Equine herpesvirus is a common virus that occurs in horse populations worldwide. The two most common strains are EHV-1, which causes abortion, respiratory disease and paralysis, and EHV-4, which usually causes respiratory disease only but can occasionally cause abortion.
EHV abortion can occur from two weeks to several months following infection with the virus. It usually occurs in late pregnancy (from eight months onwards) but can happen as early as four months. Respiratory disease caused by EHV is most common in weaned foals and yearlings, often in autumn and winter. However, older horses can succumb and are more likely than younger ones to transmit the virus without showing signs of infection.
Although EHV-1 causes outbreaks of abortion, EHV-4 has only been associated with single incidents and is not considered a risk for contagious abortions.