Foodborne transmission of Listeria monocytogenes via ready-to-eat salad: A nationwide outbreak in Switzerland, 2013-2014

From 26 October 2013 to 23 April 2014, 32 cases of listeriosis infected with an Listeria monocytogenes strain serovar 4b, sequence type 4 and belonging to a single distinct PFGE pulsotype were registered in patients from several cantons of Switzerland. L. monocytogenes was detected in blood (75%), CSF (16%), ascites (6%) and in joint fluid (3%) samples. By the end of March 2014, a food producing company reported an L. monocytogenes contamination of ready-to-eat salads to the authorities after detecting the pathogen through its in-house routine quality control. Product and environmental samples collected during subsequent investigations yielded isolates, matching the outbreak strain, thus confirming that ready-to-eat salad from this company was most likely the outbreak source. The cause for the product contamination was related to a design-inherent hygienic problem of one specific product-feeding belt. Complementary patient interviews also identified ready-to-eat green salads bought at one retailer as the likely outbreak source.