The outbreak of the midge-borne disease is in the Eastern Cape area and has spread to Port Elizabeth.
More than 180 cases have so far been reported, with more than two-thirds proving fatal.
The midges are most active at dawn and dusk, and owners have been encouraged to stable their horses at the crucial times to reduce the chances of their horses being bitten.
African horses sickness is considered one of the greatest emerging threats to British horses and has already affected parts of Europe.
The midge has been spreading north from Africa and authorities believe it is only a matter of time before an outbreak occurs in the UK.
There are concerns, too, that the disease is becoming more virulent, with some fatalities in South Africa occurring in horses which had been vaccinated.
Outbreaks in Eastern Cape are not uncommon, tending to occur annually during periods of hot and wet weather.
Zebras are known to carry the disease and it is possible the midges have transferred it from the wild population.