Retrospective Analysis of Historical Listeria monocytogenes Clinical Isolates, New York, USA, 2000-2021¹

We used whole-genome sequencing data to investigate historical Listeria monocytogenes clinical (n = 1,046) and nonclinical (n = 1,325) isolates from New York, USA. Applying a threshold of <20 single-nucleotide polymorphism differences for single-linkage clustering, 321 clinical isolates clustered into 85 clusters ranging from 2-33 isolates per cluster. Fourteen clusters included isolates with outbreak codes (4 clusters with New York codes and 10 with multistate codes). Three New York outbreak codes were assigned to isolates that were genetically highly related and from cases <2 months apart. Fifteen clusters included isolates that were obtained >10 years apart; 33 clusters included isolates from the same or contiguous counties. Seventeen clusters included food and environmental isolates highly related to clinical isolates. These findings suggest that some listeriosis clusters can be local and span a long period, demonstrating the importance of investigating small, localized listeriosis cases with closely related isolates, even over long timeframes.