Listeriosis Outbreaks in Italy in 2022-2023: Management and Source Identification

Implementation of effective teams that investigate listeriosis outbreaks is important to both limit the scope of outbreaks and to identify root causes and outbreak sources. This paper describes activities of the Italian outbreak investigation team, highlighting cross-sectoral collaboration in listeriosis surveillance and presenting a framework to strengthen national outbreak preparedness and response, using four Listeria monocytogenes (Lm) outbreaks investigated in Italy between 2022 and 2023. These outbreaks involved strains belonging to clonal complexes (CC) CC1, CC8, and CC155. Although CC1 is globally recognized as a hyper-virulent lineage predominantly associated with invasive listeriosis cases, CC8 and CC155 have been increasingly implicated in outbreaks reported in several European countries in recent years. Notably, the CC155 outbreak investigated in 2022 represented one of the largest outbreaks reported in Italy (101 cases). The implicated foodborne vehicles were meat products (chicken and turkey frankfurter, mortadella, and porchetta) and vegetables (black olive) in line with EFSA's 2023-2024 zoonoses reports, which identified pork and pork-derived products as the primary reservoirs for Lm transmission in Europe. Advanced molecular epidemiology was instrumental in outbreak detection and source attribution. cgMLST analysis facilitated large-scale surveillance and rapid comparisons, while single-nucleotide polymorphism (SNP) analysis enabled high-resolution phylogenetic characterization and discrimination of outbreak-related isolates, improving the robustness of investigations. The combination of a top-down regulatory framework with a bottom-up structure allowing rapid response and local-level outbreak detection proved critical for timely intervention. Future efforts should focus on harmonizing the One Health surveillance system through centralized data repositories, enhanced communication tools, and increased engagement of regional and local competent authorities.