At least two horses at Northville Downs Standardbred racetrack in Northville, Mich., contracted equine herpesvirus type 1 (EHV-1) and were euthanatized in December. The first case was found Dec. 14, when a 6-year-old Standardbred gelding began exhibiting classic signs of the neurological form of EHV-1 myeloencephalopathy (EHM). After the horse was taken to the Diagnostic Center for Population and Animal Health at Michigan State University (MSU), the preliminary diagnosis was confirmed. Another horse at the track tested positive for EHV-1 on Dec. 20 and was euthanatized.

The EHV-1 organism can cause several forms of disease, including rhinopneumonitis (a respiratory disease found mostly in young horses), abortions, and neurologic disease.

Elizabeth Carr, DVM, assistant professor of large animal clinical sciences at MSU, treated the first affected Standardbred when it arrived on campus. “When the horse arrived, he was dribbling urine and his hind was very weak and unsteady,” she said. “Those two clinical findings coupled with the history of rapid onset and progression of clinical signs made EHM one of our top differentials in this animal. The infected horse was euthanatized the next day.”

The track was quarantined and 25 horses have been watched closely for clinical signs. Steve Halstead, DVM, MS, Michigan’s state veterinarian and Animal Industry division director at the Michigan Department of Agriculture (MDA), said clinical signs of EHM include fever, cough, runny nose, and neurological deficits that typically appear within 21 days of exposure. The Michigan Office of Racing Commissioner and the MDA are investigating the potential outbreak and monitoring the quarantined horses