Epidemiology of human influenza A and B viruses in Myanmar from 2005 to 2007.

OBJECTIVES To perform genetic analysis of influenza A and B viruses in Myanmar from 2005 to 2007 and to determine the prevalence of amantadine-resistant influenza A viruses. METHODS Phylogenies of the HA and NA genes were analyzed and mutations in M2 that confer resistance to amantadine were screened. RESULTS Influenza in Myanmar exhibited seasonality, which coincided during the rainy season from June to August. Out of 2,618 samples, 76 influenza A and 132 influenza B viruses were isolated. Phylogenetic analysis showed that in 2005, 11 A/H1N1 isolates formed one cluster with A/Solomon Islands/3/2006 and were amantadine-sensitive strains. One A/H3N2 isolate was amantadine-resistant harboring S31N mutation in M2 and possessing S193F and D225N substitutions in HA (clade N), similar to A/Wisconsin/67/2005. No viruses were isolated in 2006 due to sample storage failure. In 2007, all 64 A/H3N2 isolates were amantadine-resistant and similar to A/Brisbane/10/2007. For influenza B, 3 Yamagata-lineage and 17 Victoria-lineage isolates were detected in 2005 and 112 Victoria-lineage viruses were isolated in 2007. All Victoria-lineage isolates were reassortants possessing NA derived from the Yamagata lineage. CONCLUSION Continuous surveillance in tropical countries is important for elucidating the seasonality of influenza and determining the molecular characteristics of circulating strains.