On June 3, a 23-year-old pony mare in Sonoma County, California, was euthanized after testing positive for equine herpesvirus myeloencephalopathy (EHM) linked to equine herpesvirus-1 (EHV-1). The mare exhibited ataxia on June 1, prompting quarantine measures for 36 potentially exposed horses on the premises.

Equine herpesvirus is highly contagious and can lead to various health issues in horses, including respiratory diseases, abortions in pregnant mares, and EHM. Initial signs of EHV-1 infection often include fever, which may go unnoticed, along with respiratory symptoms in young horses. Pregnant mares may not show symptoms before experiencing late-term abortions.

EHM typically begins with fever and respiratory signs, followed by neurological symptoms such as incoordination and paralysis. The virus spreads easily through close contact and contaminated equipment, underscoring the importance of routine biosecurity measures to prevent outbreaks. While current vaccines can reduce viral shedding, they do not protect against the neurological form of the disease, making prevention critical.