Normally horses' immune systems keep the virus at bay, but stress and an increase in contact may break down defenses.
Although most adult horses are infected with the herpes virus, stress can cause the virus to become active and contagious.
The present horse herpes outbreak centers around a show horse event that was held in Utah about a month ago.
The outbreak has resulted in at least 84 confirmed cases of the disease in horses from 10 states so far.
Equine herpes, a highly contagious infection among horses that can be fatal, may spread when stressed out show horses come together for competitions, according to animal health experts.
That appears to have helped fuel the current equine herpes outbreak, which has killed at least 12 horses and sickened 72 others in 10 states so far. These states include Arizona, California, Colorado, Idaho, New Mexico, Nevada, Oklahoma, Oregon, Utah and Washington.
"Most adult horses are infected with the virus," Philip Johnson, a professor of equine internal medicine at the University of Missouri's College of Veterinary Medicine, told Discovery News. "Like most herpes viruses -- human and animal -- infection leads to a life-long association between the virus and the host. In most healthy horses most of the time, the host's immune system prevents the virus from going active and being especially contagious."
Given "the right circumstances," however, he said "the virus can defeat the constraints of the host's immune system and go active."
Such circumstances likely were in place at the National Cutting Horse Association's Western National Championships held at the Golden Spike Event Center in Ogden, Utah, from April 29 to May 8. According to the U.S. Department of Agriculture, this event resulted in 32 confirmed cases of equine herpes, with these horses later spreading the disease to other victims.
Johnson explained that while the virus can "go active in any horse at any time, it's very likely that the immune system has something to do with it. Congregated horses coming together to compete are stressed, adversely affecting the immune system's function."
Another factor is that the virus comes in two strains, with one strain more likely to cause neurological problems than the other. Symptoms can include a fever of 102 degrees Fahrenheit or higher, nasal discharge from the nostrils, unusual tiredness, weakness, leaning, urine dribbling, reduced tail tone, and a difficulty or inability to stand.
Direct horse-to-horse contact, breathing in the virus, contaminated hands of horse workers, and equipment, tack and food harboring the virus can all lead to transmission of the disease.
Bruce King, state veterinarian of Utah, shared that "EHV-1 (equine herpes) is not transmissible to people." But due to the "highly infectious" nature of this particular outbreak, numerous secondary cases in horses have occurred, leading to the "quarantine of the veterinary teaching hospitals in Fort Collins, Colorado, and Pullman, Washington."
King said "voluntary isolation" of infected horses is encouraged, "with a minimum of two temperatures taken and recorded per day on each horse."
Acting Arizona State Veterinarian John Hunt further recommends that "isolation and monitoring continue for 28 days after any clinical signs of disease are observed."
Hunt agrees that the illness "poses no threat to humans," but Johnson said it could affect other animals, such as mules, donkeys, alpacas, llamas, giraffes and gazelles. So far, the latest outbreak seems to have only impacted horses.
Veterinarians may treat victims with anti-inflammatory drugs, anti-viral drugs and antibiotics for secondary infections. Slings and body padding could be required to help the horses stand up, while intravenous feeding and use of a urinary catheter may also provide additional medical support.
The disease has been documented for at least six decades, but it has resulted in a number of headline-generating outbreaks in recent years.
"It has been conjectured that horse owners subject their competitive horses to more intense confinement, more transport between shows, and stress and transport than they used to, making it easier for the activated virus to pass quickly between horses," Johnson said, adding that grouping "the horses more closely together in temporary housing at horse shows" could also facilitate spread of the disease.
Some vaccines provide protection against the less debilitating form of the virus, but they do not appear to protect against the neurological syndrome. Several virologists are now working on a vaccine that they hope will target this particularly virulent form of herpes.