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Horse health Equine herpes virus poses continual concern for horse owners

by Wyoming Livestock Roundup

Each year, as horse owners prepare their equine for a variety of events – from brandings and rodeos to county fairs and horse shows, concerns about exposure to disease are top of mind. In Wyoming and around the world, equine herpes virus (EHV), a group of very common DNA viruses, is often targeted as a disease of concern.

“We have had EHV-1 and other equine herpes viruses in Wyoming for decades,” says Jim Logan, Wyoming State Veterinarian. 

Endemic disease

“The great majority of horses are exposed to equine herpes viruses early in life,” Logan continues. “They become lifelong carriers of the virus, but they are latently infected and show no signs of illness.”

Logan notes 80 percent of horses have EHV but do not exhibit symptoms. 

“By two years of age, almost all horses have been infected with EHV,” Logan explains. “The initial exposure generally occurs in foals from contact with their dams.”

After exposure, the virus often becomes latent and persists in the horse for the entirety of its life. 

While they don’t show signs, he explains, “The virus can be reactivated during times of stress, such as strenuous exercise, over-exertion, long-distance transport, at weaning or during adverse weather.” 

Disease presentation

If a horse is infected with EHV and shows signs, they are often seen as one of four primary manifestations – Equine Herpes Myeloencephalopthy (EHM), respiratory disease, abortion and neonatal death.

“EHV-1 is the primary cause of EHM, the neurologic form of EHV-1, which is most often due to the mutant or neuropathogenic strains of EHV-1,” Logan says, explaining a particular mutation in the genome results in EHM. “In recent years, there has been a marked increase in the number of EHV-1 cases, especially EHM, reported in the United States.”

Outbreaks of EHM have been seen across the country at large horse facilities and events, including racetracks, horse shows grounds, veterinary clinics and boarding stables. 

Logan explains, “The large number of horses that can be exposed on such premises and the serious nature of the disease have caused significant concern within the animal health community and the U.S. horse industry.”

EHV-4 impacts the respiratory system of horses and results in rhinopneumonitis, which causes respiratory symptoms. Very rarely EHV-4 is associated with abortion. 

“EHV-4 causes a non-fatal upper respiratory tract disease in foals,” explains Logan.


Because EHV is contagious and spread by several different routes, horse owners are encouraged to be diligent when traveling to equine events.

“EHV is spread by direct horse-to-horse contact; contaminated hands, equipment and tack; and for a short time, through aerosol dissemination of the virus within the environment of the stable and stall,” Logan says. “Horses may appear to be perfectly healthy yet spread the virus via the secretions from their nostrils.”

The initial clinical signs of infection are often non-specific and may include a fever of 101.5 degrees Fahrenheit or greater. 

“Fever may be the only abnormality observed,” Logan says. “Other presenting signs may be combinations of fever and respiratory symptoms of nasal discharge and cough.”

Some horses have reddish mucous membranes, as well.

Horses with the neurologic disease become uncoordinated and weak, and they often have trouble standing and difficulty urinating and defecating. 

“Often, the rear limbs are more severely affected than the front,” he adds. “Signs of brain dysfunction may occur, as well, including extreme lethargy and a coma-like state.”

Because the incubation period of EHV is highly variable depending on the host, virulence of the virus and other environmental factors, Logan says horses may present between four and 14 days, with the majority of cases seen between three and eight days. 

“In most cases, horses exposed to EHV will develop a fever and possibly nasal discharge, then go on to recover without developing more serious symptoms.


For horse owners who are concerned about the possibility of EHV, Logan recommends several steps to minimizing the likelihood of infection.

“Stop movement of horses if an EHV infection is suspected,” he says. “This is the most important first step horse owners can take. Horses should neither enter nor leave a premises where EHV is diagnosed until cleared by a veterinarian.”

Contact between horses exposed to EHV and unexposed horses should be eliminated through quarantine, and sick horses should be isolated.

“Horses that have aborted or shown signs of fever, respiratory disease or neurologic disease should be separated from healthy horses,” Logan says. “Ideally, the sick horse should be moved into a separate building or paddock on the premises.” 

Finally, Logan cautions against sharing equipment among horses at any facility. 

“The virus can be spread from horse to horse via contaminated objects, so equipment should not be shared,” he says.

Logan concludes, “Practice proper biosecurity measures to prevent spreading the virus.”

Saige Albert is managing editor of the Wyoming Livestock Roundup and can be reached at

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