According to California Department of Food and Agriculture (CDFA) officials, the 15-year-old Warmblood mare that attended and was asymptomatic at the Sonoma County show yet displayed mild neurological signs on July 27 has received a second negative test for equine herpesvirus-1 (EHV-1) .

The mare has returned to the Sacramento County premises where the index case, which was euthanized, resided. The surviving mare is reported in stable condition and continues to improve. The three other neurologic horses that are under isolation at an offsite veterinary hospital are also in stable condition. Veterinarians retested them on July 29 and/or July 30 and confirmed them negative for EHV-1.

A 5-year-old Warmblood gelding that attended the Sonoma County show remains asymptomatic. He was tested on July 28 and confirmed positive for EHV-1 on July 30. He is under isolation at the index premises.

No additional horses from the Sonoma County show or the index case’s home premises have been reported with clinical signs. CDFA continues to monitor the outbreak.

According to a CDFA statement, statistics for this outbreak are:

• One confirmed EHM case (positive test with neurologic signs), which was euthanized;
• One confirmed asymptomatic case, which is recovering under veterinary care;
• One confirmed case with fever only, which is recovering under veterinary care; and
• Four presumed positive cases that subsequently tested negative and are recovering under veterinary care.

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.