EHV-1: Not Quite Armageddon

By June 1, 2011

Horses afflicted with full-blown EHV-1 suffer neurological damage, including loss of coordination. Fortunately, such severe cases are extremely rare.

There’s much concern—and even some hysteria and misinformation—about an outbreak of Equine Herpes Myeloencephalopathy (EHM) caused by Equine Herpes Virus 1 (EHV-1) that occurred at the National Cutting Horse Association’s Western National Championships in Ogden, Utah, from April 30–May 8. As of May 25, there were 18 confirmed cases of EHM in California; one was euthanized, seven showed neurologic signs and the rest only had a fever.

While the disease is of some concern, it is not a terrible killer, and there is little chance of an epidemic. Also, it is not new; other outbreaks have occurred this year in Florida and New York, but, for some reason, those incidents didn’t seem to gain traction like this one has. (Editor’s Note: Of the 18 affected horses in California, 16 participated in the Utah show; two were at a cutting horse show in Bakersfield on May 13. The California Department of Food and Agriculture has concluded that the outbreak is centered around horses that either competed or came into contact with participants at these events.)

Herpes viruses are among the most successful of all viruses, due largely to their ability to live in the host without causing much harm. Human cold sores are caused by a herpes virus; the virus lives in nerve sheaths and only shows up at times of stress (like a cold). Otherwise, you’d never know it was there. It’s the same way in horses; essentially every horse has been exposed to the virus by early in life, but it rarely causes disease.

Four different strains of herpes viruses have been identified in horses. The one that has been implicated in this most recent outbreak is a variation of the EHV-1 virus. EHV-1 generally causes mild respiratory disease in horses; you might not even know that your horse has been exposed. A single nucleotide difference in the DNA of the virus is the only thing that is different between the garden-variety EHV-1 and the one that has most recently made horses sick. However, this little difference in DNA seems to be more likely to cause horses to develop signs of disease related to their nervous system, such as incoordination, weakness and the inability to rise. Even so, the correlation is not 100 percent between this strain and neurologic disease. Viruses with the “normal” sequence can be associated with neurologic manifestations of the disease; on the other hand, not all horses infected with the “neurologic” strain develop neurologic disease (most of the horses that have been exposed have just developed a fever).

Like most viruses, EHV-1 is contagious and spreads from horse to horse (it cannot be transferred to humans). It usually takes 2–10 days for a horse to show signs of disease after exposure.

In most horses, the signs of EHV-1 infections are quite mild, mostly fever and nasal discharge. Only in the most severe cases is the nervous system affected; in those horses, signs include hind-end weakness, lethargy, dribbling urine, poor tail tone, a general loss of motor coordination and, in severe cases, the inability to elevate when prone. Even horses with severe incidences of such symptoms can recover, depending on the extent of the damage to the horse’s central nervous system. Treatment typically includes IV fluids, drugs to reduce the fever and other supportive care; antiviral therapy has been tried, but the effectiveness is unknown.

While there’s no specific treatment, there’s also no vaccine that’s protective against neurologic manifestations of EHV-1. In fact, so far, it’s been pretty much impossible to come up with an effective vaccine against herpes viruses of any species, and this, after several decades of work. The best way to prevent the disease is to prevent contact with infected horses and to make sure hands, tack and equipment are all clean and not passed between horses.

There are all sorts of resources available on the Web if you want to learn more about EHV-1. Unfortunately, it can be difficult to tell the difference between the good and the bad information, so there’s lots of misinformation being spread around. But here are a few good sites at which you can learn more:

American Association of Equine Practitioners Fact Sheet

California Department of Food and Agriculture

University of California Center for Equine Health

United States Department of Agriculture Resources

No one wants horses to be sick, and there’s every reason to handle your horse(s) with care and to use good hygiene measures (which should of course always be the case). If you’re concerned and want to take extra precautions, monitor your horse’s temperature daily and call your veterinarian if it is above 102 degrees.

But do know that despite the heightened publicity, relative to other causes of disease or illness, EHV-1 is not a huge threat. While one horse in California was euthanized as a result of EHV-1 in May, that number likely pales in comparison to those that have died from colic, illness or injury during the same period. These are things to keep in mind when evaluating whether to continue as usual with your normal equine activities.

A graduate of the Colorado State University School of Veterinary Medicine, Dr. David Ramey has practiced equine medicine in Southern California since 1984. He specializes in the care and treatment of sport and pleasure horses and has authored more than a dozen books. To learn more, go to


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