So far, at least 17 horses in Idaho, Utah, Colorado, California, Washington and Canada have been infected with the highly contagious Equine Herpes Virus-1, and at least three have died. The disease poses no threat to people but is easily spread among horses, alpacas and llamas because it can be airborne and transmitted by touch or by sharing feed, brushes, bits and other equipment.
The infected horses were among roughly 400 that attended the National Cutting Horse Association Western National Championships in Ogden, Utah, earlier this month.
Now, officials in several states are quarantining infected animals and asking owners of other horses that were at the event to closely monitor the animals for symptoms. Organizers also are cancelling horse shows and classes in Texas, Utah and elsewhere in an effort to stem the disease's spread.
The outbreak has horse owners across the West worried, said Preston Skaar, president of the Idaho Cutting Horse Association.
"It's a hard deal, but all you can do is have your horses stay home and wait it out," he said.
Skaar took one horse to compete at the Utah event and then brought it back to his property in Menan, Idaho, where it lives with 12 other horses. So far, he said, none of the animals have shown any signs of illness, but he's starting each day earlier so he can take their temperature and check for fever, gait problems and other symptoms of the virus.
Infected horses can appear perfectly healthy until they get stressed and the virus takes hold, he said - so none of the horses are being ridden unless absolutely necessary.
"I was kind of bummed out that I didn't make the finals (at the Ogden competition), but I'm not feeling quite as bad about it now," Skaar said. "That kept my horse from getting more stressed and fatigued, and maybe that helped."
Officials with the National Cutting Horse Association couldn't be immediately reached, but a statement on the group's website said members are closely monitoring the situation and that all NCHA-approved shows scheduled for this weekend have been canceled by the affiliates or show producers putting on the events.
"The NCHA appreciates this proactive move by show producers in a nationwide show of precaution and solidarity," the group said. "While reported cases of the virus are currently in Western states, the interstate transport of infected horses could cause a much wider spread of the virus if we are not all very cautious at this time."
Cutting competitions - in which horses and riders are judged on their cattle-handling skills - involve quarter horses or other stock-horse breeds. But all horses are susceptible to the virus.
Idaho Department of Agriculture spokeswoman Pam Juker said 26 horses from Idaho were at the Utah event. Of those, two have died of the disease and at least six others were infected, state veterinarian Bill Barton said.
All of the Idaho owners have voluntarily quarantined their animals, Barton said, and they're being told to take precautions such as disinfecting things that come into contact with the animals and limiting contact between horses.
Colorado, which has had two confirmed cases of the virus, is now requiring permits for any horses being brought into the state. One of the two horses was so ill it had to be euthanized, officials said.
The outbreak also has prompted Colorado State University's Veterinary Teaching Hospital to ban all non-emergency appointments for horses as a precaution. The university's Equine Sciences Center has cancelled two riding clinics and temporarily restricted horses from entering or leaving the campus.
Washington state veterinarian Leonard Eldridge said a horse that was treated at the Washington State University Veterinary Teaching Hospital in Pullman tested positive for the virus. Testing is being done on several other horses in the state that also attended the Utah event.
Oregon has no reported cases of the virus but is keeping an eye on 18 horses that attended the championships, said Oregon Department of Agriculture spokesman Bruce Pokarney.
Nebraska's state veterinarian has placed five horse farms under quarantine because they had horses that attended the competition. And officials in Montana are asking the owners of about 35 horses that attended the event to watch for any signs of the disease.
In Utah, the Bureau of Land Management announced it is postponing its 13th annual Wild Horse and Burro Festival until August because of the outbreak. The event, which includes horse shows and adoptions, was set to take place this weekend near Salt Lake City. But organizers said state officials asked them to contain their horses for at least two more weeks to prevent potential exposure.
The virus can usually survive for about a week on surfaces, Barton said, though under the right conditions it could last as long as 30 days. That makes it particularly tricky to fight, because even the snort of an infected horse could spray nearby equipment or feed with the virus, said Debra Sellon, a veterinarian at Washington State University's Veterinary Teaching Hospital.
Generally, fewer than half the animals that get the virus come down with the most serious neurological symptoms, Sellon said.
She added it's too soon to know exactly how many animals have been sickened in this outbreak. "There's just too much rumor out there to tell what's accurate. It may be when the worst is over before we can pull together accurate numbers," Sellon said.
Abby Yigzaw, with the USDA's Animal and Plant Health Inspection Service, said the agency is assisting state veterinarians in compiling numbers and other data about the sick animals. But she said the agency didn't have any immediate numbers on how many horses were infected.
Infected animals usually get sick between two and 14 days after they are exposed to the virus. Symptoms include fever, sneezing, staggering and partial paralysis.