On July 24, the U.S. Equestrian Federation (USEF) notified officials at the California Department of Food and Agriculture (CDFA) that a 6-year-old Warmblood mare that attended — but did not compete in — a show at the Sonoma Horse Park tested positive for equine herpesvirus-1 (EHV-1). The mare displayed neurologic signs on July 23 that indicated equine herpesvirus myeloencephalopathy (EHM). The mare was euthanized.

Show management contacted all show participants, making the recommendation of seven days of isolation, twice-daily temperature monitoring, and reporting of any elevated temperatures or neurologic signs to their veterinarians and CDFA.

Nineteen exposed horses from the mare’s barn were removed from the show facility and isolated under quarantine at their home premises in Sacramento County. The remaining 18 horses at the mare’s home facility are under enhanced biosecurity protocols and twice-daily temperature monitoring.

Six horses at the mare’s home premises that displayed fever and/or limb edema have also tested positive for EHV-1. One of those, a 15-year-old Warmblood gelding that experienced fever and began displaying neurologic signs including ataxia (incoordination), urine dribbling, and cranial nerve deficits, was moved to isolation under quarantine at a veterinary hospital.

Because EHV and EHM are reportable equine diseases in the state of California, CDFA continues to manage the outbreak on site.

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.