Investigators have been unable to make a conclusive diagnosis in the outbreak of equine neurologic disease on a single quarantined farm in Poolesville, Md. There is strong suspicion that equine herpesvirus type-1 (EHV-1) is to blame, but further diagnostic testing is under way to determine if this is the case.

Twelve horses were affected in the outbreak, four of which were euthanized. As of June 23, the situation was limited to one farm with no new cases reported since May 25.

According to the Maryland Department of Agriculture (MDA), a group of equine practitioners from Maryland and Virginia met in early June with the treating veterinarians of the horses on the private farm, which is in Montgomery County. The meeting was called to update the veterinarians in the area on the situation at the affected farm.

At this meeting, veterinarians reviewed details of the cases, including all current laboratory data on the horses involved. The difficulty in making a conclusive diagnosis based on the available data was the main focus of the discussion. There is a strong suspicion the problems were caused by the neurologic form of equine EHV-1, but due to inconsistent test results from the affected horses, a definitive diagnosis was not made. Past outbreaks of the neurologic form of EHV-1 (also called equine herpesvirus myeloencephalopathy), have been characterized by clinical signs that included fever, incoordination, and weakness progressing to recumbency. Specifics on the Maryland horses’ clinical signs are not currently available