A mutant strain of a common horse virus has resulted in the deaths of seven animals and appears to be spreading quickly in Western states, the Wall Street Journal reports. The USDA reports suspected or confirmed cases of Equine Herpes Virus-1 at 42 stables or farms across 18 states. (The Journal says cases have been confirmed in nine of those states.) The outbreak appears to have started at a national riding competition in Utah earlier this month, and now similar competitions, rodeos, and fairground expos are being canceled.
"This outbreak is much more widespread than most, or perhaps any, previous outbreak," says Colorado's state veterinarian. While there's a vaccine for equine herpes, vets think the outbreak is a resistant mutant strain. The virus is highly contagious, and horses can pick it up from water, equipment, or feces. It doesn't affect humans.