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The molecular characterization of measles virus (MeV) is a valuable epidemiological tool to monitor virus transmission and to discriminate between imported and endemic infection. There has been significant immigration into Ireland in recent years and many individuals originate from regions of high measles incidence. Ireland has had a number of outbreaks of MeV which appear attributable to sub-optimal vaccine uptake and possibly imported strains as new genotypes have been identified in recent years. To ascertain any significant changes in circulating measles genotypes we investigated 65 confirmed measles cases between the years 2002 and 2007. The laboratory diagnosis of measles was confirmed by detection of measles-specific IgM in oral fluid in conjunction with a real-time polymerase chain reaction assay targeting the MeV hemagglutinin gene. Phylogenetic analysis based on the 3′ hypervariable region of the nucleoprotein gene was performed and three genotypes, all within measles clade D, were found to be circulating during this time period. In 2002 and 2003, genotype D8 (n = 2) was observed whereas genotype D7 was dominant in 2003 (n = 31). A distinct change in the circulating MeV genotype and increased genetic diversity was observed between 2004 and 2007. All cases were within genotype D4 (n = 32) but were phylogenetically distinct from each other. These data provide important epidemiologic baseline information on MeV in Ireland and facilitates detailed examination of measles transmission. J. Med. Virol. 81:125-129, 2009. © 2008 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]