WHO declares South African Listeria outbreak ‘largest ever’

The World Health Organization (WHO) says a listeriosis outbreak in South Africa that has killed 67 people and sickened nearly 750 is the largest-ever foodborne illness outbreak recorded — and to date, a contamination source has not yet been found.

Investigators have been looking for where the implicated strain of Listeria comes from since the outbreak first began in early 2017. A major obstacle has been the bacteria’s long and inconsistent incubation period — anywhere between six hours and 10 weeks.

“You wouldn’t know what you ate three weeks ago — maybe the one particular food that made you sick three or four weeks later,” said a representative from WHO. “This is the big challenge we face in this situation.”

To support the investigation, South African authorities have made listeriosis a “notifiable disease,” meaning any patient diagnosed must be reported to health agencies. They’ve found that most reported cases are from the Gauteng province, which includes Johannesburg and Pretoria. Newborns comprise about 40% of cases, according to WHO.

Whole genome sequencing on Listeria samples taken from patients shows that one strain is responsible for the majority of infections, meaning a single source is likely — either one food product, or multiple made at a common facility. What that source is, however, is still a mystery.

Listeriosis is the illness caused by Listeria, a foodborne pathogen associated most commonly with ready-to-eat meats and unpasteurized dairy products. Its symptoms include fever, neck stiffness, confusion, weakness and vomiting.

Though usually the illness passes after a few days without treatment, it can lead to infection of the bloodstream and brain, which can be deadly.

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