Study: Some Horses ‘Resistant’ to Aspirin’s Effects

Veterinarians prescribing aspirin to thin horses’ blood might want to reconsider their decision. A recent study has revealed that aspirin’s effects on horses seem less pronounced than in humans. In fact, some horses might be resistant to it.

“Based on my data and that of others in previous published studies, there is probably less anticoagulatory efficacy of aspirin in horses,” said Katja Roscher, DrMedVet, Dipl. ECEIM, of the Justus Liebig University Department of Veterinary Clinical Science Equine Clinic, in Giessen, Germany.

In their study, Roscher and her fellow researchers tested how a five-day aspirin treatment affected coagulation (blood clotting) properties in 10 horses. They gave the horses a loading dose (4.7-5 mg/kg) orally the first morning and a maintenance dose (1-1.3 mg/kg) every day for the following four days.

Seven horses initially showed a marked reduction in platelet aggregation (clotting), ranging from a 37% to 100% reduction only six to 12 hours after the loading dose, Roscher said. But over the next four days, the researchers saw much less reduction, with a lot of variation from horse to horse, ranging from 0% to 98%.

As for the other three horses, the scientists noted no platelet aggregation changes in their blood after the loading dose. Over the next four days, they only got up to a maximum reduction of 22%. “This could be a sign of resistance,” she said.

This could have significant implications in cases in which veterinarians prescribe aspirin to treat diseases with increased platelet activation, such as endotoxemia or laminitis, Roscher said.

Whether there is “resistance” to aspirin or just a basic inefficacy of aspirin in some horses is still unknown, she added.
“Resistance was only suspected,” Roscher said. “But based on the suspected resistance I would recommend testing horses (for their platelet response to aspirin doses) when giving aspirin.”

The study, “Suspected aspirin resistance in individual healthy adult warmblood horses,” was published in the Journal of Veterinary Pharmacology and Therapeutics.