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Logan: Equine herpes virus periodically appears across western states

by Wyoming Livestock Roundup

A recent incident where four horses were tested positive for Equine Herpes Virus-1 (EHV-1) has prompted concern from horse owners across the state, but Wyoming State Veterinarian Jim Logan explains the disease is endemic across the state and around the world. 

“This disease has been around for hundreds of years,” says Logan. “It’s been in Wyoming since the inception of the state, and it’s all over the country and around the world.” 

Endemic disease

Logan estimates as many as 80 percent of the horse populations have EHV, though they don’t exhibit symptoms. 

“By two years of age, almost all horses have been infected with EHV-1,” 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. 

“Horses of any age that are carriers of EHV-1 may not show any external signs of disease when the virus is in latent form,” he explains. “The virus can be reactivated during times of stress, such as strenuous exercise, over-exertion, long-distance transport or at weaning.” 

“This isn’t new, and it isn’t even something that we see pop up every year,” he says. “We have, however, had numerous cases of EHV-1 neurologic disease in Wyoming over the years.” 

Inside the disease

EHV-1 is one of a large group of DNA viruses causing potentially serious disease in equine species, Logan comments, noting that four manifestations of disease can occur in horses – Equine Herpes Myeloencephalopthy, respiratory disease, abortion and neonatal death. 

“Rhinopneumonitis virus is very common in horses,” explains Logan. “It usually causes respiratory symptoms, and it can cause abortion in mares.” 

Logan adds, “The virus can also mutate and set up in the nervous system and the brain.”

When the virus affects the nervous system in its EHM form, horses may die, but if treated, horses may also recover. 

“EHM is most often due to mutant or neuropathogenic strains of EHV-1,” he explains. In recent years, there has been a marked increase in the number of EHV-1 cases, especially EHM, reported in the United States.”

Cases have occurred at large horse facilities and events, including racetracks, horse show grounds, veterinary clinics and boarding stables, where large numbers of horses can be exposed in a short amount of time.


EHV-1 is contagious and spread by direct contact between horses, contaminated hands, equipment and tack or through aerosol dissemination of the virus, in some cases. 

“Horses may appear to be perfectly healthy, yet spread the virus via secretions from their nostrils,” Logan explains.

Initial symptoms of the infection may include a fever of 101.5 degrees Fahrenheit or greater. Sometimes fever is the only symptom, but respiratory symptoms like nasal discharge and cough may be seen, as well. Some horses also have reddish mucous membranes. 

“Horses with neurological disease caused by EHV-1 infection can soon become uncoordinated and weak, and they may have trouble standing,” Logan says. “Difficulty urinating and/or defecating may also occur.” 

Rear limbs are often more severely affected than the front, and signs of brain dysfunction may occur. 

He continues, “The incubation period of EHV-1 infection is highly variable, depending on the host, the virulence of the virus and environmental and other factors, including stress.” 

On average, incubation occurs from four to seven days, with the majority of cases occurring within three to eight days post-exposure. 

“When the neurological disease occurs, it is typically eight to 12 days after the primary infection involving fever,” Logan adds. “In most cases, horses exposed to EHV-1 will develop a fever and possibly nasal discharge, then go on to recover without developing more serious symptoms.”

Treating affected stock

If horses are suspected to show symptoms from EHV-1, Logan notes producers should call their veterinarian immediately.

“We use non-steroidal anti-inflammatories and anti-viral drugs to treat EHV-1 under veterinary care,” he says, noting EHV is on the reportable disease list. “We must have veterinarians monitoring quarantines of EHV.” 

In more severe cases,  palliative treatments may be helpful, including fluids, vitamins, etc. 

“Particularly with the neurologic form, we see a motor impact when they lose control of their hind legs,” Logan explains. 

Additionally, Logan suspects many more horses experience EHV symptoms than are reported, but they are likely not ever seen by a veterinarian. 

“We can only take action on cases we’re aware of,” he says. “We ask that producers who suspect EHV-1 contact their veterinarians to treat horses.” 

Preventing spread

While most horses have been exposed, Logan also recommends several steps to prevent the spread of EHV-1. 

First, Logan says there are vaccines that can be used to help prevent the spread of respiratory, abortion and weak foal cases of EHV-1, but the vaccine is not reliable in the prevention of the neurologic forms of the disease.

“Stop horse movement if animals may be infected by EHV-1,” he says. “This is the most important step. Horses should neither enter nor leave a premises where EHM has been diagnosed until released from a quarantine veterinarian.” 

Exposed horses should not be commingled with un-exposed horses, and sick horses should be isolated. 

“Ideally the sick horse or horses should be moved into a separate building or paddock on the premises or be transported to a veterinary hospital with an isolation facility if pre-approved,” Logan says. 

Equipment should not be shared among horses at the facility to avoid spreading the virus from horse to horse.

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

Saige Albert is managing editor of the Wyoming Livestock Roundup. Send comments on this article to

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