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Recognizing EHV-1 early helps horses

by Wyoming Livestock Roundup

Equine Herpesvirus-1 (EHV-1) is not a new virus, but it does have widespread distribution and effects all populations of horses, according to Nicola Pusterla, DVM, an associate professor of equine internal medicine at the University of California, Davis, School of Veterinary Medicine. 

Pusterla and Robert MacKay, BVSc, PhD, professor of large animal clinical sciences and large animal internal medicine at the University of Florida in Gainesville, presented a webinar on the seriousness of the virus.

Clinical signs

EHV-1 can present itself with different clinical signs, but most commonly as an upper respiratory infection. 

Generally seen in younger horses, they will have a fever, go off feed, act depressed, have a snotty nose, cough and swollen lymph nodes. Pregnant mares may abort, but if they give birth to an infected foal, it will generally not survive, Pusterla said. 

“It will be born weak and die from respiratory complications,” he explained.  

Less frequently, a horse may develop the neurological form of the virus, which can be much more serious. 

“The neurologic form affects mature adult horses,” MacKay said. “It comes on extremely fast, so it is common to miss the early signs.”

“Usually what happens is producers will go to see their horse, and it will be down on its side or lying on its chest thrashing, looking anxious, sweating and struggling to get to its feet,” he explained. “What happens is the hind limbs are uncoordinated causing the horse to stagger, tremor and shake. It may even try to support itself against a wall. If they are down, they may be sitting like a dog attempting to get to their feet.” 

Their hind limbs and tail become paralyzed, and females may leak urine or be unable to urinate.

Disease progression

MacKay said the virus advances rapidly and may get worse for the first few days. 

“What I tell people is that if the horse is still standing 48 hours after the first signs appear, it will probably make it,” he said. “If the horse is lying on its side, paralyzed and its mind is still sharp, it is extremely stressful for the horse.”

“I don’t realistically expect those horses to survive if it goes on more than a day or two,” he added.

“It is difficult to predict which direction this virus will go during an outbreak,” Pusterla said. “It can affect young horses, young adults to older adults, breeding animals and performance horses.”

“One characteristic of EHV-1 is that it can also lay dormant in a horse after an outbreak, and if the horse becomes stressed from things like transportation, exercise or certain immunosuppressant drugs, it may shed the virus at a later time infecting other horses,” he said.


For the horses that do survive this virus, a tremendous amount of muscle mass will be lost. MacKay said they will lose muscle in their top line, neck muscles and rump, which may become bony. 

“With proper care and rehabilitation, it will fill in again over weeks and months,” he said. 

These horses will also require a lot of work and dedication to rehabilitate, which may take months. 

MacKay said during the first month, the horse will have to be isolated in a stall to ensure it doesn’t spread the virus to other horses. 

“It will need supervised walking, maybe only five minutes a day at first,” he said. “It will be extremely weak, and that will probably be all it can handle. Aqua-exercises and walking in the river can also be helpful. The next step is lunging and some riding, and finally, reintroducing the horse back into the athletics it once did.”

MacKay said most horses that survive this virus will show some residual signs from the virus and may never fully reach the level they once had.

Preventing outbreaks

If a horse owner knows of any outbreaks within 50 miles of their property, MacKay said they should consider delaying all non-essential procedures. The virus is spread from horse to horse and can also be spread through shared equipment and human contact with an infected horse through the hands or footwear. MacKay said disinfecting footbaths and sanitary hand washes can help prevent the spread of the virus. 

“Some horses can shed EHV-1 and not show clinical symptoms. It is important to treat every horse the same way during an outbreak, whether or not it shows clinical symptoms,” Pusterla said.

Horses can be tested to determine if they are shedding the virus 10 to 14 days apart. A horse may show a negative result from the test on the first sample, but the second may come back positive depending upon the incubation period.

“Contracting this virus is not a matter of distance, it is a measure of biosecurity,” Pusterla said. “It is a matter of simple measures that will prevent the spread of the pathogen. This pathogen can be transmitted via a droplet like nose-to-nose contact, through hands, shared equipment or a shared water bucket. Even if two horses are separated, there may be a common vehicle between the two.”

MacKay said for horses that are boarded, low risk and high-risk horses should be separated. Low risk horses are those that never leave the stable, and high risk are those that go to shows and clinics.

When going to a show or clinic, contact should be minimized between horses. The horse should be monitored for appetite and attitude, and its rectal temperature should be taken twice a day.

Sharing equipment should be minimized, and equipment should be cleaned and disinfected between horses. If someone is riding multiple horses, they should wash their hands between horses, he encouraged. 

MacKay said his recommendations for horses returning from events where there was a confirmed case of EHV-1 include confinement and testing at intervals. 

Test nasal secretions for viral DNA, and take a rectal temperature twice a day. The veterinarian should be contacted if the temperature exceeds 101 degrees. The horse should also be isolated from other horses for 21 to 28 days because it can shed the virus during that time.

EHV-1 Vaccination

A vaccine for Equine Herpesvirus-1 (EHV-1) is available. The vaccine will aid in the prevention of the respiratory virus, and may decrease the incidence of abortion, Nicola Pusterla said. 

“The vaccine may also decrease the severity of the clinical signs, but a horse may still contract the infection,” he explained. 

The clinical signs may be milder, there may be less viral shedding and less environmental contamination reducing the amount of susceptible horses. The vaccine may also reduce the amount of the virus present in blood.

Specific times are recommended to give the vaccine. In pregnant mares that have had their primary vaccinations and boosters, the vaccine should be administered at five, seven and nine months of pregnancy. 

“It seems to reduce the incidence of EHV-1 abortions,” Pusterla said.

Gayle Smith is a correspondent for the Wyoming Livestock Roundup. Send comments on this article to

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