A 10 year-old Warmblood gelding in San Diego County displaying neurologic signs of urine dribbling and hind limb ataxia was confirmed positive for Equine Herpesvirus -1 by the California Animal Health and Food Safety (CAHFS) Laboratory, the state’s regulatory laboratory.


Based on the clinical signs and positive laboratory test, this horse meets the case definition for Equine Herpesvirus Myeloencephalopathy, a condition which requires regulatory action. The neurologic signs were first noted on Tuesday February 7, 2017.

The positive horse was stabled in tent 21 at the HITS Show in Thermal, CA from January 21, 2017 to February 5, 2017.  The EHM horse and one additional exposed horse have been quarantined at their home premises in San Diego County.

CDFA has been in contact with HITS show management and veterinarians to ensure enhanced biosecurity measures are taken on the premises; including cleaning and disinfecting the stabling area. CDFA recommends any exposed horses be observed for clinical signs and have temperatures taken twice daily.  CDFA will continue to monitor the situation.

Equine Herpes Virus (EHV-1) infection in horses can cause respiratory disease, abortion in mares, neonatal foal death and neurological disease. The neurological form of the disease is known as Equine Herpes Myeloencephalopathy (EHM) and may be caused by damage to blood vessels of the brain and spinal cord associated with EHV-1 infection. EHM is most often due to the neuropathogenic strain of EHV-1, but may occasionally be caused by the non-neuropathogenic strain of the virus.

EHV-1 is easily spread and typically has an incubation period between 2-10 days. Respiratory shedding of the virus generally occurs for 7-10 days, but may persist longer in infected horses. For this reason, a twenty-one day isolation period of confirmed positive EHM cases is recommended along with stringent biosecurity protocols. Similar to herpes viruses in other species, the latent form of EHV-1 can reactivate at a later date, but generally with a low viral load posing a low risk of infecting other horses. Humans are not at risk of contracting the virus, however humans can act as an indirect mode of transmission.