Listeria monocytogenes post-outbreak management - When could a food production be considered under control again?

In cases of outbreaks, food business operators face inspections, recall actions and delisting by retailers. This could have happened to an Austrian meat processor whose products have been associated with a cluster of seven cases of listeriosis spread over the years 2015-2017. Sequencing of clinical and foodborne isolates by public health specialists raised the suspect of a single source outbreak since all strains were of MLST 155, cgMLST 1234. Since the family-driven business was highly motivated to save their business, a crisis management scheme was applied that was agreed upon with national authorities. An end-product-based approach testing every single lot for L. monocytogenes was set into power and only negative lots were released for delivery. We combined the active food lot controls of food authorities with a Listeria environmental transmission mapping procedure. The environmental monitoring approach included 19 sampling activities during 3.5 years resulting in 1632 samples. This scheme allowed to trace and mitigate the Listeria contamination but did not jeopardize the processing of meat products. In total, 14 measures were set into power that reduced the overall Listeria occurrence after sanitation of 50-75 % (sampling event I, II) to 0.0-3.8 % (sampling events XIII to XIX). The outbreak-associated ST155/CT1234 clone was not detected in the third sampling event onwards but popped up during the sampling event VIII again. From then on, the outbreak clone ST155/CT1234 was no longer detected in the food business operator (FBO). We conclude that an intense combined investigation of food lots and environmental samples is needed to identify the source and verify that contamination levels are under control. Initially public health authorities suspected contamination of the slicer, but the monitoring approach has localized the source of ST155/CT1234 in a Schnitzel sorting machine. Other factors leading to the contamination scenario were inadequate conveyor belt hygiene. An inadequate crate washing system and an inadequate hygiene lock led to Listeria spreading between compartments. All transmission routes could be effectively interrupted. A root cause analysis and preventive maintenance program implemented in the FPE is mandatory for food processing facilities.