Equine Herpes Virus
Nathan Voris, DVM                                                                                                                                  printer friendly PDF format

Over the last 6 years, an old nemesis of equine health has evolved into a potentially emerging disease with increasing virulence. Equine Herpes Virus (EHV-1), commonly referred to as “rhino” or more specifically the paralytic form of Herpes virus, is showing itself to be a threat equal to West Nile Virus, but more contagious among horses.

Disease surveillance reports indicate there was 1 reported outbreak in Wyoming in 2001 and 1 reported outbreak in Virginia in 2002. Since 2001, 16 states Colorado, Florida, Georgia, Kentucky, Maine, Maryland, Michigan, New Hampshire, New Jersey, New York, Ohio, Oregon, Pennsylvania, Virginia, Wisconsin and Wyoming) have reported outbreaks of EHV-1. In 2006 alone, there were 11 outbreaks in 8 states. Of the 403 known horses exposed during the outbreaks occurring from 2001-2005, there was a 40% fatality rate.

Equine Herpes Virus is primarily transmitted via nasal secretions that are aerosolized into the environment when an infected horse coughs or snorts. The virus can remain in the environment for several weeks once it has been shed by the horse. Indirect contact through sharing of equipment or transmission through human contact between infected and non-infected horses is a major threat for spreading the infection.

The incubation period, or time from infection to the horse exhibiting clinical signs, can range between 24 hours and several (4-6) days. The first clinical sign is usually a fever (normal equine temperature is between 99-101 degrees Fahrenheit). Additional signs include coughing and nasal discharge. Some horses can quickly progress into neurologic signs without showing other signs. Neurologic signs include: incoordination of the hind (and occasionally fore) limbs, urine retention/dribbling, bladder atony, recumbency (or loss of the ability to get up). Abortion in pregnant mares can occur weeks or months following infection.

Protecting your horses from EHV-1 involves vaccination and diligent monitoring of vital signs (especially temperature). Horses at greatest risk are those who attend shows or are commingled with other horses for events or activities. Because EHV-1 can be spread through indirect contact, and can last several weeks in the environment, vaccination of all horses that come in contact with high risk horses is advisable. In other words, if you take horses to show, all horses, even those that never leave the farm, are at risk and should be vaccinated.

Adequate vaccination requires repeat booster shots every 60-90 days due to the relatively short-lived protection immunization affords. The United States Equestrian Team Foundation is requiring documented proof (signed by a veterinarian) of EHV-1 vaccination between 7-90 days prior to entering USET Foundation property for clinics or events. I would expect other organizations and individual shows to follow suit in the near future.

If a horse is suspected of having EHV-1, strict quarantine procedures in accordance to the Equine Infectious Disease Outbreak: AAEP Control Guidelines should be initiated immediately (see www.AAEP.org), and a veterinarian should be contacted to assist with testing, treatment and control.

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