The horse had presented with a fever on April 4, and after several days with no change in clinical signs the veterinarian submitted a respiratory panel. The test came back positive for EHV-1, after which the stable manager began monitoring temperatures for the other 35 horses on the premises. Seven additional horses on the premises were identified with fever between April 7 and 9, and the veterinarian submitted a test for a second horse, which also came back positive for EHV-1.

One of the febrile horses on the premises began showing neurologic signs on April 11 that indicated equine herpesvirus myeloencephalopathy (EHM).

The state veterinarian’s office was notified that a group of horses from the index premises and other barns in the county had traveled together to a Scottsdale, Arizona, event from March 14 to 29. On April 12, one horse from that group, residing on a second premises, also presented with fever and developed neurologic signs.

Both premises are under official quarantine and being monitored by the state veterinarian’s office and horses’ private veterinarians.

A third premises that houses horses transported to the same event is under a hold order, although no febrile horses or EHV-1 signs have been noted.

Colorado’s state veterinarian is in communication with Arizona’s to notify other attendees from the event.

EHV 101

Herpesvirus is highly contagious among horses and can cause a variety of ailments in equids, including rhinopneumonitis (a respiratory disease usually found in young horses), abortion in broodmares, and equine herpesvirus myeloencephalitis (EHM, the neurologic form).

In many horses, the first or only sign of EHV-1 infection is fever, which can go undetected. In addition to fever, other common signs of EHV-1 infection in young horses include cough, decreased appetite, depression, and a nasal discharge. Pregnant mares typically show no signs of infection before they abort, and abortions usually occur late in gestation (around eight months) but can be earlier. Abortions can occur anywhere from two weeks to several months following infection with EHV-1.

Horses with EHM usually have a fever at the onset of the disease and might show signs of a respiratory infection. A few days later, neurologic signs such as ataxia (incoordination), weakness or paralysis of the fore- and hind limbs, urine retention and dribbling, loss of tail tone, and recumbency (inability to rise) develop.

Herpesvirus is easily spread by nose-to-nose or close contact with an infectious horse; sharing contaminated equipment including bits, buckets, and towels; or clothing, hands, or equipment of people who have recently had contact with an infectious horse. Routine biosecurity measures, including hygiene and basic cleaning and disinfection practices, should be in place at all times to help prevent disease spread.

Current EHV-1 vaccines might reduce viral shedding but are not protective against the neurologic form of the disease. Implementing routine biosecurity practices is the best way to minimize viral spread, and the best method of disease control is disease prevention.