An outbreak of equine herpes virus myeloencephalopathy, the neurological form of equine herpes virus (EHV-1), has been reported in eight western U.S. states, prompting the cancellation of 19 U.S. Equestrian Federation competitions.
The outbreak originated at the National Cutting Horse Association Western National Championship in Ogden, Utah, April 29-May 9. As of May 19, the U.S. Department of Agriculture reported 34 confirmed cases and that all of those horses attended the cutting horse event or came into contact with horses that attended the event. Of the 34 confirmed cases, 33 are horses who attended the event. On May 23 the California Department of Food and Agriculture reported another confirmed case of EHM and on May 20 the Utah Department of Agriculture confirmed two additional cases.
Cases have been confirmed in Arizona, California, Colorado, Idaho, New Mexico, Texas, Utah and Washington. Equine Canada reported one confirmed case in Alberta and three suspected cases in British Columbia, and that all of those horses were either at the Utah show or in direct contact with those horses. The USEF has compiled a list of resources for horse owners sorted by state. 
According to the May 19 report , seven horses associated with this outbreak are dead or have been humanely destroyed. The Utah Department of Agriculture reported on May 21 that one additional horse under treatment for EHM was euthanized.
The EHV-1 virus  is highly contagious and can cause abortion, respiratory disease, neonatal death and neurologic disease in horses.
This outbreak has prompted the cancellation of some equestrian events in affected states, although most are continuing as planned, many with heightened veterinary security measures. In addition to the 19 cancelled competitions, six more have rescheduled as a result of the outbreak. Read about how the outbreak is affecting horsemen in one county in California. 
- Spring Gulch Horse Trials (Colo.)  May 21-22
- Briar Fox Farm Spring Horse Trials (Kan.) May 28-29
- Golden State (Calif), May 25-29
- Morgan Horse Club Of Southern California Spring Classic (Calif.) May 20-22
- Fairway Farm May (Calif.) May 21
- Spring Dressage In Santa Barbara (Calif.) May 21-22
- Las Vegas Dressage Spring Fling (Nev.) May 21-22
- IEAHC Memorial Day Classic (Wash.) May 27-29
- WCCS Open Dressage (Calif.) June 3
- Verdugo Hills 5 (Calif.) June 4
- Diablo Arabian Horse Association (Calif.) May 19-22
- West Coast Connemara Show (Calif.) June 4
- Equestrians Institute L’Aperitif (Wash.) May 22
- Dressage In The Valley (Calif) June 4
- Rose City Opener (Ore.) May 26-29
- Millbrook Farms Dressage I (Utah) June 11-12
- Cornerstone Spring 4 (Ore.) June 5
- Fort Vancouver Dressage (Wash.) May 27-29
- Horses Unlimited Dressage Challenge I & II (N.M.) May 28-29
Rescheduled events include the Arrowhead Horse Trials in Billings, Mont., rescheduled to July 16-17.
The Event At Woodside, scheduled for May 27-29 in Woodside, Calif., is among those competitions proceeding as planned with extra health precautions in place.
“I’ve had numerous conversations with the state veterinarian’s office and with Dr. Paul McClellan (our treating vet at Galway Downs for 14 years) and Dr. Alexandra Eastman (our treating vet at Woodside), and I believe that, with the extra precautions we’re taking, our horses will be at no greater health risk than they are at any competition, at any other time,” said organizer Robert Kellerhouse.
Those precautions include requiring horses arriving at the event to have a health certificate dated within 72 hours and to be examined by an on-site veterinarian upon arrival.
News of the latest EHV-1 cases broke in the middle of High Prairie Spring Preview (Colo.), the first of two weeks of hunter/jumper shows at the Colorado Horse Park. Show manager Charlotte Skinner worked closely with Terry Swanson, DVM, and the staff at the Littleton Equine Medical Center when deciding upon a course of action.
“I spoke to Brian Curry, the manager of the Colorado Horse Park, on that Sunday, and we both agreed to do whatever Littleton told us,” said Skinner. “We left the decision up to them. We’re experts on running horse shows, Brian and his team are experts on equine facilities, and the folks at Littleton are experts in these situations. The weather had been awful during Week 1, and exhibitors were telling us that they were eager to show the second week.”
Skinner and her staff called all of the trainers planning to attend the second week of competition and discussed the situation. About 10 stables opted not to attend, marking a 45 percent drop in entries. The horse park implemented strict protocol for horses entering and leaving the facility, allowing horses to enter the park only between 7:30 a.m. and 6 p.m. Each horse was examined and had his or her temperature taken and recorded. At least one horse did have a low-grade fever and was turned away.
“The horse had a history of bad shipping, and he looked otherwise healthy and had a temperature of 102.1,” said Skinner. “We turned him away, and all the horses from his barn had to leave. It turns out he had bronchitis. The vets felt confident that because he had actually been at the show the week before and had gone home and hadn’t had any contact with the cutting horses, chances were very slim it was an infected horse.”
The veterinarians at Littleton already have plenty of experience implementing veterinary protocols at the Colorado Horse Park. They implemented successful protocols during the 1998 vesicular stomatitis outbreak during the North American Young Riders Championship held at the park.
“It was a hard decision to make,” said Swanson. “No system is perfect in preventing the spread of disease. My colleagues and I elected this approach based on what we knew, and we felt confident we could implement effective protocols. We had a controlled population of horses on the show grounds, and they hadn’t been around the horses from Utah. We made sure the horses coming back to the show grounds hadn’t been in contact with those horses.”
Because EHV-1 is an endemic disease, it’s not reportable on a national level. While the outbreak is being managed on a state-to-state basis, the USDA steps in when cases involve multiple states or the movement of horses across state lines. The American Horse Council and American Association of Equine Practitioners asked the USDA to compile a national report. They released a comprehensive situation report on May 19 that details the current information by affected state.
“They did a phenomenal job putting together this report in a timely way,” said Dudley Hoskins, director of health and regulatory affairs for the American Horse Council. “We were seeing so much misinformation out there, and we wanted their help. The USDA did a great job getting boots on the ground, compiling these reports and working to mitigate [the] effects [of this disease].”
The USEF staff have not ordered the cancellation of member competitions, and as of May 24 they do not anticipate that the outbreak will affect any national championships or qualifying competitions.
“We’re advising competitions to follow the recommendations made by their respective state veterinarians and state animal heath departments,” said USEF senior vice president of marketing and communications Kathy Meyer. “People need to make informed decisions based on their assessment of risk.”
To that end the USEF has compiled a list of state-by-state links and phone numbers  for the veterinarians for each of the affected states, as well as other useful links about the outbreak.
All National Cutting Horse Association shows have been cancelled through June 5 as a precautionary measure. Both the American Quarter Horse Association and the Arabian Horse Association have reevaluated their qualifying requirements for upcoming regional and national championships to reflect cancelled competitions in states affected by the outbreak.
Previous neurological EVH-1 incidents included several cases in New York State  and New Jersey  in April 2011 and an outbreak in late 2006 and early 2007 in Florida and elsewhere in the country.