WASHINGTON — The world’s largest known listeria outbreak has spread throughout South Africa for 15 months, killing 189 people. Health officials believe they have identified the source: bologna.

Since January last year, 982 confirmed cases of listeriosis had been recorded, the National Institute for Communicable Diseases in South Africa reported on Thursday. The infection, caused by food that has been contaminated with the bacterium Listeria monocytogenes, is often lethal.

A cluster of gastroenteritis cases among toddlers in a Johannesburg hospital this January led authorities to the sandwich meat in a day care center’s refrigerator — and in turn, to a meat production facility in the northern city of Polokwane. There, officials said they detected traces of LST6, the listeria strain identified in 91 percent of the outbreak’s cases.

The South African meat processor, Enterprise Foods, issued a recall of some of its processed products in early March. Food safety experts at the World Health Organization plan to review the company’s exports to 15 countries across Africa, many of which lack reliable disease surveillance systems and diagnostic tools. Namibia recently reported one listeriosis case; its link to South Africa’s outbreak is uncertain.