Phylodynamic analysis of Salmonella Enteritidis ST183 in Aotearoa New Zealand finds no evidence for introduction via European hedgehogs (Erinaceus europaeus)

Salmonella enterica subsp. enterica serovar Enteritidis (S. Enteritidis) is the second most common serovar causing human salmonellosis in Aotearoa New Zealand (NZ). Sequence type (ST)183, which includes phage types 9a and 11, was the second most frequently isolated S. Enteritidis strain from human cases between 2020 and 2023. This ST is considered endemic in NZ as well as in mainland Europe and Great Britain, where the European hedgehog (Erinaceus europaeus) is a recognized wildlife reservoir. Hedgehogs were introduced to NZ in the late 19th century; however, their role in the ecology of ST183 in NZ has not been formally evaluated. The aim of this study was to investigate whether hedgehogs act as a reservoir for S. Enteritidis ST183 in NZ and to assess the evolutionary history and epidemiology of this strain across human, animal and environmental contexts. We analysed human, animal and environmental ST183 isolates, including hedgehog carcasses opportunistically sampled in NZ, using Bayesian phylogenetic methods integrated with national epidemiological data. Although S. Enteritidis ST183 was isolated from three of 45 hedgehog carcasses during our study, consistent with recent prior detections of ST183 in NZ hedgehogs, Bayesian phylogenetic analysis supports a most recent common ancestor for currently circulating ST183 strains in the late 20th century, ~100 years after the introduction of hedgehogs into NZ, providing no evidence that the strain was introduced concomitantly with hedgehogs. Epidemiological analysis revealed that, unlike in Europe, ST183 infections in NZ are more common in people aged over 60 years compared with non-ST183 S. Enteritidis infections, with rural residence and contact with farm animals identified as key risk factors. Together, these findings suggest that S. Enteritidis ST183 is established within the NZ rural environment, with evidence of interspecies transmission. While hedgehogs may contribute to the maintenance of ST183, they are unlikely to represent the original source of introduction, indicating a complex, multi-host ecology.