Molecular epidemiology of carriage and disease associated strains of serogroup C Neisseria meningitidis, in the Irish Republic following the introduction of the meningococcal conjugate vaccine.
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In the Irish Republic during the 1990s incidence meningococcal disease caused by serogroup C increased dramatically. During the 1999-2000 epidemiological year Ireland had the highest reported incidence of meningococcal disease in Europe (www.hpsc.ie). The national menC vaccination programme was introduced into the infant schedule in October 2000 and also offered to all persons under 23 years.
At the same time, as part of a period of enhanced surveillance following the vaccine introduction, the National Meningococcal Carriage Survey was launched and continued each winter for the next 4 epidemiological years. The results of the vaccine campaign were impressive; a decrease in both carriage and incidence of serogroup C meningococci were observed.
There were 29, 24, 13, 8 and 3 serogroup C organisms (n=77) recovered fromthe survey during the 2000 - 2001 - 2004 - 2005 epidemiological years respectively, which were available for this study. To gain a better understanding of the relationships between these organisms they were serotyped and serosubtyped using a conventional panel of antisera. They were also typed using Multi Locus Sequence Typing (MLST) and Multi Locus Restriction Typing (MLRT), a less expensive and more rapid alternative to MLST. BURST (Based Upon Related Sequence Type), is a grouping algorithm which uses the halotype data generated from MLSTIMLRT to form a visual depiction of the organisms inter-relatedness. BURST and other tools can be found in the START package (Sequence Type and Recombination Tests). In addition PAUP (Phylogenetic Analysis Using Parsimony) was applied to further elucidate these organisms relatedness. PAUP uses parsimony and concatenated MLST sequence data to generate relationships.
Surveillance of the disease associated meningococcal population is hampered by the fact that only approximately 30% of the time is an organism recovered from the blood or CSF. The remaining 70% are usually assigned a capsule type by PCR with other relevant epidemiological typing data absent. MLST by nested PCR can help identify the genetic background of these organisms, but because most of the cases are infants and young children (< 4 years) it is often the case that insufficient clinical DNA extract is available. To address this problem a nested multiplex PCR to generate an MLST result from a minimal clinical sample was designed and applied to 60 clinical samples (blood1CSF DNA extracts) fi-om the 1997-1998 - 2006-2007 epidemiological years, resulting in 40 complete MLST profiles. In addition to these 40 extracts, a total of 42 disease associated serogroup C organisms were recovered during the same period. This allowed a phenotypic and genotypic comparison of pre and post vaccine era. ST-I 1 meningococci still exist in the disease associated Irish meningococcal population, but at greatly decreased numbers.
In addition to this work it was noticed at the time, that a large number of organisms recovered from the first year of the carriage survey did not give a group by PCR. A representative collection of these were also analysed by MLST/..MLRT/PAUP. Further, several possible 'capsule switch' phenotypes from the first year of the survey were investigated by MLST to find if they shared a similar genetic background.