More than 140 cases of African horse sickness, with 100 deaths, have been recorded in KwaZulu-Natal since an outbreak in December last year, media reports said yesterday.
Experts said this was the highest incidence of the disease, AHS, in the country.
The disease is transmitted by a type of midge and happens mostly during the warm, rainy summer season.
African Horse Sickness Trust chief executive Douglas Welsh said an outbreak of the disease "generally happens every year".
Northern KwaZulu-Natal, Limpopo and Mpumalanga were the worst affected.
Horse owners near areas where the outbreaks had occurred were urged to have their animals vaccinated and to treat them with insect repellant daily.
Racing SA chief executive Peter Gibson said the summer months were a high-risk period.
"We should start seeing a decline towards the end of April."
On Monday the Western Cape department of agriculture said horse movements into the province would be restricted.
"These restrictions have been instituted due to the increasing number of reported and suspicious cases of AHS in South Africa," said spokesperson Alie van Jaarsveld.
No cases have been reported in the Western and Northern Cape. - Sapa