Horse Equine Herpesvirus: Neurological Risk and Biosecurity

Equine herpesvirus poses serious welfare and economic risks to horses, with neurological outbreaks causing severe suffering and difficult decisions about treatment and euthanasia.

Key Facts

Equine herpesvirus type 1 (EHV-1) causes respiratory disease, abortion, and neurological disease (equine herpesviral myeloencephalopathy)
EHV-4 primarily causes respiratory disease and is less commonly associated with abortion or neurological signs
Neurological EHM (equine herpesviral myeloencephalopathy) causes ataxia, weakness, and bladder dysfunction, sometimes progressing to recumbency
EHV-1 is endemic in horse populations worldwide; most horses carry latent infections that can reactivate under stress
Outbreaks are most common at events where horses from different premises mix, facilitating transmission
No vaccine fully prevents EHM, though modified-live vaccines may offer partial protection against neurological disease
Recumbent horses with EHM face severe welfare compromise; euthanasia decisions are among the most difficult in equine medicine
Strict biosecurity — isolation, temperature monitoring, hand hygiene — is essential during outbreaks

Welfare Considerations

EHV-1 neurological outbreaks present among the most challenging welfare situations in equine medicine. Horses with EHM experience progressive neurological deterioration that may leave them unable to stand, urinate, or eat normally. The suffering is acute, and the prognosis for severely affected horses is poor. Treatment with antiviral drugs (valacyclovir, acyclovir) and supportive care can support recovery in less severe cases, but there is a point at which continuing treatment prolongs suffering rather than offering a reasonable chance of recovery. Veterinarians and owners navigating EHM outbreaks need clear, compassionate guidance about when to pursue aggressive treatment and when euthanasia is the welfare-centered choice. Biosecurity culture in the equine community is an ethical obligation to prevent outbreak spread.

What You Can Do

Follow event biosecurity requirements: temperature checks, documentation, and isolation of any horse showing symptoms
Vaccinate for EHV-1/4 according to your veterinarian recommendations, particularly horses that travel to events
Report suspected EHV-1 cases to your veterinarian immediately and implement strict isolation protocols
Monitor horses returning from events for 21 days for respiratory signs, fever, or neurological changes
Work with your veterinarian to develop a farm biosecurity plan before attending high-risk events
Support research into more effective EHV-1 vaccines that prevent neurological disease
Learn to recognize early neurological signs — mild hindquarter weakness, bladder distension — to enable early intervention