Experts share opinions and research on guidelines for deworming and vaccinating aging equines.

Do older horses build immunity over the years, or lose it? Do they build a resistance to worms as time passes, or to deworming agents? With more equines than ever living long into their golden years, these are questions an increasing number of horse owners are having to face. Due to a lack of hard scientific research on older equids, the answers aren’t cut-and-dry.

WHAT IS OLD?

Just when does a horse cross the threshold from "mature" to senior citizen? Like so many other factors affecting elderly equines, there is no clear-cut answer. Once upon a time, a horse was considered aged after 16, says Joseph Bertone, DVM, MS, Dipl. ACVIM, a professor of equine medicine at the Western University of Health Sciences’ College of Veterinary Medicine. But that timeline has moved up.

In one recent study, owners generally perceived their horse as old at about age 22.2 However, notes Bertone, that impression also depends on the horse’s activity. "A 6-year-old racing Thoroughbred filly is a little older, but as a broodmare, she’s young," he says

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