Not all horses infected with EHV-1 will show neurological symptoms. The neurologic form of EHV-1 is called Equine Herpes Virus Myeloencephalopathy (EHM). The most common symptoms are poor co-ordination, weakness, the horse may lie down and be unable to rise, and may suffer bladder weakness.
APHIS reports that owners of horses known to have been exposed in this incident have been contacted by state animal health officials.
Suspected and confirmed cases are under voluntary or state quarantine, as are known as exposed horses.
In all, 308 horses were exposed at the Ogden, Utah, cutting horse event from which it has spread, and APHIS lists 729 secondarily exposed.
The confirmed cases of EHV-1 are in Arizona, California, Colorado, Idaho, New Mexico, Oregon, Texas, Utah and Washington.
Of the 34 confirmed cases, 33 cases are horses that were at the Ogden, Utah event.
The death toll remains at seven.
The outbreak has led to widespread event cancellations across many states, and experts have urged equestrians to avoid non-essential transport of their horses.
Dr David Wilson
Another equine veterinarian at the hospital, Gary Magdesian, urged owners to avoid any non-essential transport of their horses, mules and donkeys.
He added that alpacas, llamas and other camelid species are rarely affected by equine herpes virus.
Authorities urge veterinarians issuing Certificates of Veterinary Inspection for interstate movements to contact the state of destination to ensure horses meet requirements for entry.
Some western states, among them Colorado and Wyoming, have amended their requirements for horses entering their states.