A 9-year-old Quarter Horse mare at a training facility in San Luis Obispo County, California, has been confirmed positive for equine herpesvirus myeloencephalopathy (EHM) secondary to EHV-1. The mare, who was undervaccinated, became acutely neurological on February 22 and was unable to rise. She was subsequently euthanized due to the severity of her clinical signs.

The mare, who competed in reining, had recently attended a large western discipline event in Tarrant County, Texas, from February 10 to February 20. Clinical signs were not observed during the event or travel. Event management and the Texas state animal health officials have been notified. Twice-daily temperature checks for 14 days following departure from the event grounds are recommended for all horses that attended the event.

Twenty-six horses at the mare’s home premises have potentially been exposed. They are quarantined, and the California Department of Food and Agriculture was onsite to help implement enhanced biosecurity measures.

EDCC Health Watch is an Equine Network marketing program that utilizes information from the Equine Disease Communication Center (EDCC) to create and disseminate verified equine disease reports. The EDCC is an independent nonprofit organization that is supported by industry donations in order to provide open access to infectious disease information.

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 EHM.

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.