Describing Australian listeriosis outbreaks, 2012 to 2022

Foodborne listeriosis outbreaks occur occasionally in Australia and can lead to severe outcomes for at-risk populations. Outbreaks also have the potential to cause illness in a large number of people in a short period. We identified invasive listeriosis outbreaks investigated in Australia from 2012 to 2022. We summarised the key features of these outbreaks and assessed the implications for food safety and future outbreak investigations. Outbreak data were extracted from the national OzFoodNet Outbreak Register and described by year reported; size; severity; type of evidence; food implicated; setting in which the food was prepared and eaten; and likely cause of contamination. Twelve listeriosis outbreaks were identified. These outbreaks involved a total of 94 cases, with 20 deaths reported (an overall case fatality rate of 21.3%). The median number of cases per outbreak was three (range: 2-34) and the median number of deaths was one (range: 0-7). Except for one outbreak with a median age of 32 years, the median age per outbreak ranged within 62-92 years. The most common food type implicated was pre-prepared composite foods (25%), including frozen meals and sandwiches. Ten outbreak investigations (83.3%) identified microbiological evidence of the same aetiological agent in the cases and the suspected food vehicle, including using whole genome sequencing as an emerging laboratory method. Most outbreaks (ten outbreaks, 83.3%) were caused by contamination of the product in the production environment, with one outbreak associated with extreme weather events. Use of novel microbiologic techniques has increased listeriosis outbreak detection and has also improved the ability to identify causes of outbreaks. It is important that public health communication emphasises the risks of consuming high-risk ingredients in composite foods, not just as standalone products. Food safety protocols should undergo ongoing review to ensure they are responsive to a changing climate.