The death toll is now 13, and results expected from Pretoria this week should confirm the cause. It is possible the deaths have resulted from equine encephalosis and not African Horse Sickness.
Horse events have been called off and strict movement controls are in place as authorities work to contain the outbreak.
AHS is spread by a midge. Confirmation of AHS will be a blow to South Africa's bloodstock export industry, with a six-month ban likely.
The European Union lifted its ban on horses from South Africa only in November last year after an outbreak in 2004.