The 15-year-old Warmblood mare in Sacramento County was confirmed positive for equine herpesvirus-1 (EHV-1).
The mare had initially shown signs of colic on April 21 and was transported to a veterinary clinic for treatment.
While in isolation at the clinic she started displaying neurologic signs and lay down. She was subsequently euthanized due to the severity of her neurological problems.
Quarantine biosecurity measures were immediately implemented at her home premises.
The California Department of Food and Agriculture said 31 horses were under quarantine, having potentially been exposed to the virus. Required biosecurity measures included twice daily temperature-taking and observation of horses.
No exposed horses have shown signs of the disease, the department said in a statement dated April 28.
The department said it was continuing to monitor the situation.