A horse in Virginia has tested positive for the neurological form of equine herpes virus-1, the state veterinarian has confirmed.
Dr Richard Wilkes said the affected horse, from northern Virginia, was euthanized late on Thursday.
He said diagnostic samples were submitted for testing by a private practice veterinarian because the horse had experienced a fever for three days and began to have neurological signs compatible with the disease.
Field veterinarians are assessing potential contacts the horse had with other equines.
All horse owners in Virginia were urged to minimize non-essential contact with other horses and to be vigilant over biosecurity measures.
Symptoms of the disease may include a fever, nasal discharge, wobbly gait, hind-end weakness and dribbling of urine. The disease is often fatal.
The virus is easily spread by airborne transmission, horse-to-horse contact and by contact with nasal secretions on equipment, tack, feed and other surfaces.
Caregivers can spread the virus to other horses if their hands, clothing, shoes or vehicles are contaminated.