Equine Herpesvirus

Help keep this infectious disease from inhibiting your show season.

From AQHA Corporate Partner Pfizer Animal Health

“It won’t happen to my horse.”

“The disease hasn’t been in my area.”

“It has affected horses in a different discipline than what I ride.”

Many horse owners think their horse is immune to threat of infectious equine diseases, but they may want to think again.

A year ago, in April 2011, news broke about horses attending an equine event in Ogden, Utah, who were exposed to equine herpesvirus-1. Three months later, the USDA Animal Plant and Health Inspection Service declared the outbreak contained, but by then, more than 2,000 horses had been exposed¹.

Of those, 90 tested positive for the virus or its neurologic form, equine herpesvirus myeloencephalopathy¹. Ultimately, 13 horses died or were euthanized as a result of the disease¹.

Just more than half of the 90 horses actually participated in the Ogden event, demonstrating the highly contagious nature of equine herpesvirus-1 and its ability to spread quickly. The remaining 36 horses contracted the virus due to secondary exposure to those horses who attended the show, such as airborne exposure or contamination of water buckets, tack or grooming equipment. Cases were confirmed in 10 states, stretching from Oklahoma to California¹.

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In January 2012, 17 cases of equine herpesvirus-1 were confirmed in California1. On April 6, the California Department of Food and Agriculture confirmed that a mare from Santa Barbara County, California, tested positive for the non-neurological form of equine herpesvirus-1². In addition, a Los Angeles County mare diagnosed on March 30 with equine herpesvirus-1 was the fourth California horse confirmed positive with the non-neurological strain, which stemmed from the HITS Thermal Horse Show, which took place March 5-11, 2012².

“Equine herpesvirus is highly contagious and can affect a number of horses before horse owners and veterinarians realize there is problem,” says Dr. Tom Lenz, senior director of equine veterinary services for Pfizer Animal Health. “The disease is transmitted through both direct and indirect contact with infected horses and can be spread via water buckets, feed tubs, tack, grooming equipment and even on the hands and feet of people caring for affected animals.”

Equine herpesvirus-1 is most likely to create clinical disease, such as respiratory distress, fever, nasal discharge, cough, anorexia and abortions. Recent outbreaks of equine herpesvirus-1 have also involved an uncommon but devastating secondary disease known as equine herpesvirus myeloencephalopathy, which causes neurologic symptoms such as hind limb ataxia or weakness, decreased tail tone, urinary incontinence and death. The prognosis for recovery in horses that do not go down is favorable, but poor for horses that remain recumbent for more than two days. Equine herpesvirus myeloencephalopathy occurs rarely with equine herpesvirus-4 respiratory infections and can also occur with no previous signs of respiratory disease.

Equine herpesvirus is particularly hard to manage because a horse may become latently infected and not display symptoms for many years after infection. The disease may then be activated by periods of stress caused by training, competition, transportation or other environmental situations.

While there are no vaccines labeled for the prevention of equine herpesvirus myeloencephalopathy, horse owners should discuss equine herpesvirus vaccination protocols with their veterinarian.

Pfizer Animal Health offers the FLUVAC INNOVATOR® EHV-1/4 vaccine to help prevent equine rhinopneumonitis due to equine herpesvirus-1 and equine herpesvirus-4, and equine influenza due to type A2 viruses. In addition, PNEUMABORT K®+1b helps prevent respiratory diseases caused by the equine herpesvirus-1 viruses.

When administered prior to exposure to stressful situations such as horse shows, transportation or exposure to disease, immunomodulators can help stimulate and prepare a horse’s immune system to function more efficiently. ZYLEXIS® from Pfizer Animal Health is a demonstrated safe and effective immunomodulator that stimulates the immune system and helps to reduce upper respiratory disease caused by equine herpesvirus-1 and equine herpesvirus-4. As with all vaccinations, owners should discuss their options with a veterinarian. Immunomodulators should not be used in horses with a fever or showing signs of clinical disease.

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While vaccination programs and hygiene management practices may not completely prevent the spread of equine herpesvirus, informed owners can help provide their horses with the best level of protection. Ongoing herd management practices, such as isolating new horses from the main herd, not sharing feed pans or water buckets between horses and thoroughly disinfecting tack and grooming equipment should be part of a facility’s daily routine, especially during proximity to an outbreak.


1. Looking Back: 2011 EHV-1 Outbreak. March 31, 2011. Available at: Accessed on April 23, 2012.

2. Larson, Erica. EHV-1: Additional Non-Neurological Case Identified in California. April 10, 2012. Available at: Accessed on April 23, 2012.