Electron micrograph of EHV-1.
"While I have not yet placed any restrictions on the movement of animals, I strongly suggest that horse owners isolate animals that attended the Western National Championships in Ogden, Utah," he said.
"For the protection of other horses, these owners are advised to keep their animals home for a couple of weeks."
Two horses that attended the event have tested positive for the disease in Colorado, with six more showing symptoms. One has tested positive in the veterinary hospital at Washington State University.
Blood samples from several other Washington horses that attended the Utah event are currently being tested at the university's Washington Animal Disease Diagnostic Laboratory.
The Western National Championships of the National Cutting Horse Association was held in Ogden, Utah, was held from April 30 to May 8.
The Colorado Department of Agriculture said it was working with the Utah State Veterinarian to investigate the location of the Ogden championships "as a point of interest for the infection".