Crotch-Harvey, L.; Thomas, L.A.; Worgan, H.J.; Douglas, J.L.; Gilby, D.E.; McEwan, N.R. The effect of administration of fenbendazole on the microbial hindgut population of the horse. J Eq Science 2018, vol. 29, no.2, pp. 47-51].

Five horses with greater than 200 eggs per gram of feces were treated with fenbendazole paste dewormer; five were left untreated as controls in the study. The results indicate that this dewormer “has relatively little impact on microbial and protozoal populations of the hindgut−based on fecal samples as a representation of organisms recently present in the animal’s digestive tract.” Further, there were no differences in metabolite patterns.

The authors concluded that, “The only effect to be had from treatment with this deworming agent is its intended purpose of killing intestinal worms.”