Genomic Epidemiology of Salmonella enterica Serovar Hadar Strain Linked to Poultry-Associated Salmonellosis Outbreaks, United States

Nontyphoidal Salmonella enterica serovar Hadar causes poultry-associated salmonellosis outbreaks in the United States. One persisting strain of Salmonella Hadar caused 5 multistate outbreaks resulting in ≈2,000 human cases during 2020-2023. The Centers for Disease Control and Prevention designated the strain as reoccurring, emerging, or persisting (REP), related within 26 core-genome allele differences. That REP strain has caused human illnesses by consumption of commercial poultry food products or contact with backyard poultry. To investigate the REP strain's evolution and identify possible markers for source attribution, we performed phylogenetics and molecular clock analysis on 404 genomes subsampled from routine surveillance and outbreaks. The most recent common ancestor likely emerged in early 2018. We identified 2 clades: clade 1, associated with backyard poultry and other food sources, and clade 2, predominantly linked to commercial poultry products. We found 2 clade-specific single-nucleotide polymorphism markers; in silico screening of additional isolates supported their use for source attribution.