Authorities in Switzerland are investigating to find the source of a suspected foodborne outbreak of Listeria with 12 cases and two deaths.

The Federal Office of Public Health (BAG) said since June this year it has recorded an unusual surge in Listeriosis cases with serotype 4b. The agency told Food Safety News two people had died.

According to analysis using next generation sequencing (NGS) the cases show a close relationship with each other and can be considered linked. BAG officials said it is likely an outbreak due to the same food source.

Of the 12 cases, five men and seven women have fallen ill with an age range between 0 to 99 years old. In one case, there was transmission between mother and child.

Cases occurred in six cantons: Argovie (or Aargau), Neuchâtel, Schwyz, Tessin, Valais and Zurich. A canton is comparable to a state in the United States.

The last major outbreak of listeriosis serotype 4b was in 2013-2014 and was linked to consumption of ready to eat packaged salad.

BAG and the Federal Food Safety and Veterinary Office (BLV) are looking for the source of infection with help from the Competence Center for Epidemiological Outbreak Investigations (KEA) and the National Reference Centre for Enteropathogenic Bacteria and Listeria (NENT). The KEA, which is attached to the Tropical and Public Health Institute (Swiss TPH), is responsible for this investigation.

From the start of 2018, 41 listeriosis cases have been declared in Switzerland. The largest listeriosis outbreak ever recorded in the country with 122 illnesses and 33 deaths was caused by the 4b serotype from 1983-1987 and was traced to contaminated Vacherin Mont d’Or, a cow’s milk cheese.

Listeriosis is an infectious disease caused by Listeria monocytogenes. Foodborne infection occurs through contaminated products like unpasteurized milk, soft cheeses, vegetables and prepared meat products such as pâté. Time between becoming infected and developing symptoms varies between three and 70 days but is three weeks on average.

Last year, 45 cases of Listeria confirmed by laboratory diagnosis were reported to BAG, according to the annual zoonoses report for the country. Serovar could be detected in 40 of these 45 cases. The most common was 4b in 18 cases followed by 1/2a in 15 infections.

Campylobacteriosis remained the most frequently recorded zoonosis in humans in 2017 with 7,219 cases confirmed by lab diagnosis. A slight decrease from the 7,688 cases recorded in the previous year. In most cases, infections happen by eating contaminated food, with poultry meat being the primary source.

The second most common zoonosis is salmonellosis with 1,848 cases of lab-confirmed cases reported in humans. The number of cases in 2017 increased compared to the 1,517 in the previous year.

Recorded Verotoxigenic E. coli (VTEC) infections increased 50 percent from the previous year from 463 to 696 lab-confirmed cases for 2017. This rate is the highest since reporting requirements were introduced in 1999. The main cause of this is likely new methods of analysis resulting in labs testing more VTECs which means more cases are detected, according to the report.

In 2017, 18 foodborne outbreaks were reported compared to 11 in the previous year.

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