On Feb. 18, California Department of Food and Agriculture (CDFA) officials confirmed another horse positive for equine herpesvirus-1 (EHV-1) at the Riverside County event venue where an outbreak began Feb. 10.

The horse was stabled outside the index quarantine barn and displayed fever but no neurologic signs. It was isolated with other positive horses, and other exposed horses were quarantined and isolated in a separate area of the premises. Two positive horses that previously exhibited fever only have shown neurologic signs.

The next day, two more horses on the premises but housed outside the index quarantine barn displayed fever but no neurologic signs and were confirmed positive for EHV-1. They were isolated, and all potentially exposed horses remain quarantined and isolated on the index premises.

One of the previously EHV-1 positive horses was euthanized due to the severity of its signs.

To date, five EHV-1 cases with fever only and three confirmed equine herpesvirus myeloencephalitis (EHM, the neurologic form) cases (EHV-1 with neurologic signs) have been confirmed.

CDFA continues to monitor the 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 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.