Nearly 1,000 cases of listeriosis have been reported in South Africa since Jan. 1, 2017, resulting in 189 deaths.

According to the World Health Organization, this is the largest outbreak of listeriosis that has ever been detected—and the source of the outbreak has just recently been identified.

A report released last week from the National Institute for Communicable Diseases (NICD) explained that ready-to-eat processed meat products from the South African meat processor, Enterprise Foods, were contaminated with various strains of listeria. The bacteria was most prevalent in the meat product known in South Africa as polony, also referred to as bologna.

The 15-month long outbreak affected all provinces in South Africa, but Guateng Province was a hot spot with 59 percent of all reported cases.

The Ministry of Health announced on March 4 that meat products manufactured at Enterprise Foods’ Polokwane production facility were the source of the outbreak, which led to an immediate recall on the company’s foods.

However, new cases may continue to be reported despite the recall, according to the NICD. The incubation period of listeriosis is 70 days, and the food products have a long refrigeration shelf life. These factors, as well as the possibility of cross-contamination of other types of foods in both retail and home settings present a lingering threat of infection. Twenty-three confirmed cases have been reported since the March 4 recall, but health officials believe the initial exposure in all these cases occurred prior to the recall.

The company exports to 15 countries across Africa. Namibia also recently confirmed a case of listeriosis, but officials still need to determine if this case is linked to the outbreak.

Scientists conducted whole genome sequencing on isolates from a large subset of patients and found that 91 percent of the listeria strains belonged to Listeria monocytogenes Sequence Type 6 (ST6). The same ST6 sequence type was identified in the polony meat product, and was also found in the processing environment of the Enterprise Foods location, according to the WHO.

But nine percent of the reported patients were infected with different strains of listeria than the predominant ST6 outbreak strain. This may indicate that more than one outbreak is ongoing, a WHO report states.

As of March 26, there was a total of 982 laboratory-confirmed listeriosis cases reported to the NICD since January 2017. Patients ranged in age from newborn to 93-years-old. Outcome data is currently available for 70 percent of all reported cases—of which, 28 percent died.

Listeriosis can result in flu-like symptoms, including high fever, vomiting and diarrhea. Young children and the elderly are particularly vulnerable. Forty-two percent of cases in the South African outbreak were newborns infected during gestation. According to the CDC, pregnant women are 10 times more likely than others to become infected.

A class action lawsuit was filed last week against South Africa’s Tiger Brands, the parent company of Enterprise Foods. The company has suspended production at its Polokwane, Germiston, Pretoria and Clayville sites in South Africa.