Update: Multi-County EHV-1 Outbreak In California

On Jan. 28, an Equine Herpesvirus Myeloencephalopathy (EHV-1) outbreak caused a 300-horse barn to be quarantined in San Mateo County, Calif. The California Department of Food and Agriculture (CDFA) is actively monitoring the situation as it evolves.

On Feb. 16, one additional horse on the property, a 19-year-old Draft cross gelding, was confirmed to have EHV-1. He has been isolated. To date, two EHM cases and 17 EHV-1 febrile-only cases have been confirmed. The quarantine will be released when all positive EHV horses have had two negative tests 7 days apart.

On Feb. 16, the EHV-1 index case in Alameda County was released from quarantine at her home farm in Sonoma County, Calif., after testing negative for the disease. The 8-year-old Thoroughbred mare had a confirmed EHV-1 diagnosis on Jan. 19, but had been removed from the racetrack where she was stabled in Alameda County and shipped to her home barn in Sonoma County on Jan. 12 after she began to show signs of illness. Fourteen other horses from the mare's training barn and 13 from an adjacent barn were isolated and quarantined, but no others were diagnosed with the disease.

One additional case of EHV-1 was found on Feb. 17 in Alameda County in an adult Warmblood gelding. The horse was afebrile, but was displaying neurologic signs. He was quarantined along with 21 other equids on the property. There is no known epidemiological link between this EHM case and others in the state as no horses have moved on or off the property in the last 14 days.

On Feb. 11, a 12-year-old Zangersheide was confirmed positive for EHM after showing neurologic signs the day before. The gelding was at an event in Riverside County and stalled with 45 other horses. All 45 horses were quarantined. To date, there have been three confirmed EHM cases and three confirmed EHV-1, fever only cases. CDFA is onsite and monitoring the situation.