The property is in Tuolumne County. The department said the nine horses showing symptoms were in isolation awaiting test results from nasal swabs and blood samples.
The initial confirmed positive mare has shown improvement, the department said, and had returned to the premises from a veterinary referral hospital to continue her recovery.
An investigation is ongoing into the outbreak and the owner of the premises, as well as department staff, have contacted owners of potentially exposed horses that visited the affected premises over the past two weeks.
The 150 samples were collected yesterday by University of California Davis veterinary school staff, including from the nine horses demonstrating showing clinical signs.
Test results are expected within the next few days.
To date, one event at which exposure could have potentially occurred has been identified - the September 10 ACTHA Ride for Mustangs.
A horse participating in the ride was exposed to the positive confirmed case on the Tuolumne County premises.
The exposure occurred before the ride and the exposed horse has not displayed any signs of disease.
However, "out of an abundance of caution", the department has contacted all ride participants to recommend isolation and temperature-monitoring of horses which participated in the ride for 14 days from the last exposure.
A temperature greater than 102F or other compatible clinical signs should be reported to a veterinarian, who can collect samples for diagnostic testing.
The cases follow a May outbreak centred on the National Cutting Horse Association Western National Championship held in Ogden, Utah, which had 400 competitors.
Infected horses subsequently returned home to about a dozen western states.
Horses suffering from EHV-1 can suffer from any or one or more of the following clinical signs: fever, nasal discharge, ataxia, hind-end weakness, diminished tail tone, and/or recumbency.