An EHV-1 outbreak in California worsened over the weekend, with one horse euthanized and eight total cases now confirmed at Desert International Horse Park in Thermal, including three horses with neurological symptoms. Two shows planned for this week, at DIHP and at Los Angeles Equestrian Center, have been canceled in response.

The situation evolved rapidly over the weekend.

On Friday night, Feb. 18, one horse who previously had been confirmed with EHV-1 was euthanized due to the severity of its neurological symptoms. In addition, two more cases were confirmed in horses outside the index barn where the original cases were reported Feb. 11, according to the California Department of Food and Agriculture. The new cases have fevers but are not showing neurological symptoms.

In total, there have been three confirmed cases of equine herpesvirus myeloencephalopathy, or horses with EHV-1 and neurologic symptoms, and five cases with fever only. Horse park CEO Steve Hankin said they currently are awaiting the results of three retests and seven new tests, but that horses in isolation, pre-isolation and quarantine remained stable that day.

“The situation at the horse park is very serious,” Hankin said in a Saturday statement. “And it is constantly—almost hourly—evolving. It is nearly impossible to create an accurate picture of the situation on an on-going basis. Horses that tested negative test positive two days later. Horses that tested positive test negative as they recover. Horses with the non-neurologic strain of EHV-1 develop neurologic symptoms and are then classified as EHM. But they do not carry the neurologic strain of EHV-1. This is a complex virus. We are working closely with the experts, and at their direction, on priorities, actions, and resources.”

DIHP initially announced that it was closing to new arrivals and would hold a smaller, modified show for horses already on grounds this week. However, with the weekend’s developments, the horse park announced Sunday, Feb. 20, that it will not hold shows this week and has implemented new protocols for the week to help avoid further transmission.

Also over the weekend, West Palms Events Management, which was running the LA February Horse Show from Feb. 18-20 at LAEC, first announced it would add an extra week of showing, “LAXtra” from Feb. 24-27, then canceled that plan “[g]iven the EHV-1 situation at Desert Horse Park and our commitment to equine health.”

LAEC organizers also announced they learned that three horses who attended the LA February show had previously been in Thermal and traveled to Los Angeles before completing a recommended seven-day isolation and observation period:

“[T]hree horses attended this show that had been at DIHP, returned to their home barn, and then came to our show after five days of isolation instead of seven days,” officials said in an email. “As soon as we became aware, we notified the trainers, and those horses left the property that same day. Furthermore, both trainers assured us that they were monitoring temperatures and all horses in their care had normal temperatures.”

Officials said they contacted Dr. Katie Hatch, equine health program research scientist for California Department of Food and Agriculture. Hatch characterized the risk of EHV-1 at LAEC as “fairly low” due to the timing of when the three horses in question departed the Thermal venue.

Later Sunday, West Palms issued a second statement saying one horse who had been stabled in Barn 1 during the February show had just reported an elevated temperature. The horse has been isolated and EHV-1 test results are pending.

Both the California Department of Food and Agriculture and the U.S. Equestrian Federation are urging horse owners to take precautions and observe all bio-security guidelines.

In Thermal, Hankin implored those on the grounds to act with utmost care and be thoughtful about leaving the property and potentially exposing other horses.

“As we communicated yesterday, what the state and USEF want us to do is ‘hunker-down,’ ” Hankin wrote Saturday. “We have stopped horses from coming in and where possible turned away horses that were in transit. While everyone is free to leave, it also creates challenges, as we want to ensure the best monitoring of any potential exposed horses that remain asymptomatic. We also want to reiterate the critical importance, and routine best practice, of isolating horses returning from shows from all other horses for at least [seven] days to continue monitoring.

“In the end, you should do what you and your vet think is important to care for your horses,” he continued. “We share a simple goal with everyone in our community: ending this EHV-1 infection with as few horses getting ill as possible.”