We take it for granted when administering drugs to horses that their intestinal tracts can handle the medications just fine. At the 12th International Equine Colic Symposium (2017), a paper was presented about the effects of moxidectin on the equine hindgut microbiome [Daniels, S.P.; Ellis, R.; Swann, J.R.; Moore-Colyer, M.J.S.; and Proundman, C.J. The Effect of Moxidectin Treatment on the Equine Hind Gut Microbiome, Metabonome and Feed Fermentation Kinetics in Horses with Very Low Parasite Burdens. Equine Veterinary Education, 29: 6].
The researchers looked at three questions:
Does moxidectin alter bacterial composition?
Does moxidectin get metabolized by bacteria?
Does moxidectin alter hindgut fermentation?
The study included 17 pastured horses fed only hay and no concentrates. The horses had negative fecal egg counts prior to medication administration. Nine horses were given the oral anthelmintic; eight horses served as controls.
Results of the study reported that the presence of moxidectin in the hindgut altered fermentation kinetics for hay by 16 hours following treatment. Gas production of oats was evaluated in vitro, not in the live horses, with similar findings.
In conclusion, there was no significant change in bacterial diversity or in bacterial metabolic output by moxidectin for dietary hay or oats.