Equine Herpesvirus Infection

Equine herpesvirus (EHV) infections have gained much attention for their role in outbreaks of neurologic disease or abortions. EHV can be divided into several different types based upon the disease that each causes.

Equine herpesvirus-1 (EHV-1) is a well-known cause of late-term abortion, neurologic disease, and respiratory tract infections. EHV-1 is more likely to cause abortion or neurologic disease than respiratory signs. The biggest difference between EHV-1 and the other EHV is that some strains of EHV-1 can multiply rapidly and invade lymphocytes, a type of white blood cell. Lymphocytes transport EHV-1 throughout the body where it induces vasculitis (inflammation of the blood vessels). It is the inflammation, damage, and resultant decrease in blood flow in the uterus that causes abortion because the fetus is unable to get enough nutrients to survive. Foals born with EHV-1 infections are very ill and often die despite aggressive therapy. Vasculitis in the central nervous system leads to ataxia (unsteady gait) that is usually more severe in the hind limbs. Other neurologic signs include dribbling urine and bladder, tail, and penile paralysis. There are no equine vaccines labeled to prevent neurologic disease due to EHV-1 infections, but some EHV vaccines may provide some protection. Vaccination against EHV-1 will reduce the incidence of EHV-1 abortions.

Equine herpesvirus-3 (EHV-3) is coital exanthema, which is a true venereal disease of horses. Lesions of EHV-3 initially appear as blisters, but after the blisters rupture they produce painful ulcerations and scabs. Once the lesions heal, there may be residual areas of depigmentation (vitiligo). The biggest problem with EHV-3 is that affected horses are too sore to breed until the lesions have healed.

Equine herpesvirus-4 (EHV-4) is a major cause of equine viral respiratory infections, along with influenza virus. Rarely, abortion can be due to EHV-4.  Rhinopneumonitis (inflammation of the nose, throat, and lungs) is a commonly used term for EHV-4 respiratory infections. This is confusing terminology because respiratory tract infections are often caused by rhinoviruses, which are similar to common cold viruses in humans. Horses affected with EHV-4 have fever, cough, and nasal discharge. There are many equine vaccines against EHV-4 available.

The role of equine herpesvirus-5 (EHV-5) is not completely understood, but it is becoming more recognized as a potential cause of disease. Many horses have antibodies against EHV-5, indicating that they have been exposed to the virus. Equine multinodular pulmonary fibrosis is a newly reported condition of progressive respiratory failure that has been associated with EHV-5. Clinical signs of equine multinodular pulmonary fibrosis include fever, inappetence, weight loss, coughing, rapid breathing, and respiratory distress (similar to severe heaves). Some affected horses have responded to treatment with antiviral drugs and anti-inflammatory therapy, but the prognosis is poor.