A suddenly clumsy gelding has become aggressive toward a favorite pasture buddy, is salivating profusely, and wants to cuddle with his handlers, which for him is unusual. It’s July in the Northeast, and the mosquitoes are out. It’s clear he is neurologic, but what tests should you run? This is the kind of diagnostic puzzle that frustrates equine veterinarians. All they and the horse’s owners want is a clear diagnosis, so that potentially life-saving treatment can begin, but the truth is that many equine neurologic diseases can only be definitively diagnosed on post-mortem examination.

Amy L. Johnson, DVM, Dipl. ACVIM, in her role as an assistant professor of Large Animal Medicine and Neurology at University of Pennsylvania School of Veterinary Medicine’s New Bolton Center, admits that much of what she has learned about distinguishing one neurologic disease from the next has been from examining severe cases that have succumbed to neurologic illness, and she’s applying what she’s learned in studying these horses to help keep other horses out of necropsy. To help veterinarians navigate the confusing landscape of laboratory diagnostics for neurologic disease in the live horse, she presented guidelines for diagnosis at the 2015 American Association of Equine Practitioners Convention, held Dec. 5-9 in Las Vegas.

“If you know the best sample to collect, as well as the best test to request, it should improve your ability to diagnose neurologic diseases in horses,” said Johnson, who focused primarily on equine protozoal myeloencephalitis (EPM) and Lyme disease in her presentation, but also touched on other infectious neurologic diseases as well. “Not only is choosing the test important, but you need to know how to interpret the test, and that’s not necessarily as easy as it sounds.