Vets: Signs Consistent With Neurologic Equine Herpesvirus

Necropsies from the two horses affected by neurologic signs at Churchill Downs showed the horses had lesions consistent with equine herpesvirus myeloencephalopathy (damage to the brain and/or spinal cord). Three other horses are currently showing neurologic signs, according to Dr. Peter Timoney, head of the University of Kentucky's Gluck Equine Research Center.

Dr. Lenn Harrison, head of the Kentucky Livestock Disease Diagnostic Lab, discussed the necropsy cases with Timoney, but they said there is not a confirmation of herpesvirus at this time.

However, Dr. Doug Byars, an internal medicine specialist from Lexington who is consulting on some of the cases, said while there isn't laboratory confirmation, he is confident they are dealing with the neurologic form of herpesvirus. After initially being called in by one of the attending veterinarians, insurance companies are now paying Byars to consult on some cases.

Timoney concurred, saying based on clinical signs, histories, progress of the illness in the first two fatal cases, the absence of any other logical diagnosis, and consistency of lesions with equine herpesvirus myeloencephalopathy, he believes it is an outbreak of neurologic equine herpesvirus.

More tests are pending at the Gluck Equine Research Center that would confirm the type of herpesvirus affecting the horses. Dr. George Allen of the Gluck Center, an expert in the field of equine herpesviruses, is working to confirm the diagnosis.

Horses affected by equine herpesvirus often will develop a fever as an initial sign of disease onset. Within 24-48 hours of showing neurologic signs, veterinarians can usually define the outcome of the case based on how severe signs become and how quickly they come on. If a horse becomes recumbent (can't get up), prognosis is very poor. Horses that become recumbent and recover often have residual permanent neurologic damage.

"If they show neurologic signs and don't go down within 48 hours, they usually get better," Byars said.

There is no vaccine licensed to prevent the neurologic form of equine herpesvirus.

Of the two horses that have been euthanized, the first horse began showing neurologic signs May 10-12. The second horse began showing neurologic signs May 13. The latter horse had a fever three to five days before becoming recumbent, then was humanely euthanized.

Timoney said the horse industry is seeing an increase in the confirmation of cases of the neurologic form of herpesvirus. In March of this year, Allen stated that researchers discovered that a mutation within a single EHV-1 gene is associated with strains of the virus recovered from neurologic disease cases. He said further research is needed to determine if this mutation arises anew with each neurologic outbreak, or if there is a subpopulation of horses that carry the mutant virus in the latent state with the capacity for reactivation.

Byars said all horses in the barns with active cases are being treated with acyclovir, which some veterinarians feel attenuates disease during outbreaks. He said there aren't any downsides using the drug. He said if there are no new cases at Churchill Downs in the next week, then probably the outbreak is over.

One Kentucky veterinarian said he believed the outbreak was brought under control so fast because of the recent problems with strangles in Kentucky that forced trainers, veterinarians, and track officials to work more closely.

Dr. Catherine Kohn, a specialist in internal medicine and a professor in the Department of Clinical Sciences at The Ohio State University School of Veterinary Medicine, answered user-submitted questions about equine herpesvirus (EHV) on the May 18th edition of's "Talkin' Horses" live discussion series.

A transcript of that discussion is available here.