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Equine Herpes Virus (EHV-1)

Equine Herpes Virus (EHV-1) 

Equine Herpes Virus (EHV-1) is found worldwide and infection is common. It is believed that almost all horses older than two years of age have been exposed to it. Many horses never develop any clinical symptoms; however, the virus will become latent, creating a lifelong dormant infection in the horse. Latent infections can become re-activated, often as a result of stress, causing disease and/or shedding of the virus long after the initial infection.

Reporting the disease

EHV infection is a provincially notifiable disease; therefore any operators of a laboratory, licensed veterinarians or other persons must report the disease within 24 hours of confirmation to the Saskatchewan Chief Veterinary Officer.

The incubation period

Respiratory disease is the most common clinical presentation of EHV-1 infection. The incubation period is usually two to eight days, although incubation periods as short as 24 hours and as long as 14 days have been reported. Initially, only fever may be observed; in some cases, this may be the only sign of infection. Coughing and nasal discharge follow. Some horses may have less common symptoms such as conjunctivitis, reddened mucous membranes or lower leg edema. In pregnant mares the virus can cross the placenta and cause the foal to be aborted anywhere from two weeks to several months after infection. If a mare is infected during late pregnancy, her foal may be stillborn or born alive, but die within a few days of birth. Foals infected in-utero are usually abnormal from birth with weakness, jaundice, respiratory distress, neurologic signs and death within several days.


Equine Herpes Myeloencephalopathy (EHM) occurs far less frequently than respiratory disease. Usually, it is caused by a particular mutation in the EHV-1 genome which results in the neurotropic strain (nEHV-1). While nEHV-1 is the strain most likely to cause EHM, the non-neurotropic strain of EHV-1 can also cause neurologic disease. Approximately 15 per cent of cases of EHM are caused by non-neurotropic strains, while 85 per cent are caused by neurotropic strains. Although observations over recent years indicate that the incidence of EHM is increasing, it is not a new disease. nEHV-1 has been present in the North American equine population for decades, but the molecular tools needed to differentiate it from other strains have only become available recently. The ability to accurately diagnose nEHV-1 infections is likely at least partly responsible for the apparent increased prevalence of this disease.

EHM occurs when the virus causes injury to the small blood vessels in the brain and spinal cord. Damage to blood vessels is widespread and includes the blood-brain barrier. The resulting damage and the formation of clots obstruct blood flow and/or causes hemorrhages into the nervous tissue. The ensuing loss of blood flow and lack of oxygen to the spinal cord and brain lead to dysfunction of the neural tissues.


Neurologic disease may or may not be preceded by respiratory signs. As with respiratory disease, fever is usually the first clinical symptom observed in cases of EHM. Neurologic signs may include incoordination, weakness, difficulty standing, lethargy, poor tail tone, urine dribbling or inability to urinate with resulting bladder distension. Severely affected horses may become recumbent and unable to rise; "dog-sitting" may be seen. Horses often remain bright and continue to eat and drink despite their neurologic deficits.

Treatment and Prevention of EHV-1 Related Disease

There is no specific treatment for EHV-1 infection. Respiratory disease due to EHV-1 infection is often mild and no specific treatment is required. Anti-pyretic medication is recommended when fever is 40 C or above. Antibiotics should be given if secondary bacterial infection is suspected.

In the case of EHM, treatment is mostly supportive and includes anti-inflammatory medication such as corticosteroids and intravenous (IV) fluid therapy if the horse is not drinking sufficient water. Antiviral medications such as acyclovir and valacyclovir may potentially be beneficial. Horses should be kept in a safe, well-bedded area, especially if they have severe incoordination and have difficulty rising. Other supportive treatment is dependent on the specific symptoms displayed in each case. For example, the use of a sling may help recumbent horses, while horses that have difficulty urinating may need to be catheterized to relieve bladder distention and prevent bladder rupture.

Horses with mild EHM have a generally good prognosis and recover without incident. On the other hand, prognosis is poor for horses that become recumbent for days. It may take several weeks or months for symptoms to resolve completely. Some horses may have neurologic deficits that endure for the rest of their lives.


There are several vaccines licensed in Canada for the respiratory and abortogenic forms of EHV; however, none claim any protection against nEHV-1. The protection provided by these vaccines is incomplete, and thus the usefulness of vaccination is questionable. However, vaccination of pregnant mares to reduce the potential for EHV-related abortion is strongly recommended.

The most effective way to protect any horse is to apply good biosecurity and hygiene practices every day. Isolation of new horses, using separate equipment for sick horses, minimizing contact with outside horses and not sharing equipment between horses and especially between facilities are just a few ways to protect horses from a myriad of infectious diseases, including EHV-1.

If you travel with your horse to attend equine shows and events, keep tabs on its health. Check your horse's temperatures before attending events; if your horse has a fever, keep it at home. Also, monitor your horse's temperature during events; if a horse develops a fever during an event, isolate it from other horses immediately to prevent the further spread of contagious disease. When the event is over and horses return home, keep them isolated from other horses on the farm. If EHV is confirmed in any horse on the farm, a voluntarily quarantine of your premises will help prevent the spread of the virus to other facilities.

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