On Wednesday, one of the positive horses which initially displayed a fever and limb swelling lay down and the decision was made to euthanize the animal.
Necropsy results as to the cause of the recumbency are pending, the department said.
The property, described as a large multi-discipline facility, remains under quarantine and all exposed horses are being monitored.
The first of the cases was confirmed on January 11, when a gelding displaying neurologic signs was confirmed positive for the neurological strain of the virus.
The department's Animal Health branch is conducting a full epidemiologic investigation at the affected premises.