Scott, Kevin J.
Molecular epidemiological characterisation of carried Neisseria meningitidis isolates in Scotland, 1974 - 2004 and a comparison with an invasive meningococcal disease strain collection.
PhD thesis, University of Glasgow.
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Neisseria meningitidis is an important cause of meningitis and septicaemia worldwide. Invasive meningococcal disease (IMD) is cyclical and varies by age group, being more common in children, especially those under 5 years. However, IMD is a rare outcome relative to asymptomatic carriage of the organism by the host and disease-causing isolates represent the tip of the iceberg in terms of the overall meningococcal population biology. The emphasis of previous research into N. meningitidis has rested firmly on the study of IMD isolates. In more recent times, however, there has been a more conscious effort to re-dress this balance to aid in our understanding of the relationship between carriage and disease.
The Scottish Haemophilus, Legionella, Meningococcus and Pneumococcus Reference Laboratory (SHLMPRL) have accrued an extensive isolate archive dating back to the 1960s. Only recently with the expansion of molecular techniques has the SHLMPRL begun to investigate this valuable resource. For around a decade or more the SHLMPRL have routinely used genotyping techniques to characterise all IMD isolates it receives. However, these methods have hitherto not been employed to investigate isolates of the archive not obtained from cases of IMD.
This study sought to characterise a collection of carried meningococci assembled from the isolate archive of the SHLMPRL that was obtained during the 31-year period, 1974 – 2004. Multi-locus sequence typing (MLST), porA Variable Region (VR) subtyping and a panel of genogrouping PCRs was used to characterise 791 carriage isolates. Temporal analyses of the data revealed how the presence of individual serogroups, clonal complexes, Sequence Types (STs), PorA subtypes and individual strain types in carried meningococci had changed in Scotland. Furthermore, these data were used in a comparison with a previously characterised collection of IMD isolates obtained during the same 31-year period to investigate the association of individual serogroups, clonal complexes, STs, PorA subtypes and individual strain types either with a carriage phenotype or with invasive disease.
Nongroupable isolates [Odds Ratio: OR 17.66; 95% Confidence Interval: CI (12.71 to 24.54)] and those of serogroups W135 [OR 2.49; 95% CI (1.72 to 3.60)], Y [OR 6.26; 95% CI (4.18 to 9.39)], X [OR 3.13; 95% CI (1.20 to 8.14)], Z [OR 131.89; 95% CI (18.00 to 960.76)] and 29E [OR 21.30; 95% CI (4.76 to 95.36)] were significantly associated with a carriage phenotype. In contrast, serogroups A [OR 3.64; 95% CI (1.96 to 6.76)], B [OR 2.65; 95% CI (2.25 to 3.12)] and C [OR 1.92; 95% CI (1.58 to 2.33)] were significantly associated with invasive disease.
The carriage strain collection reported herein was also observed to be highly diverse with the majority of STs identified only once. This diversity observed within the carriage strain collection [0.981; 95% CI (0.955, 1.006)] was significantly greater than the diversity within an IMD strain collection [0.938; 95%CI (0.934, 0.942)] from the same period. Temporal changes in the most prevalent clonal complexes (ccs) were observed throughout the 31-year period with increases in cc22, cc41/44 and cc269 and decreases in cc1, cc5, cc8, cc11, cc32, cc35, cc37, cc254, cc334 and cc364. Furthermore, for several ccs a significant association with a carriage phenotype (cc22, cc23, cc35, cc92, cc167, cc174, cc212, cc213, cc254, cc461, cc750, cc1157 and meningococci unassigned to a clonal complex) or with invasive disease (cc1, cc8, cc11, cc32, cc41-44 and cc269) was observed. Four lineages were identified amongst capsule null locus-containing meningococci, two of which, cc53 and cc1117, contained a unique allele (cnl-8) that was distinct from isolates of these lineages reported elsewhere.
Despite significant associations at the level of cc, distinct differences in those associations were apparent for individual STs within a given clonal complex; most notably the significant association of ST41 and both ST43 and ST44 with invasive disease and a carriage phenotype, respectively. A feature of the carriage strain collection was the concentration of cc8 isolates during the period 1984 – 1986 and the high proportion of isolates obtained from individuals resident in Lanarkshire at a time when an episode of increased disease was experienced within that region. In this study, cc8 was found to be significantly associated with invasive disease. Furthermore, whilst ST8 was also significantly associated with invasive disease the most common strain type within cc8, C:8:8:5,2,36-2, [OR 1.68; 95% CI (1.25, 2.27)] was however, significantly associated with a carriage phenotype. The strain types B:213:22,14,36 [OR 2.38 (95% CI 1.40, 4.07)] and B:43:19,15-1,36 [OR 2.88 (95% CI 1.42, 5.88)] were also significantly associated with a carriage phenotype.
Due to the heterogeneity of PorA subtypes in meningococci in Scotland the potential coverage by experimental or licensed PorA-based OMV vaccines would be limited. Therefore the introduction of monovalent or multivalent PorA-based vaccines in Scotland may be of little benefit. Improved strain coverage as a whole, not just against those of serogroup B, may require the addition of other vaccine antigens. Several other vaccine targets, including factor H-binding protein, are currently under investigation to improve coverage. Surveillance of these antigens and of the different lineages and serogroups in carriage and IMD isolates is essential to accurately monitor the effects that future vaccines will have on the meningococcus. We must remain vigilant despite a downward trend in cases of meningococcal disease in more industrialised countries.
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