A total of nine equid herpesviruses have been named, but only five are found in domestic horses. (The other four affect primarily donkeys and other equine species.) The five primary equine herpesviruses can be further divided into two subgroups: EHV-1, -3 and -4 are alphaherpesviruses, which are the fastest to multiply and take over cells in the body. They also cause the more serious diseases. EHV-2 and -5 are gammaherpesviruses, which multiply and spread much more slowly.
EHV-1 is a highly contagious respiratory virus that penetrates the cells lining the horse’s airways, causing the inflammatory airway disease called rhinopneumonitis. Signs include cough, fever, nasal discharge and loss of appetite. Most sick horses recover with no ill effects. However, certain strains of EHV-1 can infect certain white cells and circulate throughout the body. If the virus infects epithelial cells lining the uterus, it can cause pregnant mares to abort, and if it infects the cells lining the central nervous system, the resulting localized inflammation can inhibit circulation to the brain and spinal cord, leading to a potentially fatal neurologic disease called equine herpesvirus myeloencephalopathy (EHM).
EHV-4 is also a respiratory virus that causes rhinopneumonitis. It is closely related to EHV-1 and was once believed to be a subtype of that virus until DNA analysis showed that it could be classified as a separate viral species. However, EHV-4 replicates more slowly than EHV-1 and causes less serious illness. Although it can also travel throughout the body, abortion and neurological disease are much less common after infection with EHV-4.
EHV-3 causes a venereal disease called equine coital exanthema, which passes by skin-to-skin contact, usually between mares and stallions while mating. The disease causes pox-like pustules to appear on the external genitalia of affected horses. The disease is not considered serious and does not affect fertility, but it is recommended that infected horses be rested until the lesions heal.
EHV-2, also called equine cytomegalovirus, is found in the respiratory secretions as well as the tears of nearly all horses. Mares commonly pass it to their foals soon after birth. The role of EHV-2 in disease is not fully understood but it is suspected to contribute to the inflammation of the cornea (keratitis).
EHV-5 is a respiratory virus that has been linked with a disease called equine multinodular pulmonary fibrosis (EMPF), although the role the virus plays, if any, in causing the disease is unclear. EMPF is characterized by formation of fibrous nodules that damage the lung’s alveoli, the air sacs where air exchange takes place. Signs include exercise intolerance, coughing, fever and weight loss. The disease is progressive, and while supportive care can help, most horses with EMPF are eventually euthanatized.