Written by: Ali Miletic

Ask the Vet: Equine Herpes

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Equine Herpes Myeloencephalopathy (EHM) is an alternative name for a neurologic disease associated with Equine Herpes Virus (EHV). There are multiple herpes viruses with which horses can be infected; however, EHV-1 and EHV-4 are the most common. Both viruses are prevalent in horses worldwide, and both cause initial upper respiratory infections. It is easily contagious horse-to-horse and spreads via aerosol (sneezing, nose-to-nose contact). Clinical symptoms including fever, depression, nasal discharge, and inappetence are indicators of EHV infection.

Strains and Signs of EHM

Where the two strains differ, however, is the manifestations of the virus beyond the initial infection. EHV-4 is more common in weanlings and yearlings (think breeding farms or racetracks) but can affect a horse of any age. Symptoms rarely go beyond a snotty nose and mild-to-moderate fever. This is, of course, dependent on immune and/or pregnancy status of the horse and viral strain of EHV-4.

Certain strains of EHV-1 result in neurologic disease (EHM) after the initial infection and upper respiratory clinical signs appear. The horse will experience a range of clinical signs from mild incoordination and paresis (weakness) of the hind limbs, loss of bladder and tail control, to total paralysis and recumbancy. If the horse is immunocompromised, infection with these strains can result in death. Prognosis post-infection depends on severity of clinical signs, and if the horse is recumbent, the length of time it is unable to stand.

EHV-1 strains also cause abortion in pregnant mares and foal death. Pregnant mares infected with EHV-1 often don’t abort for 2-12 weeks after infection and usually between seven and nine months of pregnancy. Mares that contract EHV-1 in late pregnancy may not abort, but will deliver a foal that has also contracted the virus. These foals are very susceptible to bacterial infections and usually die within hours or days. Thankfully, there is no residual damage to the mare’s reproductive tract and her next pregnancies are unaffected, barring another infection.

Treatment Options

Diagnosis of EHV is difficult based on clinical signs alone. The initial respiratory infection mirrors many other viral infections including equine influenza. There are laboratory tests to confirm EHV, such as virus isolation from nasal swabs or cerebral spinal fluid during initial infection. If EHV-1 abortion is suspected, samples of the aborted fetus can be sent to the lab for virus isolation as well.

There is no specific treatment for EHM other than general nursing and supportive care. Anti-pyretics to control fever are essential, as well as rest, and antibiotics if there is a concurrent bacterial infection. Horses with the neurologic strain of EHV-1 require more intensive care to prevent bladder rupture and rectal tear, as they are unable to urinate, defecate, or stand. Pneumonia is also an increased risk due to a compromised respiratory tract from the initial herpes virus infection.

The good news is that neurologic signs rarely progress after the first 24 hours of onset. Horses that are severely affected are frequently hospitalized to ensure round-the-clock care and the best chance of survival. Horses mildly affected by EHM usually make a full recovery with supportive care. Those that show more severe clinical signs may have residual neurologic signs if they survive the initial infection.

Unfortunately, vaccination does not protect from the neurologic form of the disease, although the horse will be better able to protect itself from the virus if he is up-to-date on other vaccinations. This is simply because his immune system will be able to fend off any other viruses to which the horse may be exposed.

Prevention of EHM lies within biosecurity measures, especially if the horse is travelling to and from shows, or other horses in the barn are. Limit nose-to-nose contact, sharing of water buckets, tack or brushes with horses from a different farm while at a show. If a new horse is coming into the barn, a minimum one week to 10-day quarantine is the best way to limit spread of any disease he or she may bring in.