Orange County has experienced three premises with EHV, the third of which shows no epidemiological link to the previous two.

At Orange County premises No. 1, one horse was confirmed with EHM and 18 with fever only.

At Orange County premises No. 2, two horses were confirmed with equine herpesvirus myeloencephalopathy (EHM, the neuropathogenic strain of EHV-1). Six horses there are confirmed with fever or mild signs only.

In Los Angeles County, one horse was confirmed with EHM.

In San Mateo County, two more horses that displayed fever only were confirmed with EHV-1 on March 4 and are isolated at the index premises, an event venue in Riverside County. As of March 5, three horses in San Mateo County were confirmed with EHM, and 30 with fever only (EHV-1).

In Riverside County, where the outbreak began at an event in late January, three more horses at the index premises, but not in the index quarantine barn, were confirmed positive for EHV-1 after displaying fever without neurologic signs. As of March 5, three horses had been confirmed with EHM and 30 with EHV-1.

CDFA continues to monitor the outbreak, and quarantines, which involve only premises with EHM cases or that involve EHV-1 cases associated with an EHM incident, will remain in effect until all positive horses have had two consecutive negative tests seven days apart.

EHV 101

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 equine herpesvirus myeloencephalitis (EHM, the neurologic form).

In many horses, the first or 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 EHM 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.

Herpesvirus is easily spread by nose-to-nose or close contact with an infectious horse; sharing contaminated equipment including bits, buckets, and towels; or clothing, hands, or equipment of people who have recently had contact with an infectious horse. Routine biosecurity measures, including hygiene and basic cleaning and disinfection practices, should be in place at all times to help prevent disease spread.

Current EHV-1 vaccines might reduce viral shedding but are not protective against the neurologic form of the disease. Implementing routine biosecurity practices is the best way to minimize viral spread, and the best method of disease control is disease prevention.