Equine herpesvirus (EHV) is common throughout horse populations worldwide and can occur in a number of different forms. The two most common forms are EHV-1 and EHV- 4.
EHV-1 can cause respiratory disease in young horses, abortion in pregnant mares and in some cases neurological disease ranging from mild incoordination to total paralysis.
EHV-4 is typically associated with mild respiratory disease but can also cause occasional cases of abortion.
The virus can be spread via airborne particles, close contact with affected horses or aborted foetuses. Once exposed, up to 80% of horses will develop latent or silent infections of the virus. This means that months or even years later the virus can be reactivated and shed leading to spread of the virus. In a similar manner to cold sores in people, this most commonly occurs at times of stress. These latently-affected carrier horses act as a major reservoir for infection within the population and can transmit infection without displaying any signs of disease.
Herpesvirus in the racing/performance horse population
The disease: Both EHV 1 and 4 can cause respiratory disease and poor performance. Signs in clinically-affected horses can include coughing, nasal discharge and fever and are especially common in younger horses. Some animals, particularly older horses can become sub-clinically affected simply resulting in below-expected performance. Treatment is generally not required and horses will go on to recover uneventfully with rest and time. Occasionally, secondary bacterial infections are identified which may warrant antibiotic treatment.
Diagnosis: Definitive diagnosis can be achieved by a range of tests including nasal swabs and blood-sampling. Horses can be screened for active virus before entering a yard and a period of isolation following movement of horses between premises is advisable as movement and transport can precipitate shedding of virus in latently-affected horses. Due to the nature of these silent carriers, however, it is possible for even horses which have tested negative to become contagious at a later date.
Prevention: A vaccine is available against EHV 1 and 4 which requires a primary course of two vaccinations 4-6 weeks apart followed by 6 monthly boosters. While this vaccine cannot provide complete protection as many horses will already be carriers, it can be useful to prevent respiratory disease and limit the severity of signs and the spread of virus should an outbreak occur.
Herpesvirus in breeding horses
The disease: Abortion due to EHV may occur from 2 weeks up to several months following infection. It can occur following first-time exposure or by reactivation of latent virus in a carrier horse. It is typically seen in mares from eight months of gestation but can occur earlier in the pregnancy. Occasionally, affected mares can foal down with weak or sick foals as a result of the virus. Herpesvirus should be suspected until proved otherwise in every case of abortion. Aborted mares and foetuses are a huge source of infection and “abortion storms” can occur where large numbers of unvaccinated mares are exposed to aborted material. It is therefore vitally important that every aborted mare is immediately isolated and examined by a vet.
Diagnosis and biosecurity: The foetus and placenta should be submitted to the Irish Equine Centre for diagnosis, this service is currently available free of charge. Anything which has been in contact with the aborted mare (bedding, equipment etc) should be thoroughly disinfected or disposed of immediately as it can act as a source of infection. Staff and handlers should maintain strict biosecurity with the use of protective clothing, footbaths and careful disinfection.
Prevention: Affected mares which abort due to EHV usually won’t abort again in subsequent years however they can become a source of infection if they become silent carriers. Managing mares in small, stable groups and avoiding mixing, transportation and stress in late pregnancy will help to reduce the likelihood of virus shedding in these carriers.
Vaccination is highly recommended in pregnant mares. While it cannot provide total protection against abortion in individual animals, it greatly helps to reduce the spread of virus and the number of abortions should an outbreak occur. Broodmares should be vaccinated at 5, 7 and 9 months of pregnancy. Contact between pregnant mares and other horses (youngstock, barren mares, horses in training) should be avoided in order to reduce potential exposure of mares to the virus. It is advisable that all non-pregnant horses resident on a stud farm be vaccinated with a primary course and 6-monthly boosters in order to maintain herd immunity.