According to veterinary services at UC Davis many managers of equine facilities and events have imposed EHV-1 vaccination requirements for incoming and resident horses in the hope that EHV-1 infection, particularly the neurological form, can be prevented.
The efficacy of this approach remains to be proven. In fact, frequent revaccination of mature horses to prevent the neurological form of EHV-1 is not clearly justified in most circumstances for the following reasons:
• Most mature horses have been infected previously with EHV-1 and are latent carriers of the virus.
• EHM is a relatively rare disease from a population standpoint.
• EHM has been observed in horses vaccinated against EHV-1 regularly at 3- to 4-month intervals.
• Currently available vaccines do not reliably block infection, development of viremia (virus in the bloodstream), or establishment of latency.
• Currently available vaccines make no claim to prevent EHM.
On the other hand, regular vaccination of pregnant mares and other horses on breeding farms to reduce the risk of EHV-1 induced abortion is strongly recommended.
Commercially available vaccines for EHV-1 include two single-component inactivated vaccines (Pneumabort K and Prodigy) marketed for the prevention of abortion in pregnant mares; several multicomponent inactivated vaccines (Prestige, Calvenza, Innovator); and one MLV vaccine (Rhinomune) for the prevention of respiratory disease induced by EHV-1 and EHV-4.
Each of these vaccines induce some, but not all, of the desired components of the immune response in the horse. Therefore, it is not surprising that NONE induces sterile immunity or complete protection from clinical disease. The best that can be hoped for is a reduction in the severity of clinical signs and in the amount of EHV-1 shed by vaccinated horses that do become infected.
Ultimately, enforcement of strict biosecurity measures and hygiene practices are likely to be more effective than widespread vaccination in most cases.