A subtle effect of endurance competition

A surprising physical change in endurance horses is linked to their finish times.
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A surprising physical change in endurance horses is linked to their finish times.

A Kansas State University study shows that the physiological effects of endurance competition can sometimes be found in an unexpected part of a horse’s body: the eyes.


The researchers monitored the intraocular pressure (IOP) of 69 horses in five 50-mile endurance events in 2009. IOP is the fluid pressure inside the globe of the eye and is measured with a tool called a tonometer in millimeters of mercury (mmHg).

“IOP is not static and actually fluctuates by a few millimeters of mercury over the course of the day,” says Rachel Allbaugh, DVM. “In many species it is highest in the morning and lower in the afternoon/evening, but this is opposite in horses.”

Prior to each competition, the study horses received a brief ocular exam, during which IOP was measured. Eye pressure was again measured at two points during each ride and once at the conclusion.

The data showed that IOP fluctuated in a manner that reflected finishing times: Most horses experienced an overall IOP decrease by the end of the ride, but those with shorter finishing times had a greater IOP decrease than did competitors who took longer to finish. In addition, horses who received “Best Condition” awards showed less IOP reduction than did horses who did not receive that award.

These differences could be due to hydration status and fitness level, says Allbaugh, although her study wasn’t specifically designed to investigate those factors. “It would be very interesting to repeat an equine IOP and exercise study in a research setting with control of more variables,” she says.

Reference:“Intraocular pressure changes in equine athletes during endurance competitions,” Veterinary Ophthalmology, July 2014

This article first appeared in EQUUS issue #450, March 2015.