Equine herpesvirus-1 disease is "one of the costliest equine diseases worldwide," said Rebecca McConnico, DVM, PhD, Dipl. ACVIM, associate professor of veterinary medicine at Louisiana State University. She and Paul Lunn, BVSc, MS, PhD, Dipl. ACVIM, professor of equine medicine at Colorado State University, presented a discussion of controversial issues and myths surrounding the neurologic form of equine herpesvirus-1 (also called equine herpesvirus-1 myeloencephalitis, or EHM) at the 2010 American Association of Equine Practitioners convention, held Dec. 4-8 in Baltimore, Md.

Equine herpesvirus-1 (EHV-1) can cause a variety of problems including respiratory disease, abortion, and neurologic problems. The virus is highly contagious among horses, asymptomatic carriers can shed it readily, and the disease is endemic worldwide; the neurologic form was declared an emerging disease in the United States in 2007 based on increasing incidence of outbreaks. Cases in such neurologic outbreaks are often characterized by fever, weakness, ataxia (incoordination), difficulty urinating and moving the bowels, tail and anal-tone deficits, and a dog-sitting posture.

McConnico and Lunn first detailed several aspects of EHV-1 neurologic disease/infection and control as follows:

• There is no definitive test for EHV-related neurologic disease in live horses; diagnosis is generally based on a history of acute-onset myeloencephalopathy, an outbreak scenario, and the presence of EHV-1 in blood and/or nasal swab samples.

• The incubation period for the disease (time between exposure and clinically apparent illness) is one to 14 days.

• Infected horses usually will shed virus into the environment for seven days or less, but in some cases they can shed for two weeks or longer.

• Horses can be infected with EHV-1 without showing signs of disease, but they can still shed the virus and infect other horses (as asymptomatic carriers).

• Equine herpesvirus can become latent (inactive) in the horse's body, but reactivate in response to stress and cause disease at a later time.

• The disease can be treated successfully.

Minimizing economic losses associated with EHM outbreaks requires agreement among veterinarians and officials on disease/strain nomenclature, case definition, test interpretation, and appropriate biosecurity response, said McConnico and Lunn