Two outbreaks of EHV-1 in California

Horse owners and veterinary health officials are grappling with two separate and significant outbreaks of equine herpes virus type 1 (EHV-1) in California.

Horse owners and veterinary health officials are grappling with two separate and significant outbreaks of equine herpesvirus type 1 (EHV-1) in California.

EHV-1 most often causes mild-to-moderate respiratory illness (rhinopneumonitis), but the infection occasionally leads to the life-threatening neurologic disease equine herpesvirus myeloencephalopathy (EHM). The mechanisms through which EHV-1 produces neurologic disease are not yet understood.

microscopic view of equine herpes virus
Most cases of equine herpesvirus-1 infection cause only mild to moderate respiratory illness but occasionally some lead to life-threatening neurologic disease.

Learn more: Read “When EHV-1 Turns Deadly.” 

San Mateo County: A total of 26 EHV cases have been confirmed and 300 horses may have been exposed to the virus at a boarding facility in San Mateo County, according to the Equine Disease Communication Center (EDCC). The first horse affected was a 22-year-old Paint gelding that developed a high fever and ataxia in late January. He was euthanatized. Of the 22 horses who have since developed signs and been diagnosed, all are still alive at this time. According to the California Department of Food and Agriculture (CDFA), three of the horses developed neurological signs associated with EHV-1 and 23 have fever only. The facility is under quarantine and will released “when all positive horses have had two consecutive negative tests seven days apart,” according to CDFA.

Riverside County: A second outbreak in California is associated with a show facility in Riverside County. Nineteen EHV cases were confirmed and five more suspected as of February 24, according to the EDCC. The first case at the facility was 12-year-old gelding that developed neurologic signs on February 10. Two other horses in the same barn had a fever at that time and 45 potentially exposed horses were quarantined. Since then, cases have been identified in horses kept at different barns at the facility as well as horses who had returned to their home stables. Of the 19 confirmed cases, three have involved neurological signs; one of those horses was euthanatized, according to the CDFA. Sixteen cases are fever-only cases. The CDFA reports that positive horses have been placed in isolation, and all horses in the exposure cohorts are isolated and under quarantine. The facility management has put new biosecurity protocols in place to help control the outbreak.




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