The mare had no recent travel or show history and was vaccinated for EHV-1. She was euthanized on June 2 due to severity of clinical signs. 300 horses at the mare's home premises in San Mateo County have been quarantined, and CDFA will continue to closely monitor the situation

A 7-year-old Holsteiner mare in San Mateo County displaying fever followed by mild neurological signs was confirmed positive for Equine Herpesvirus Myeloencephalopathy (EHM) secondary to EHV-1 (non-neuropathogenic strain) on June 6th.

She was referred to a veterinary hospital at onset of neurological signs, and isolated and quarantined. She attended an equine event in Sacramento County the weekend prior, asymptomatic throughout the show, and departing on Saturday May 28th with 5 other horses from her home barn.

Event management has notified all show attendees to observe best biosecurity practices and isolate any horses returning from the event for 14 days following departure from the show grounds. CDFA was onsite at the index mare’s home premises, and 9 potentially exposed horses were also quarantined and enhanced biosecurity measures implemented.

There is no known epidemiological link between this case and the previously confirmed active EHM incident in San Mateo County. CDFA will actively monitor the situation.

No additional cases have been confirmed. CDFA continues to monitor the situation.

One (1) additional horse on the premises, a 15-year-old Irish Sport Horse mare displaying neurological signs was confirmed positive for Equine Herpesvirus Myeloencephalopathy (EHM) secondary to equine herpesvirus-1 (EHV-1 non-neuropathogenic strain) on June 6th. The mare was euthanized on June 3rd due to severity of clinical signs.

Promoting Healthy Equine Venues

Equine Herpes Virus (EHV-1) infection in horses can cause respiratory disease, abortion in mares, neonatal foal death and neurological disease. The neurological form of the disease is known as Equine Herpes Myeloencephalopathy (EHM) and may be caused by damage to blood vessels of the brain and spinal cord associated with EHV-1 infection. EHM is most often due to the neuropathogenic strain of EHV-1, but may occasionally be caused by the non-neuropathogenic strain of the virus.

EHV-1 is easily spread and typically has an incubation period between 2-10 days. Respiratory shedding of the virus generally occurs for 7-10 days, but may persist longer in infected horses. For this reason, a twenty-one day isolation period of confirmed positive EHM cases is recommended along with stringent biosecurity protocols.

Similar to herpes viruses in other species, the latent form of EHV-1 can reactivate at a later date, but generally with a low viral load posing a low risk of infecting other horses. Humans are not at risk of contracting the virus, however humans can act as an indirect mode of transmission.

Recommendations for Participants at Equine Events

CDFA Animal Health Officials are continually monitoring for EHV-1 cases. CDFA reminds horse owners traveling with horses to participate in an equine event, that there is always disease risk when horses of unknown health status are commingled for a show or competition. CDFA strongly recommends that horse owners practice proper biosecurity when attending an equine event.

Compliance with basic biosecurity practices is an important factor in reducing risk of exposure to all contagious equine diseases. Basic biosecurity measures to follow to decrease potential disease spread at equine events include:

• Limit horse-to-horse contact
• Limit horse-to-human-to-horse contact
• Avoid use of communal water sources
• Avoid sharing of equipment unless thoroughly cleaned and disinfected between uses
• Monitor your horse for clinical signs of disease and report any temperature over 102°F to a veterinarian

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Press release by CDFA Animal Health and Food Safety Services,