JOHANNESBURG (Reuters) - An outbreak of African swine fever, a severe hemorrhagic disease of pigs, has spread to a fourth province in South Africa, the agricultural ministry said on Monday, following a spate of outbreaks last month.

The ministry said the latest outbreak of the disease was reported in the Heilbron area of the central Free State Province, after the disease was detected in the country’s North West, Mpumalanga and Gauteng provinces last month.

The affected areas have been placed under quarantine and investigations were being carried out to trace the origin of the outbreaks and identify other farms that may be affected, the agriculture, forestry and fisheries ministry said in a statement.

The disease is incurable in pigs but harmless to humans. It has spread to China since last year, causing major losses to the world’s largest hog herd.