Update: Three additional cases have been confirmed at Desert International Horse Park in Thermal, California, and two more at an Orange County barn which stables horses who previously competed at the horse park, the California Department of Food and Agriculture reported late Wednesday, March 3. This brings the total number of cases for DIHP to 29 (three with EHM and 26 with fevers only) and 17 at the Orange County barn (one with EHM and 16 with fever only).

Correction: The asymptomatic horse that tested positive for EHV-1 at Murieta Equestrian Center after traveling from DIHP within the 14-day window also attended Northern Classics Winter I, held Feb. 16-20, during that time.

The U.S. Equestrian Federation on Monday released new protocols for riders wishing to return their horses to competition from show venues in California where equine herpesvirus cases have been confirmed. Meanwhile, more cases of the highly contagious disease were recorded as an outbreak first reported at Desert International Horse Park has spread to eight counties around the state.

Any horse that has competed at either DIHP in Thermal, where the first EHV-1 case was confirmed Feb. 11 and where the majority of cases have been reported since, or Murieta Equestrian Center in Rancho Murieta, where a subsequent case was confirmed Feb. 25 in a horse who had been at DIHP during the outbreak, will have to meet PCR testing requirements before being allowed to participate in USEF-sanctioned shows elsewhere.

Asymptomatic Case At Rancho Murieta

The case at Rancho Murieta was discovered in a horse who was attending the Northern Winter Classic II, held Feb. 23-27, after previously attending Desert Circuit IV, held Feb. 8-13. The horse was not showing any signs of illness but had traveled from DIHP within 14 days.

An asymptomatic EHV-1 horse may or may not be able to transmit the virus to others, California Department of Food and Agriculture representative Dr. Katie Hatch said.

“Yes, horses can be asymptomatic carriers of EHV-1,” she wrote in an email. “Sometimes horses can be shedding dead virus (which is not infectious to other horses), but sometimes it is live, infectious virus that is being shed, and other horses can then become infected and show clinical signs.”

When USEF established a mandate Feb. 24 that all horses that had been at DIHP within the past 14 days be isolated and receive two negative PCR tests seven days apart before returning to competition, the asymptomatic horse was isolated and tested at the Sacramento County show grounds, officials said.

“It was a test given to start the process once the mandate was in place,” USEF spokesperson Carly Weilminster said. “The horse was asymptomatic and is isolating at home.”

The horse and its exposed cohort are being quarantined at their home barn in Placer County.

While the USEF mandate did not go into place until Feb. 24, show management at Rancho Murieta has been criticized for allowing a horse or horses that had recently competed in Thermal to enter the Northern Winter Classics Series, which began Feb. 16.

“As USEF has reported there was an asymptomatic case that tested positive on Friday, February 25, 2022,” said Tim Postel, organizer of the Northern Winter Classic show series. “The entire two weeks of the Northern Winter Classic Shows we did not have an elevated temperature due to EHV, and this horse was only tested due to the new mandate required by USEF on February 24.  Affected exhibitors were informed immediately of the positive case, and the announcement was made the next day to cancel Weeks III and Weeks IV out of precaution and safety of the horses. CDFA will be updating should any further cases arise.”

For its part, while USEF has said it is investigating reports of horses that were shipped from DIHP to other shows against protocols, Weilminster would not comment on how long those investigations will take or what penalties might be levied.

“Penalties for substantiated violations are within the Federation’s discretion,” she said. “USEF cannot comment any further on pending disciplinary matters.”

CFDA does not legally require horse owners and caretakers to follow quarantine rules unless a barn or cohort of horses has been officially quarantined.

“As far as compliance goes, biosecurity practices are only legally required when the horse/exposure cohort is under official CDFA quarantine,” Hatch said. “We certainly can strongly recommend that everyone complies with the biosecurity best practices, but we cannot legally require it unless under quarantine.”

No other cases have been reported from the showgrounds.

The California counties above—excluding Alameda County, where an unrelated outbreak happened at a large boarding facility—all have active cases of EHV-1 that trace back to horse shows held since Feb. 11, when the first case was reported at Desert International Horse Park. Courtesy Of California Department Of Food And Agriculture

New Cases At DIHP

At DIHP, which canceled the remainder of its winter show season but continues to stable horses affected by the outbreak, two additional horses were confirmed positive for EHV-1, with fevers only, on the show grounds.

Two more who had left already, one of which returned to its home barn in Sonoma County and another which had gone to a San Diego veterinary facility for treatment of an unrelated condition, also tested positive. All have been isolated in their respective locations.

In what have become near-daily online updates from DIHP CEO Steve Hankin, he reported Tuesday that 17 test samples were collected Monday.

“Only two of the 17 tests are initial tests for new horses exhibiting a fever,” he said in the daily statement. “Three of the tests are retests for previously reported positive EHV-1 horses. These tests are done to understand the trend of the virus in these horses and, if negative, to begin the seven-day cycle for being cleared from isolation.

“Twelve of these tests are of horses that have had previous negative results on their first-time test. These tests are done to ensure the horses remain negative for EHV-1,” he continued.

To date, the California Department of Food and Agriculture lists 26 cases—23 with fever only and three neurologic cases of equine herpesvirus myleoencephalopathy—associated with the Thermal outbreak. The horse in Rancho Murieta is included in this number.

However, that total does not include a group of horses at a barn in Orange County, where on Feb. 24 one horse who previously had been at DIHP tested positive for EHV-1 and a mare exposed to that horse tested positive for EHM and had to be euthanized.

As of Tuesday, CDFA reported that eight more horses in that barn had fevers and were confirmed positive with EHV-1. According to the Equine Disease Communication Center, 86 horses have been exposed there.

No New LA Cases

In a bit of good news, CDFA reported Monday that no new cases or clinical signs have been reported in Los Angeles County, where one horse who attended the LA February Show (Feb. 18-20) subsequently developed EHM and was euthanized.

Although show organizers determined several trainers shipped horses from DIHP to LAEC after the outbreak began, thus far CDFA has reported no known relationship between the Los Angeles case and the Thermal ones, and USEF is not requiring the same return-to-competition measures for horses who showed at LAEC as they are for horses that were at the other two venues.

“The horse at LAEC exhibited no symptoms while on competition grounds and never confirmed positive while on the grounds,” Weilminster said. “Additionally, at this time, there has been no identifiable link between this horse and any cases confirmed positive at DIHP.”

Looking Ahead

With show managers voluntarily canceling competitions, the next USEF-sanctioned hunter/jumper show scheduled to take place in California is Blenheim Spring Classic I, beginning March 30 at Rancho Mission Viejo Riding Park in San Juan Capistrano.

Show managers on Monday announced entry requirements for that show that mesh with the USEF’s new return to competition protocols but require PCR testing (either two, seven days apart or one test after 21 days of isolation) for any horse that has been to a show in California or been exposed to a horse that has showed in California since Jan. 28, or comes from a barn with a confirmed EHV-1 case.

Outside of the hunter/jumper world, organizers of the Twin Rivers Winter Horse Trials, scheduled to take place March 3-6 in Paso Robles, California, announced Monday that the event is canceled.

“Unfortunately, after much deliberation the Twin Rivers Winter Horse Trials is canceled due to the EHV-1 Outbreak in California,” organizers announced on Facebook. “We felt that as a facility we had every precaution in place to make a safe environment for competitors to be at the Ranch. However, it has become apparent that the willingness to be transparent about EHV exposure and vaccination status of horses is troublesome. As show management to ensure a safe environment for horse welfare we felt it was in everyone’s best interest for safety to cancel.”