EHV-1: Minimizing Costs, Dispelling Myths (AAEP 2010)

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. They addressed several myths surrounding the disease and drove home the following points:

  • Two strains of EHV-1 (D752 and N752) have been termed "neuropathogenic" and "non-neuropathogenic" in the past. The D752 strain appears to be more commonly detected in neurologic disease outbreaks, causes higher levels of viremia (virus in the bloodstream), and appears to cause disease more readily than the N752 strain. However, both can cause neurologic disease and the response in outbreaks of each is the same (isolate and treat ill horses, quarantine ill and exposed horses, and monitor fevers every 12 hours for at least two weeks).
  • Perform testing via PCR (preferably real-time) and viral isolation when possible to help diagnose disease and document which strain(s) are present.
  • The D752 strain has been more commonly isolated in recent years from Central Kentucky populations. Whether this means this strain's prevalence is increasing in the general EHV-1 viral pool is not yet clear.
  • Neither strain is definitively considered to be a "wild-type" or original strain, and these viruses do not spontaneously mutate from one strain to the other.
  • Since virus is recovered from ill horses for up to 16 days after infection, the AAEP recommends a 28-day quarantine period for ill/exposed animals. In some states, officials might allow a shortened quarantine period of 14 days if animals "test clean" via real-time PCR. However, it might be cheaper to simply continue to board horses at the quarantine location until 21 days have passed since the latest clinical case was identified or became asymptomatic (not retesting if no new cases have presented).

"In conclusion, EHV-1 testing plays a vital role in managing equine neurologic disease," said McConnico. "Outbreaks are manageable, and testing should be done to document which EHV-1 strain(s) are present; this could change the situation's risk assessment."

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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