Deworming Horses: Can You Overdo It?

Q: Can you overdo deworming?

A: Yes, the concept that "more is better" is not a sound decision when it comes to deworming. As a matter of fact, many of the studies documenting resistance on a farm have been where there has been a history of very frequent deworming, especially with the same class of dewormer.

Deworming programs, including decisions on frequency of use, types and schedules of dewormers, should be based on a complex number of factors such a geographic location, weather, density of horses, age of horses, exposure of those horses to outside untreated horses (for example, performance horses on the road), management practices (pasture rotation, etc) and fecal egg counts of animals. It is also important to know if there is a problem of resistance on the farm, which can be determined with pre- and post-fecal egg count testing (Fecal Egg Count Reduction Tests, or FECRT).

Based on our studies at Texas Tech, we recommend a 4-way rotation for many farms and ranches. Some conditions may warrant a six way rotation only if needed. The quarterly rotation includes dewormers targeting adult parasites, larval parasites, tapeworms and bots. We believe it is important to consider the range of parasites including ascarids, and not be based only on small strongyle control. Several very recent studies have documented resistance of equine ascarids to the ivermectin class of parasites. This is very alarming due to the serious consequences ascarids can pose to foals. In high numbers, blockage of the gut can occur with fatal results.

Because there is no new class of dewormer on the horizon, we believe it is very important to preserve all of the current classes of dewormers, which have different applications in parasite control in the horse. Other researchers suggest even a less frequent schedule of anthelmintic administration based on a targeted approach, where only the heavy egg shedders are given the dewormers. In the targeted approach, the high shedding horses as identified by fecal egg count (FEC) testing are selectively dewormed where as the low shedding horses are not. Further scientific trials and farm testing need to be performed in the immediate future to investigate all approaches to parasite control.

The bottom line is that many types of deworming regimens are being proposed. There need to be further research studies on the effectiveness of these programs throughout the US as well as around the globe. In addition, remember that each farm needs to be considered individually and decisions need to be made based on testing and management information.

Learn more about deworming horses and read more Q&A with Dr. Brady in our free on-demand webinar: Strategic Deworming!

About the Author

Heidi Brady, PhD, Dipl. ACAP

Heidi Brady, PhD, Dipl. ACAP, is an Associate Professor in the Department of Animal Science and Food Technology at Texas Tech University.

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