Equine herpesvirus (EHV) is a family of DNA viruses belonging to the Herpesviridae, with five characterized subtypes in horses. EHV-4 causes primarily respiratory illness — rhinopneumonitis — most severe in young horses and resembling moderate influenza. EHV-1 causes respiratory disease but is distinguished by its capacity to produce two additional and more serious syndromes: abortion in pregnant mares and equine herpesvirus myeloencephalopathy (EHM), a neurological form that can cause ataxia, paralysis, and death.
Both EHV-1 and EHV-4 establish lifelong latency in the trigeminal ganglia and lymphocytes after primary infection. A horse that has recovered from EHV illness carries latent virus indefinitely and may reactivate shedding under stress — weaning, competition, transport, or concurrent illness. This means herd immunity is never complete and outbreak control depends on biosecurity and stress management rather than elimination of the virus from exposed populations.
EHM outbreaks at competition venues have become a recognized event-management concern. Affected horses display progressive hindlimb ataxia, bladder dysfunction, and in severe cases quadriplegia within days of fever onset. Case fatality in neurological EHV-1 infections ranges from 20 to 40 percent in outbreak settings, making prompt quarantine and reporting to regulatory authorities essential. The index of suspicion should rise for any horse that develops fever and neurological signs within 14 days of attending a competition or event.
Vaccination for EHV-1 and EHV-4 is available and recommended in the core vaccination schedule for horses with any exposure to other horses. Current vaccines reduce respiratory shedding and provide partial protection against abortion but do not reliably prevent EHM. Biosecurity during transport — checking temperatures during travel, limiting nose-to-nose contact — is the primary mitigation for horses moving between facilities. See also the core vaccination schedule for EHV-1 and EHV-4 timing and the limitation of current vaccines against neurological disease, and the Equus genus entry for the host-specificity context that gives all five EHV subtypes their name.
Further Reading: The five EHV subtypes, their clinical syndromes, and the EHM neurological form are covered on Wikipedia’s Equine herpesvirus article. Utah State University Extension provides a practical guide to EHV biosecurity and outbreak management at USU Extension: Equine Herpesvirus.