On Nov. 27, California Department of Food and Agriculture (CDFA) officials confirmed that another exposed horse on the San Bernardino County index premises first identified on Nov. 6, a 12-year-old Quarter Horse mare, became febrile (displayed an elevated fever) and subsequently tested positive for equine herpesvirus-1 (EHV-1).
The outbreak on the index premises now involves four horses that have developed neurologic signs. Three of these were euthanized due to the severity of their signs. Six horses have become febrile and confirmed positive for EHV-1, and an additional 10 horses remain in exposed status following quarantine and implementation of enhanced biosecurity measures, including twice-daily temperature monitoring.
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 myeloencephalopathy (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.