Following an equine herpesvirus-1 (EHV-1) outbreak that began Jan. 24 at a San Mateo County premises, officials at the California Department of Food and Agriculture (CDFA) confirmed two additional horses at the premises with equine EHV-1 on Feb. 8.

The affected horses, a 20-year-old Warmblood mare and an 11-year-old Warmblood gelding, displayed fever but no neurologic signs. They remain isolated at the index premises.

On Feb. 7, a 10-year-old Quarter Horse gelding that began showing fever but no neurologic signs on Feb. 5, was confirmed positive for EHV-1 and was isolated at the index premises.

As of Feb. 9, two of the facility’s confirmed cases are positive for equine herpesvirus myeloencephalopathy (EHM, the neurologic form of EHV-1), and 11 cases have displayed fever only.

The premises will remain under quarantine until all positive horses have received two consecutive negative tests seven days apart. CDFA continues to oversee the outbreak.

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.