More than a month since the first cases of EHV-1 and equine herpesvirus myeloencephalopathy were reported at Desert International Horse Park in Thermal, California, triggering an outbreak that sickened dozens of horses there and spread through the state, the state continues to battle the disease, but cases definitively linked to the horse park are dwindling.

Only one new EHV-1 case has been reported at DIHP in the past week, bringing total cases there to three EHM and 32 EHV-1 with fever only since Feb. 11, according to statistics kept by the California Department of Food and Agriculture.

As of Sunday, horse park officials reported that seven EHV-1 horses remain in isolation and five trainers’ groups of horses are still in quarantine at DIHP.

In Orange County, only one new case has been confirmed in the past week at an 86-horse barn that had its first confirmed EHV-1 and EHM cases on Feb. 24, related to horses who had returned from showing at DIHP. That barn now has had two EHM and 21 fever-only EHV-1 cases, in total.

California counties with active EHV-1 and EHM cases as of March 13. Courtesy Of California Department Of Food And Agriculture

A barn in Placer County that is home to an asymptomatic horse which tested positive for EHV-1 after showing at DIHP also has been released from quarantine. That horse was tested while attending a show at Murieta Equestrian Center, in Rancho Murieta, under return-to-competition protocols instituted last month by the U.S. Equestrian Federation because it competed at Murieta a week after showing at DIHP.

New Cases Reported

EHV-1 continues to spread at other barns in the state, however.

In the past week, barns in Santa Clara and San Mateo counties have been placed under quarantine after horses at each were euthanized with severe neurological symptoms related to EHM, according to CDFA. Two horses, ages 21 and 18, have been euthanized with EHM at the Santa Clara barn, among 168 horses exposed. In San Mateo, one 15-year-old horse has been euthanized and two cases of EHV-1 confirmed among 52 horses exposed.

Two barns in Orange County also continue to report new cases.

At a 350-horse facility where the first cases of EHM were confirmed by CDFA on March 3 in a pair of 22-year-old horses (from different barns on the site) who were euthanized, four additional EHV-1 cases have been confirmed in horses showing fever only or mild clinical signs. The total cases confirmed at that facility is now two EHM and 14 EHV-1.

At another Orange County barn where two cases of EHV-1 and one case of EHM first were confirmed on March 4, multiple new cases have been reported in the past week. As of March 13, the total confirmed cases there is two EHM and 14 EHV-1 with fever or mild clinical signs.

New Entry Requirements For April Hunter Show

Blenheim EquiSports, which canceled the first week of its Spring Classic Series in San Juan Capistrano in response to the outbreak, released updated entry requirements for its Spring Classic II show, scheduled to start April 6.

In addition to implementing the testing required under USEF’s return to competition mandate, all horses entering the show (including those not exposed to horses from DIHP or Murieta) will have to provide a negative PCR test within 28 days, temperature readings for three days leading up to show entry, and other measures.

EHV-1 Resources:
• Read all of the Chronicle’s coverage of the 2022 California EHV-1 outbreak.
• Learn more about biosecurity best practices recommended by USEF.
• See the most recent case numbers and quarantine alerts from CDFA.
• Learn more about EHM and EHV-1 from CDFA’s EHM factsheet.