The latest updates on the EHV-1 outbreak in California from the Equine Disease Communication Center

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According to the Equine Disease Communication Center (EDCC), it is currently monitoring the Equine Herpes Virus-1 (EHV-1) outbreaks that have been reported in a number of counties in California.¹ EHV-1 usually causes a mild and self-limiting respiratory disease. However, in rare cases, EHV-1 (and EHV-4) can cause equine herpesvirus myeloencephalopathy (EHM), a neurologic disease that affects the spinal cord and is potentially fatal.²

As of February 25, 2022, the EDCC has confirmed EHV-1 cases in San Mateo, Riverside, Orange, and Los Angeles counties. All information on the current outbreak status has been provided by the EDCC, while the situation in all counties is currently being monitored by the California Department of Food and Agriculture (CDFA).¹

San Mateo County

In total, 26 horses have been confirmed positive in this region. Three EHM cases (EHV-1 positive with neurologic signs) and 23 EHV-1 fever-only cases have been confirmed.

Riverside County

A second outbreak in California is associated with the Desert International Horse Show. To date, 21 horses have been confirmed positive. Three are confirmed EHM cases and 18 are confirmed EHV-1 fever-only cases associated with this incident.

Orange County

Two horses have been confirmed positive in this county. A 16-year-old Warmblood mare in Orange County displaying neurologic signs on February 22, 2022 was confirmed positive for EHM secondary to EHV-1 on February 24, 2022. The horse was euthanized due to the severity of the clinical signs, the EDCC says. In addition, a 10-year-old Warmblood gelding on the same premises displaying fever only with no neurologic signs was confirmed positive for EHV-1 on February 24, 2022. The febrile gelding was part of a cohort of horses that attended the event premises in Riverside County that has an active EHM incident, and the neurological mare was exposed to the cohort of horses returning to the home premises from the event in Riverside County. Eighty-six exposed horses on the home premises have been isolated and quarantined.

Los Angeles County

A 20-year-old Warmblood gelding in Los Angeles County displaying neurological signs on February 22, 2022 was confirmed positive for EHM on February 24, 2022. The horse was also euthanized due to severity of clinical signs, according to the EDCC. This gelding attended a show in Los Angeles County last week and event management has been notified. An investigation is pending to determine if this incident is epidemiologically linked to any other active EHM incidents. The exposed horses on the gelding’s home premises have been isolated and will be quarantined.

The EDCC recommend those in an area of a suspected outbreak, horse owners, and participants of all competitions should practice the following best practices:

• Limit horse-to-horse contact
• Limit horse-to-human contact
• Avoid use of communal water sources
• Monitor your horse for clinical signs of disease, take temperatures daily and report any temperature over 102° F to a veterinarian

References

1. Equine Herpesvirus. Equine Disease Communication Center (EDCC). Accessed February 28, 2022. https://equinediseasecc.org/disease-information/equine-herpesvirus
2. Equine Disease Communication Center providing latest updates on EHV-1. News release. February 28, 2022. Accessed February 28, 2022. https://equinediseasecc.org/disease-information/equine-herpesvirus