Editor’s note: This story has been updated to reflect an additional febrile horse identified as part of the euthanized mare’s group at Sonoma Horse Park.

On July 25, the California Department of Food and Agriculture announced that a 6-year-old warmblood mare who was stabled July 19-24 at the Sonoma Horse Park in Petaluma, California, has tested positive for equine herpes myeloencephalopathy, a form of equine herpesvirus, and has been euthanized. Three other febrile horses have been identified; two are awaiting test results, the third tested positive for EHV-1.

The sick mare was on the Sonoma Horse Park grounds during two horse shows, the Adequan/USEF Junior Hunter National Championship—West (July 20-21) and the SHP MGT July Classic (July 22-25). She did not compete in either.

Nineteen exposed horses from the index mare’s barn were removed from the horse park to be isolated and quarantined at their home premises in Sacramento County, California, CDFA said in a press release. Those horses, plus an additional 18 at the home barn are being monitored with twice daily temperature checks and “enhanced biosecurity measures,” the release said.

“Six (6) horses from the home premises displaying fever and/or limb edema have also tested positive for EHV-1,”  according to the release. “A 15-year-old Warmblood gelding at the home premises that was febrile and tested positive for EHV-1 began displaying neurological signs, ataxia, urine dribbling, and cranial nerve deficits and was moved to isolation under quarantine at a veterinary hospital for care. CDFA continues to actively monitor the situation.”

Sarah Appel, manager of the Sonoma Horse Park, sent a letter July 25 to competitors who had horses on grounds during the index mare’s stay at the horse park, stating that a second horse presented with a fever on the grounds and was sent to the University of California, Davis School of Veterinary Medicine. Results are expected by the end of day on July 26.

On Monday, Appel sent out an update saying an additional horse from the index mare’s barn also had a “slight fever.”

“There was a [second] horse from the Sacramento group of horses that left the property that had a slight fever [and] after being reviewed by the state veterinarian the recommendation was to send that horse home to quarantine and not send it to Davis,” she wrote. “It has been tested for EHV-1 and we will also share those results, in the meantime that horse has not had a fever and has shown no signs of symptoms.”

In her July 25 letter, Appel advised Sonoma Horse Park users that the state veterinarian has said horses can enter and leave the park, but any who leave should quarantine for at least seven days, with 14 days being preferable.

“We are not aware of any additional horses with symptoms at this time,” she wrote. “We will notify you immediately if we do hear of additional horses with symptoms.”