On Monday, March 14, 2022, the California Department of Food and Agriculture released an updated report on the state’s ongoing equine herpesvirus outbreak. The outbreak, which began at the Desert International Horse Park in Palm Springs, has resulted in the death of several horses and equine event cancellations throughout the state. The California state veterinarian is requesting horse owners limit the transportation of their animals between facilities.

The state’s report accounts for horses exhibiting neurologic signs (equine herpesvirus myeloencephalopathy, or EHM), as well as those exhibiting fever and mild signs of the disease. Each entry represents one affected premises:

• Orange County No. 1—23 confirmed (2 EHM, 21 fever), no change;
• Orange County No. 2—16 confirmed (2 EHM, 14 fever/mild signs);
• Orange County No. 3—16 confirmed (2 EHM, 14 fevers);
• San Mateo County No. 1—39 confirmed (4 EHM, 35 fevers), no change;
• San Mateo County No. 2—3 confirmed (1 EHM, 2 fever), no change;
• Riverside County—35 confirmed (3 EHM, 32 fevers), no change; and
• Santa Clara County—2 confirmed (2 EHM), no change.

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.