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Virus outbreak sickens horses, cancels events

Issue Date: May 25, 2011
By Ron Miller

Horse events have been cancelled in California and other Western states, after an outbreak of a potentially fatal disease.

As of Monday, equine herpes myeloencephalopathy had been diagnosed in 18 California horses, and in horses from several other states. The disease is caused by equine herpes virus, known as EHV-1. The virus does not infect humans, but it can kill horses or cause permanent neurological damage. One horse in California has died from the disease.

According to the California Department of Food and Agriculture, confirmed California cases have been located in Amador, Glenn, Kern, Los Angeles, Marin, Napa, Placer, Plumas, Sacramento, Shasta, Stanislaus and Ventura counties.

All of the infected horses had participated in the National Cutting Horse Association Western National Championships between April 30 and May 8 in Ogden, Utah.

The horses that have been confirmed as having the disease have been placed under a state quarantine by CDFA. Thus far, there is no evidence that the disease has spread outside the cutting horses that participated in the Utah event. Equine herpes virus is contagious and may spread quickly among horse populations.

CDFA Branch Chief for Animal Health Dr. Kent Fowler said that horse owners can prevent the spread of the virus by using good biosecurity.

"The main way the virus is spread is through nose-to-nose contact," he said, adding that horses should not share water and keepers should not wear dirty boots that could carry the virus.

Any horse suspected of having the disease should be isolated and be seen by a veterinarian.

"We encourage owners to avoid any non-essential transport of their horses, mules and donkeys," said Gary Magdesian, an equine veterinarian at the University of California, Davis.

David Wilson, an equine veterinarian and director of the UC Davis Veterinary Medical Teaching Hospital, said horse owners and coordinators of upcoming equine events should educate themselves about the virus and "exercise the utmost caution as they determine whether to participate in or host events that could increase their animals' exposure to this potentially life-threatening disease."

Although many scheduled events for horses had been cancelled as a result of concern about spreading the virus, Fowler said thus far no horse races had been cancelled.

CDFA has contacted all 54 exhibitors from California who participated in the Utah event. The horse owners were asked to isolate their horses and monitor them for clinical signs of the disease. The department suggested owners with potentially exposed animals take temperatures on each individual horse twice a day. If a temperature of 102 is detected, the horse's private veterinarian should be contacted immediately for evaluation and laboratory testing.

The one California horse to be euthanized after showing severe neurological signs associated with the disease had also attended a Kern County cutting horse event May 13 in Bakersfield. One other horse known to have attended the Kern County event was also diagnosed with EHV-1. Owners of other horses at that event have been notified about the disease.

States including Colorado and Wyoming have begun requiring that horses have health certificates before entering their state. CDFA recommends that horse owners contact officials in the state they plan to visit to find out what is required. California borders are open to horses, but they need a health certificate and to have had a negative Coggins test for the disease.

The quarantine will remain in effect until 21 days after the last infection is found. The horse show cancellations have been done as a precaution to keep the infections from spreading.

"The incubation period for the virus is from two to 10 days," Fowler said.

Early detection and treatment are the best tools to use so the horse will survive, and Fowler said the best early detection is to check horses for a fever.

CDFA offered the following biosecurity recommendations for horses that attended the cutting horse events in Utah or Bakersfield:

• Isolate an exposed horse a minimum of 30 feet away from all other horses for 21 days.

• Monitor temperature twice a day for 14 days.

• Immediately report temperatures over 102 degrees to your private veterinarian.

• Use separate equipment, bucket and halters/leads for isolated horses.

• Use protective clothing when handling an isolated horse: coveralls, boot covers, gloves. Do not use the same clothing with other horses.

• Ideally, use separate personnel for isolated horses.

• Restrict movement.

Fowler said that the disease-causing virus is a mutation that happened relatively recently. It was first reported in the 1970s. As a result, there are no specific medicines to treat EHV-1, but veterinarians have general medicines to help horses recover. Three of the infected animals were being treated at the UC Davis Veterinary School.

This is not the first time equine herpes has been found in California. Fowler said that three years ago, one infected horse came into California from the East Coast. The animal arrived on the East Coast from Europe and must have had the disease when it entered the country. The animal was then sent to California. Once diagnosed with the disease, the horse was quickly isolated and no other horses were infected.

More information about the EHV-1 virus and the current outbreak may be obtained at the following websites:


• UC Davis:

• U.S. Department of Agriculture:

(Ron Miller is a reporter for Ag Alert. He may be contacted at

Permission for use is granted, however, credit must be made to the California Farm Bureau Federation when reprinting this item.

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