• Orange County 1, 23 confirmed cases (two equine herpesvirus encephalomyopathy—EHM, the neurologic form of EHV-1—cases and 21 with fevers): premises released March 24.
• Orange County 2, 16 confirmed cases (two EHM, 14 fevers/mild signs): premises released March 30.
• Orange County 3, 27 confirmed cases (three EHM, 24 fevers): One horse remains in isolation pending two negative tests seven days apart, all others released.
• Riverside County, 35 confirmed cases (three EHM, 24 fevers): premises released March 23.
• San Mateo County 1, 40 confirmed cases (four EHM, 36 fevers): premises released March 31.
• San Mateo County 2, three confirmed cases (one EHM, two fevers): premises released March 26.
• Santa Clara County, two confirmed EHM cases: premises released March 31.
• Sonoma County, one confirmed EHM case: premises released April 1.
• San Diego County, one confirmed EHM case: premises released April 14.

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 EHM.

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.