On March 16, officials at the California Department of Food and Agriculture (CDFA) updated case counts for the equine herpesvirus-1 (EHV-1) outbreak affecting Orange, San Mateo, Riverside and San Mateo counties. Numbers reflect one premises in each named county.

• Orange County premises No. 1: 2 horses with equine herpesvirus myeloencephalomyelitis (EHM, the neurologic form of equine herpesvirus-1 (EHV-1) and 21 with fever only;
• Orange County premises No. 2: 2 with EHM and 14 with fever and/or mild signs;
• Orange County premises No. 3: 2 with EHM and 22 with fever only;
• San Mateo County premises No. 1: 4 with EHM and 36 with fever only;
• San Mateo County premises No. 2: 1 with EHM and 2 with fever only;
• Riverside County: 3 with EHM and 32 with fever only; and
• Santa Clara County: 2 with EHM.

Orange County premises No. 3 is the only location at which numbers have changed since CDFA’s last update on March 15, at which time the count was 2 horses with EHM and 18 with fever only.

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.