Equine herpesvirus is a common virus that occurs in horse populations worldwide. The two most common types are EHV-1, which causes respiratory disease in young horses, abortion in pregnant mares and paralysis in horses of all ages and types, and EHV-4, which usually only causes low-grade respiratory disease but can occasionally cause abortion. Following first infection the majority of horses carry the virus as a latent (silent) infection that can reactivate at intervals throughout life. EHV-3 is a venereal disease that causes pox-like lesions on the penis of stallions and the vulva of mares (Equine Coital Exanthema) and EHV-5 is a virus that is currently associated with unusual sporadic cases of debilitating lung scarring (Equine Multinodular Pulmonary Fibrosis) in adult horses.
EHV abortion can occur from two weeks to several months following infection with the virus, reflecting either recent infection or recrudescence (re-activation) of latent infection in a carrier horse. Abortion 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 clinical signs of infection. It is the continual cycling of EHV respiratory disease in young horses and the periodic reactivation of latent EHV in older horses that maintains the risk of EHV abortion in pregnant mares and EHV neurological disease in horses of all types and ages.
Although EHV-1 may cause outbreaks of abortion, particularly in non-vaccinated mares, EHV-4 has only been associated with single incidents and is not considered a risk for contagious abortions.
Occasionally, EHV-1 can cause paralysis, which ranges in severity from a mild incoordination of the hindlimbs to quadriplegia (total paralysis where the horse is unable to stand). The most important risk factors for this form of disease include animals greater than 5 years of age, season (autumn, winter and spring when animals are more likely to be stabled or UV light levels are low) and perhaps the strain of virus involved. However, although so-called ‘neurological’ strains have been identified, paralytic signs are seen with both these and ‘non-neurological’ strains, so this differentiation cannot be relied upon. Therefore, characterising EHV-1 on the basis of the ‘neurological’ strain marker is not considered useful and control measures should be adopted consistently irrespective of the strain of EHV-1 involved. Clinically, the onset of paralysis may be sudden, with no prior clinical signs of respiratory disease and usually occurs in the second week following infection.