Two cases of EHV-1 were confirmed in the last week in horses in Colorado who competed at the National Cutting Horse Association Western National Championships in Ogden, Utah.
Other states are also assessing their situation, with horses from the Ogden event having crossed state borders in their return home.
The confirmed cases forced the cancellation at the weekend of another major cutting horse competition in Tulsa, Oklahoma.
Washington's Veterinary Hospital said the horse had been admitted for evaluation of unrelated problems and was later found to have competed at the show.
Subsequent diagnostic testing confirmed that the horse was positive for EHV-1.
"Due to the potential for spread of the virus, access to the veterinary teaching hospital is currently restricted," it said in a statement.
"During this time, no new equine or camelid patients may be admitted to the hospital except for critical emergencies. We are taking every precaution to ensure the health and well-being of animals"
It is expected that the period of isolation will last at least two weeks.
There are currently no horses exhibiting signs of EHV-1 at the hospital.
Equine Herpes Virus does not affect cattle, sheep, goats, pigs or birds, and the remainder of the hospital remains open.
There is no risk of transmission to people.
There are several strains of the virus, with EHV-1 and EHV-4 being most often involved in clinical disease.
EHV-1 can cause respiratory disease, abortion and neurologic disease, while EHV-4 typically causes respiratory disease.
Neurologic disease, which is generally caused by specific strains of EHV-1, is called equine herpesvirus myeloencephalopathy (EHM).
Fever is one of the most common clinical signs and often precedes the development of other signs. Respiratory signs include coughing and nasal discharge. Abortions caused by EHV generally occur after five months of gestation.
Neurologic signs associated with EHM are highly variable, but often the hind end is most severely affected.
Horses with EHM may appear weak and unco-ordinated. Urine dribbling and loss of tail tone may also be seen. Severely affected horses may be unable to rise.
None of these signs are specific to EHV, and diagnostic testing is required to confirm EHV infection. Also, many horses exposed to EHV never develop clinical signs.
Shedding by the respiratory route typically lasts for 7 to 10 days, although occasionally it may persist longer.
EHV is transmitted primarily by aerosol and through direct and indirect contact. Aerosol transmission occurs when infectious droplets are inhaled.
The source of infectious droplets is most often respiratory secretions.
In the case of abortions, the virus may be present in the placenta, fetal membranes and fluid, and aborted fetuses.
Direct horse-to-horse contact is a common route of transmission of the virus, but indirect transmission is also important. This occurs when infectious materials (nasal secretions, fluids from abortions, etc) are carried between infected and non-infected horses by people or fomites (inanimate objects such as buckets, etc).
Anyone suspecting their horse has been exposed to EHV should contact their veterinarian.
In general, exposed horses should be isolated and have their temperatures monitored twice daily for 10 days.
If an exposed horse develops a fever, diagnostic testing should be performed.
While there are several testing methods available, the most frequently recommended test is a PCR (polymerase chain reaction test) to detect the viral genetic material done on a nasal swab.