The role of pigs and slaughterhouses in the transmission of Listeria monocytogenes into the meat processing chain.

Listeria (L.) monocytogenes is one of the most dangerous foodborne pathogens. Many different food products are known to be the source of listeriosis including pork. However, the information on the role of pigs in L. monocytogenes epidemiology is very limited. In this study, we analysed the occurrence of Listeria spp. in 800 faecal samples from Austrian pigs, in swab samples from the farm environment (n = 280) and from two corresponding slaughterhouses (n = 305). L. monocytogenes was rarely present in pigs (1.5%), more often in their farm environment (7.86%) and commonly in the two slaughterhouses (27.1 and 20.7%). We isolated a diverse set of L. monocytogenes (n = 104). Whole genome sequencing (WGS) and single nucleotide polymorphism (SNP) analysis revealed the presence of 31 strains (SNP ≤ 20), assigned to 13 sequence types (STs). ST451, ST37, ST9, and ST4 were the most common STs. Mining the Austrian Listeria Genome Database for identical strains (SNP ≤ 20) in food products and food processing environment identified co-occurrence and transmission of at least eight strains from the farm environment and/or the slaughterhouse to the meat processing facility and to the final product. We observed a wide spread of these eight strains in different meat processing facilities and diverse final products over many years (from two to six years). One ST9 strain even caused two listeriosis cases in different years. Our study demonstrates the potential of L. monocytogenes originating from the pig environment to persist in the slaughterhouses and along the food chain, leading to contamination of food products and posing a potential risk for listeriosis.