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AQHA issues statement about equine herpesvirus outbreak

May 17, 2011     E-mail this page to a friend!

By Don Treadway, Jr., AQHA Executive Vice President

I suspect several of you may have heard or are hearing about an outbreak of EHM and I want to provide you with the most current information we have.

Yesterday afternoon, we participated on a conference call with the NCHA Executive Committee and Dr. Jerry Black about the outbreak of Equine Herpesvirus Myeloencephalopathy (EHM) caused by EHV-1 (Equine Herpes Virus) that occurred at the NCHA Western National Championships in Ogden, Utah, from April 29 through May 8, 2011.

Dr. Black gave us a current update as to the number horses, shows and states affected by this outbreak and explained if a horse is going to come down with clinical signs of EHM they will do so in 2 to 10 days. Once showing clinical signs, they will be contagious within 12 to 24 hours.

Note: download the PDF file from APHIS that explains the disease at the following web address:

According to Dr. Black, it is believed the index horse (the initial carrier) came from Western Canada. Currently there are nine states and western Canada with confirmed cases. It was reported last Friday in Colorado there were two confirmed cases; both horses had been at the show in Utah. One horse died and another is in CSU’s isolation hospital. There were horses that left the show in Utah and went to a show in Bakersfield, California, and one of those horses has died; there are three others in isolation at UC Davis. Washington State University has one horse in isolation as well. For precautionary measures, all three vet schools have restricted non-emergency equine veterinary appointments.

In a preventative move, the board of directors of the Breeders Invitational canceled the cutting event scheduled for May 14-28 in Tulsa, Oklahoma. NCHA canceled the Mercuria-NCHA World Series of Cutting that was scheduled in conjunction with the Breeders Invitational. At this time, Dr. Black reported there were no horses showing any clinical signs that had been in Tulsa.

The show managers for the AQHA/NCHA Weekend in Montana and Oregon have voluntarily cancelled the events. The NCHA, with advice from Dr. Black are monitoring the other states before making further cancellations.

NCHA further recommends that all its approved events for May 20-22 be cancelled.

At this time no states have closed their borders. Per Dr. Black, it appears USDA will be taking control of tracing the outbreak as they have requested the list of participating horses from Utah.

Additionally, following is a press release from AAEP regarding this subject.

Urgent response information and resources

Currently, there are numerous reports of equine herpesvirus myeloencephalopathy (EHM) affecting horses and farms across the U.S. and Canada. This outbreak appears related to initial cases at a cutting horse show in Ogden Utah, which was held from April 29 - May 8. Horses at that event may have been exposed to this virus and subsequently spread the infection to other horses. While the true extent of this disease outbreak is uncertain, there is clearly a very significant elevated risk of EHM cases at this time. At this time control of the outbreak is critically dependent on biosecurity.

Laboratory submission of nasal swabs and whole blood samples collected from the exposed horse can be utilized for virus detection and isolation. Please consider testing any suspected cases.

The EHV-1 organism spreads quickly from horse to horse but typically only causes neurological disease sporadically. However, in an outbreak of EHV-1 neurologic such as we are experiencing now, the disease can reach high morbidity and case fatality rates. The incubation period of EHV-1 infection is typically 1-2-days, with clinical signs of fever then occurring, often in a biphasic fever, over the following 10 days. When neurological disease occurs it is typically 8-12 days after the primary infection, starting often after the second fever spike. In horses infected with the neurologic strain of EHV-1, clinical signs may include: nasal discharge, incoordination, hind end weakness, recumbency, lethargy, urine dribbling and diminished tail tone. Prognosis depends on severity of signs and the period of recumbency.

There is no specific treatment for EHV-1, although antiviral drugs (i.e. valacyclovire) may have some value before neurological signs occur. Non-specific treatment may include intravenous fluids, and other appropriate supportive therapy; the use of anti-inflammatory drugs (NSAIDs) is strongly recommended. Currently, there is no equine vaccine that has a label claim for protection against the neurological strain of the virus.

Horse-to-horse contact, aerosol transmission, and contaminated hands, equipment, tack, and feed all play a role in disease spread. However, horses with severe clinical signs of neurological EHV-1 infection are thought to have large viral loads in their blood and nasal secretions and therefore, present the greatest danger for spreading the disease. Immediate separation and isolation of identified suspect cases and implementation of appropriate biosecurity measures are key elements for disease control.

Please report any cases or suspect cases to your state/provincial animal health department as soon as possible.

In order to assist you and your clients further, visit for Frequently Asked Questions, resource information from the AAEP, USDA, state and provincial animal health departments, and other related information regarding this outbreak and the disease.

—William Moyer, DVM, 2011 AAEP President