A study has provided more detail on a multi-year outbreak of Listeria linked to meat products from one company in Germany. The United States was one of 20 countries that received the meat.

The aim of the work was to verify and describe a cluster of listeriosis patients and identify factors leading to the outbreak. The cluster was named “Sigma1.” Findings were published in the journal Clinical Microbiology and Infection.

The outbreak underlines the vulnerability of hospitalized patients for foodborne diseases and shows foods that may pose a high risk for Listeria monocytogenes contamination should not be offered to vulnerable people, according to the researchers.

In Germany, recommendations on catering for people in health care facilities and other institutions exists. Experts said the burden of health care facility acquired listeriosis could be reduced if existing guidance for safe catering was properly implemented by operators.

Three deaths in Germany; no illness elsewhere
The listeriosis outbreak had 39 genetically related cases between 2014 and 2019. Three people died. Illnesses occurred in 11 of 16 federal states in Germany. No further patients were detected after a recall of meat products and closure of Wilke Waldecker Fleisch und Wurstwaren in October 2019.

The United States was one of 20 countries that were listed as having received meat from Wilke but no illnesses were reported by any of them. Researchers said this was surprising given the export volume of the company’s products and long-term contamination in the production plant.

In Germany, one early illness in 2014 was retrospectively detected, as well as a few in 2016 and 2017. Twenty-one outbreak cases occurred in 2018 and nine in 2019. The last known onset of symptoms in a Sigma1 patient was mid-July 2019.

Twenty-one outbreak cases were men and 18 were women. They were 31 to 91 years old. There were no pregnancy associated cases. Eighteen people died and listeriosis was reported as the primary cause of death for three people. Twenty-five people were hospitalized because of listeriosis. Two contracted meningitis, and 16 developed sepsis.

From 39 outbreak cases, 25 were invited for an interview but 19 refused. Data from six — one case interview and five interviews with relatives or care givers — were retrieved. From these interviews there was no indication of commonly consumed foods. However, several cases had been in health care facilities such as hospitals or rehabilitation centers before the onset of listeriosis for other medical reasons. Outbreak cases were not all in one health care facility but spread in many different ones, mostly with only one patient per institution.

Supply to health-care facilities
In total, 13 cases had an inpatient stay in 12 different facilities during the incubation period, eight did not and the information for six cases was not known. Twelve other patients reported a stay in a health care facility before the listeriosis but it could not be verified if that was during the outbreak incubation period.

Distribution of products from the company to supermarkets and restaurants could explain the outbreak cases without known links to a health care facility.

Information about the food supply in six health care facilities was retrieved by local food safety authorities. Ready-to-eat (RTE) foods that have a higher probability for Listeria monocytogenes contamination were prioritized and their supply to health care facilities was analyzed.

This identified Wilke as the company that supplied a range of different RTE sausage products to the six investigated health care facilities through different firms.

Listeria monocytogenes was isolated from products from Wilke and sequencing showed a very close relationship between the clinical Sigma1 isolates and six isolates from sausage products and one isolate from the production environment.

Follow-up investigations by authorities suggested extensive hygiene breaches and widespread contamination with Listeria monocytogenes in the production plant.

The work received funding as part of the NOVA project in the One Health European Joint Program. This three year project that began in 2018 is developing surveillance tools and methods and harmonizing and optimizing the use of existing surveillance system data.

About Listeria infections
Food contaminated with Listeria monocytogenes may not look or smell spoiled but can still cause serious and sometimes life-threatening infections. Anyone who has symptoms of Listeria infection and believes they could have been exposed should seek medical treatment and tell their doctors about the possible exposure.

It can take up to 70 days after exposure to Listeria for symptoms of listeriosis to develop.

Symptoms of Listeria infection can include vomiting, nausea, persistent fever, muscle aches, severe headache, and neck stiffness. Specific laboratory tests are required to diagnose Listeria infections, which can mimic other illnesses.

Pregnant women, the elderly, young children, and people such as cancer patients who have weakened immune systems are particularly at risk of serious illnesses, life-threatening infections, and other complications. Although infected pregnant women may experience only mild, flu-like symptoms, their infections can lead to premature delivery, infection of the newborn, or even stillbirth.

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