The deadly and record-setting listeriosis outbreak in South Africa isn’t showing any signs of slowing down.

Food is the suspected cause, but health officials have not yet identified a specific source.

Since the new year began, the country’s National Institute for Communicable Diseases (NICD) has confirmed six more listeriosis deaths in the outbreak, bringing the death toll to 67. The South African listeriosis outbreak is already the deadliest in recorded history.

Previously, the world’s largest listeriosis outbreaks were in the United States in 2011, and in Italy in 1997.

The Italian outbreak was traced to cold corn and tuna salad and sickened more than 1,500 people, mostly children at two elementary schools. Public health officials traced the 2011 U.S. outbreak to cantaloupes from Jensen Farms in Colorado. In that outbreak, 147 people across 28 states were sickened and at least 33 people died.

Other Listeria outbreaks have resulted in fatality rates as high as 20 to 40 percent. The best known of those was the high-profile Listeria outbreak linked to deli meats from Canada’s Maple Leaf Foods, which caused the deaths of 22 mostly elderly Canadians.

The World Health Organization (WHO) says the South African listeriosis outbreak is now the largest ever reported. The organization officially declared the outbreak status on Dec. 5, 2017.

With 748 cases reported, the fatality rate for the overall outbreak is just less than 9 percent. However, the death toll among so-called traced patients is much higher, topping 40 percent.

Neither South Africa nor WHO have yet identified the source of the deadly pathogen, but the investigation is continuing. The South African government suspects a food source is responsible for the outbreak that has touched all nine of the country’s provinces.

As in the United States, listeriosis is a reportable disease in South Africa, meaning every patient with the diagnosis must be reported to federal health authorities. However, its possible some cases have not yet been diagnosed and reported.

Two-thirds of the confirmed cases so far involve the Gauteng province in northeastern South Africa, which includes the cities of Pretoria and Johannesburg,

South Africa’s 23 private food testing labs and labs operated by the South African Meat Processors Association, South African Milk Processors Association, Milk South Africa, Consumer Goods Council and the National Laboratory Association have all agreed to help in the investigation.

Both public and private hospitals have reported cases, and WHO says those infected represent diverse socio-economic backgrounds.

In this image from the Pasteur Institute, Listeria monocytogenes (shown in red) is in the process of infecting tissue cells.

The incubation period for Listeria, the time from exposure to experiencing symptoms, can be as long as 70 days after exposure. That is complicating the investigation because it is harder for people to recall possible food sources. Officials are making home visits to help people remember to practice basic food safety measures and report illnesses. Investigators are specifically looking at unpasteurized dairy products and ready-to-eat foods.

Pregnant women and people with weakened immune systems are especially at risk from listeriosis. WHO says pregnant women are 20 times more likely to get listeriosis than regular the healthy adults. Pregnant women can experience stillbirths from listeriosis and they can pass the infection to their babies before birth.

Symptoms of Listeria infection include mild flu-like complaints, headaches, muscle aches, fever, nausea and vomiting. If the infection spreads to the nervous system, it can cause stiff neck, disorientation or convulsions.

Statistics South Africa tracks and reports the country’s leading causes of death. Foodborne illnesses such as listeriosis are not at the top of the list:

  • tuberculosis  causes 8.8 percent of total deaths;
  • influenza and pneumonia cause 5.2 percent;
  • HIV/AIDS causes 5.1 percent;
  • cerebrovascular diseases cause 4.9 percent;
  • diabetes mellitus causes 4.8 percent;
  • other forms of heart disease cause 4.6 percent; and
  • hypertensive disorders cause 3.7 percent.

Some of the waterborne diseases that pose a high risk to South Africans include gastroenteritis, cholera, viral hepatitis, typhoid fever, bilharziasis and dysentery. Malaria is endemic in small areas within the provinces of Kwazulu-Natal, Limpopo, and Mpumalanga.

Listeriosis is caused by the bacterium Listeria monocytogenes, which is found in the environment in water, soil, vegetation and in certain animal feces. It can contaminate animal products, including meat and dairy, seafood and fresh produce. Once Listeria becomes established in a food production facility it can be very difficult to remove.

Until the listeriosis outbreak ends, South Africans are being urged to practice hand washing before and during food preparation; separate raw meat, poultry and seafood from other foods; and cook meats, poultry, eggs and seafood thoroughly. Consumption of unpasteurized, raw milk and products made from it is also discouraged.

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