Anne M. Eberhardt

EHV-I Confirmed in Horses at Maryland Farm

The neurologic form of equine herpesvirus type-1 has been confirmed as the cause of disease in two Maryland horses infected during a deadly outbreak this spring.

Twelve horses exhibited signs of neurologic illness in the outbreak at a single Montgomery County farm, and five of them died. While the virus has not been confirmed in the other 10 horses, EHV-1 is suspected.

Dr. Amy Polkes, one of the veterinarians involved in the case, said it took several weeks of extensive testing at multiple laboratories to make the confirmation, which was announced July 8. The confirmations were made using samples from a deceased horse and one that survived.

The first signs of illness were detected in March, and the Maryland Department of Agriculture put the farm under quarantine. Veterinarians are confident the disease was contained, and the quarantine was lifted June 23 after five weeks passed with no additional cases of neurologic disease.

Polkes said no one is sure how EHV-1 was introduced to the farm, which houses mainly polo horses. "It's one of those contagious diseases that is usually seen as the respiratory form, and there's no way that we could track it back to exactly how these horses got it," she said.

"This has been a very interesting and frustrating presentation of equine herpesvirus," Polkes said. "Clinical signs ranged from mild hind limb ataxia to acute recumbency and coma to cranial nerve signs. (While this covers the gamut of possibilities for EHV-1 neurologic clinical signs), it's just unusual to have such a wide range.

"Not one of these horses had any bladder paresis (which often causes urine dribbling in affected horses), and the diagnostic testing was not all consistent in several aspects--you would hope that you might have gotten more positive test results out of that number of horses."

Veterinarians don't have molecular information on the EHV-1 strain, so they can't yet compare this strain to others from recent U.S. EHV-1 neurologic outbreaks.