As of Mar. 31, California Department of Food and Agriculture (CDFA) officials report the following numbers for equine herpesvirus-1 (EHV-1) cases in various counties:

• Orange County premises #1, which was released from official quarantine Mar. 24—two confirmed equine herpesvirus myeloencephalopathy (EHM, the neurologic form of EHV-1) cases and 21 cases with fever only
• Orange County premises #2, which was released Mar. 30—two EHM cases and 14 with fever and/or mild signs only
• Orange County premises #3, which remains under official quarantine—three EHM cases and 24 with fever only. These numbers include one new case of EHV-1 with neurologic signs that was confirmed Mar. 30. The affected horse is an 8-year-old Selle Français mare that experienced onset of clinical signs on Mar. 29. Signs included ataxia (incoordination) and urine dribbling. She is recovering under veterinary care.
• Riverside County, which was released Mar. 23—three cases of EHM and 32 with fever only
• San Mateo County premises #1—four EHM cases and 36 with fever only
• San Mateo County premises #2, released Mar. 26—one EHM case and two with fever only
• Santa Clara County—two confirmed EHM cases
• Sonoma County—one confirmed EHM case
• San Diego County—new premises

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.