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 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.