A 3-year-old Standardbred mare suffering from signs of neurologic disease tested positive for equine herpesvirus type-1 (EHV-1) and was euthanatized at the Allegan County Fairgrounds in Allegan, Mich., on Jan. 18. The horse had been shipped in from Canada to train at the harness racing training facility. Two of the fairgrounds’ barns were quarantined and officials have been locating all horses that might have been in contact with the mare.

Susi Baer, DVM, field veterinarian with the Michigan Department of Agriculture, described the mare’s clinical signs. “It happened very quickly…she basically showed neurologic signs on the 16th,” said Baer. The horse’s treating veterinarian initially saw the horse and called the MDA. “I went out on the 17th and did a pharyngeal swab (that she submitted for testing), and on the 18th she was diagnosed and euthanized. She actually was down in the rear quarters, but she was very bright and alert and was eating well, and that’s why they (the owners) chose to wait until they had a specific diagnosis. They had hoped she had just hurt herself and would come out of it.”

The mare’s diagnosis was confirmed at the state’s Diagnostic Center for Population and Animal Health at Michigan State University on Jan. 18. Michigan agriculture officials placed a 21-day quarantine on the fairgrounds, and they are monitoring animals at the facility for signs of illness. Disinfection and other biosecurity measures are in place, and confirmation of any additional cases would reset the 21-day quarantine. Baer said all the other horses at the facility at the time of the diagnosis were up-to-date on their vaccinations and received herpes vaccine boosters immediately.

“They all appeared healthy,” she said. There are no EHV vaccines labeled for preventing the neurologic form of EHV-1.
There had been little animal movement from the fairgrounds prior to the herpes case. Officials determined that three horses had left the fairgrounds to go to Hanover Fairgrounds in Harrisburg, Pa., the weekend before the mare became ill. Michigan officials alerted the Pennsylvania state veterinarian’s office that the horses had been at the fairgrounds where a herpes case had been detected. When officials told the owners, the owners chose to euthanatize those three horses rather than undergo quarantines in Pennsylvania