RMR July 2011 Article - Recovery from EHV-1

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Recovery from the neurologic form

of Equine Herpesvirus Type 1

by Dorinda Troutman, RMR Staff Writer

July 2011 issue

An outbreak of EHV-1, including a more severe neurologic form of equine herpesvirus (EHM), began at the National Cutting Horse Association Western National Championships in Ogden , Utah , April 29 to May 8, 2011.

Out of 421 horses competing there, as of June 15, 2011, there were 100 suspected or confirmed cases of either EHV-1 or EHM. Twenty-six developed EHM, the most severe form of disease. All were quarantined and ten died or were euthanized.¹

EVH-1 can cause four manifestations of disease in horses, including a neurological form, respiratory disease, abortion, and neonatal death. The neurological form (EHM) is most often due to mutant or neuropathogenic strains of EHV-1, so called because of a particular mutation in the genome.²

The American Association of Equine Practioners states that EHV-1 is endemic, and most horses will have contracted the disease without showing symptoms by the time they are two years old. Neurologic cases can occur singly, or as outbreaks affecting 20% to 50% of the population.

According to Debra Sellon, DVM, PhD, Professor of Equine Medicine at Washington State University in Pullman , Washington , the neurologic form can occur with any of the EVH-1 strains, and can be brought on by stress caused by trailering and showing.

“We know that most horses have been exposed to the disease and can be latent carriers,” says Dr. Sellon. “EVH-1 is a ‘large’ virus – in that it has a lot of genes in it. A single change in one gene may cause the neurologic form to develop, or not. We know that there are outbreaks of the neurological form that do not have the neuro gene in the virus. It can happen with any of the EVH-1 strains. There is just a lot that we need to learn. Every experience, such as this recent outbreak, gives us more understanding of the virus.”

Questions remain

If a horse that has had the neurologic form recovers, does it recover fully enough to compete or breed, or is there permanent damage?

Dr. Sellon explains that most horses which show overt signs of the disease, and even those with neurologic symptoms that do recover, recover fully. However, a small number may have lasting problems.

Does the horse remain a carrier?

Dr. Sellon says that most horses that recover remain latent carriers or have dormant virus markers. They will not be contagious unless their virus becomes active again, which leads to the next question:

Can the disease come back?

“Horses may start shedding virus after recovery from initial infection, especially if they are stressed. However, we have no evidence at this time that horses that have recovered from neurologic disease (EHM) will have recurring disease or that they pose an increased risk to other horses. I am not aware of any horses that have had EHM, recovered, and then developed EHM again,” says Dr. Sellon.

For more information, visit APHIS at www.aphis.usda.gov/vs/nahss/equine/ehv/

1 Equine Herpesvirus (EHV-1) Situation Report, June 16, 2011, USDA, APHIS, Veterinary Services.

2 A brochure: Equine Herpesvirus (EHV) Myeloencephalophathy, A Guide to Understanding the Neurologic Form of EHV Infection, from USDA-APHIS, Fort Collins, Colorado, December 2008.

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