This study describes the epidemiology of listeriosis in New Zealand (NZ) between 1999 and 2018, as well as the retrospective whole genome sequencing (WGS) of 453
Listeria monocytogenes
isolates corresponding to 95% of the human cases within this period. The average notified rate of listeriosis was 0.5 cases per 100,000 population and non-pregnancy associated cases were more prevalent than pregnancy-associated cases (average 19 and 5 cases per annum, respectively). Analysis of WGS data was assessed using multi-locus sequencing typing (MLST), including core-genome and whole-genome MLST (cgMLST and wgMLST) and single-nucleotide polymorphism (SNP) analysis. Thirty-nine sequence types (STs) were identified, with the most common being, ST1 (21.9%), ST4 (13.2%), ST2 (11.3%), ST120 (6.1%) and ST155 (6.4%). A total of 291 different cgMLST types were identified, with the majority (n = 243) of types observed as a single isolate, consistent with the observation that listeriosis is predominately sporadic. Amongst the 49 cgMLST types containing two or more isolates, 18 cgMLST types contained 2-4 isolates (50 isolates in total, including three outbreak-associated isolates) that shared low genetic diversity (0-2 whole-genome alleles), some of which were dispersed in time or geographical regions. SNP-analysis also produced comparable results to wgMLST. The low genetic diversity within these clusters suggests a potential common source but incomplete epidemiological data impaired retrospective epidemiological investigations. Prospective use of WGS analysis, together with thorough exposure information from cases will potentially identify future outbreaks more rapidly and possibly those that have been undetected for some time over different geographically regions.
Genetic species identification and low genetic diversity in Pseudocalanus acuspes of the Baltic Sea
