The California Department of Food and Agriculture (CDFA) reported that two additional horses in residing at the Orange County facility quarantined for equine herpesvirus-1 (EHV-1) tested positive for the disease last week.

“A mare displayed a fever the morning of April 17 and a horse which was febrile for one day on Sunday, April 8,” have tested positive, the CDFA said on its website. “Both horses have been placed in separate quarantine isolation stabling on the property. All exposed horses under quarantine continue to be monitored and have temperatures taken twice daily.”

The index case—a 17-year-old Thoroughbred gelding that was diagnosed with equine herpesvirus myeloencephalopathy (EHM, the neurologic form of EHV-1) after developing neurologic signs of disease—remains quarantined and isolated offsite and continues to show clinical improvements, the CDFA said.

As of April 22, no additional EHV-1 cases have been confirmed, the CDFA said.

Herpesvirus is highly contagious among horses and can cause a variety of ailments in equids, including rhinopneumonitis (a respiratory disease usually found in young horses), abortion in broodmares, and EHM. In many horses, the only sign of EHV-1 infection is fever, which can go undetected.

In addition to fever, other common signs of EHV-1 infection in young horses include cough, decreased appetite, depression, and a nasal discharge. Pregnant mares typically show no signs of infection before they abort, and abortions usually occur late in gestation (around eight months) but can be earlier. Abortions can occur anywhere from two weeks to several months following infection with EHV-1.

Horses with the neurologic form usually have a fever at the onset of the disease and might show signs of a respiratory infection. A few days later, neurologic signs such as ataxia (incoordination), weakness or paralysis of the fore- and hind limbs, urine retention and dribbling, loss of tail tone, and recumbency (inability to rise) develop.