Investigations of Equine Herpesvirus 1 (EHV-1)

EHV-1 can be a major threat to the breeding industry, having the ability to cause epidemic and sporadic abortions usually in the last trimester of pregnancy, respiratory disease and myeloencephalitis. EHV-4 (closely related to EHV-1) can cause respiratory illness that can adversely affect performance. In 1997 DuvaxynTM (a whole virus EHV-1 and EHV-4 vaccine), was released in Australia and was adopted on a number of stud farms. However, prior to this study, there have been few investigations into the effectiveness of the vaccination in the stud situation.

An Australian study (J M Whalley, C E Foote, D N Love, J R Gilkerson, Investigations of Equine Herpesvirus 1 Cycle of Infection in Foals, RIRDC Project US 96-A, May 2003) aimed to examine whether routine vaccination has an effect on the circulation of EHV-1 and EHV-4 in vaccinated populations of mares and their young weaned and unweaned foals.

How EHV-1 can be Transmitted

  • The transmission of EHV-1 from the mare to the foal is a key element in the cycle of infection, with lactating mares having the potential to infect their suckling foals within 30 days of age. These foals can then spread EHV-1 to other foals within their group prior to and throughout weaning.
  • As the weaning process involves the separation of the foal from its dam and intensive human contact, it places the foals under stressful conditions, which is a contributing factor in the reactivation of EHV. Previously infected foals when placed under these conditions allows for the virus to be reactivated, therefore providing the opportunity for other foals to be infected as they socialise.
  • Farm management practices, such as routine handling of mares and foals and the mixing of paddock groups, can also encourage the introduction and spread of EHV.

Outcomes of the Study

  • Vaccinated mares were still able to transmit EHV-1 and EHV-4 to their unweaned unvaccinated foals, despite having received three vaccinations during the previous gestation.
  • There was a spread of infection from foals that had been previously infected to other foals during weaning.
  • The vaccine may result in a reduction in the period of excretion of the virus , however it has not been sufficient to prevent EVH-1 or EVH-4 infecting new foals.

This information is extremely valuable for the breeder/stud farm manager when preparing management strategies for the handling of mares and their foals, and ways to optimise vaccination regimens.

The full report on Investigations of Equine Herpesvirus 1 Cycle of Infection in Foals can be obtained from the RIRDC website (