Epidemiologic and Phylogenetic Characteristics of a Dominant Clone of Neisseria Meningitidis Serogroup W in Florida, 2008-2014

BACKGROUND: Beginning in 2008, a clone of Neisseria meningitidisserogroup W (NmW), multilocus sequence type 11 clonal complex, was identified in southeast Florida. Since its detection, the clone has continued to spread and became the predominant cause of invasive meningococcal disease (IMD) in southeast Florida. The aim of this study is to describe the epidemiology of NmW IMD and to conduct whole genome sequencing to evaluate the evolutionary relationships of NmW isolates. 

METHODS: All confirmed IMD cases in Florida from 2008-2014 were aggregated. Isolates of NmW were sent to the Florida Department of Health, Bureau of Public Health Laboratories, for serogrouping, pulsed-field gel electrophoresis (PFGE), and genomic DNA extraction. Genomic DNA was sequenced on the Illumina MiSeq. A recombination free single nucleotide polymorphism (SNP) alignment was used for Bayesian phylodynamic and phylogeographic analysis using BEAST v1.8.2.

RESULTS: From 2008-2014, NmW caused 115 of 354 (32.5%) confirmed IMD cases reported in Florida. The median age of NmW cases was 49 years, compared with 29 years for other IMD cases (p <0.001) and were more likely to be Hispanic (p <0.001), and non-Hispanic black (p <0.001). NmW patients did not differ significantly by sex, hospitalization, or mortality. NmW was reported to cause more bacteremia (74.8% versus 59.0%; p = 0.004) and fewer meningitis (20.9% versus 46.0%; p <0.001) infections. Clonal PFGE patterns were observed for 93 of 102 (91.2%) NmW isolates.

Minimum spanning trees elucidated several clusters of genetically related isolates. NmW isolates had a mean pairwise nucleotide SNP distance of 12.7 (SE 1.2). In maximum likelihood and Bayesian trees, Florida NmW isolates clustered within a highly supported monophyletic clade, phylogenetically related to South American strains. Molecular clock analysis showed that the most recent common ancestor of NmW isolates in Florida dated back to 2007 that subsequently diversified in three independent clades simultaneously emerging in 2008, which strongly suggests at least three separate introductions in specific geographic areas leading to sustained transmission.

CONCLUSIONS: NmW IMD continues to be identified in Florida and its geographic distribution is expanding. NmW Infections are associated with older age, Hispanic ethnicity, black non-Hispanic, bacteremia and septic arthritis. The percent of patients with bacteremia was higher for NmW than patients infected by other serogroups, although the mortality did not differ. Phylogenetic analysis has putatively identified multiple introductions of closely related NmW strains from South America, dating back to 2007, followed by ongoing transmissions within separate chains, possibly including unrecognized transmission events.