Emergence of dengue virus type 1 and type 3 as dominant serotypes during 2017 in Pune and Nashik regions of Maharashtra, Western India.

India witnessed dengue outbreaks during 2017 in different parts with more than 180000 cases. There is no data on the serotypes/genotypes of dengue virus (DENV) associated with the 2017 outbreak season. The present study investigated DENV circulating in Pune and Nashik regions of Maharashtra, Western India at molecular level. IgM negative samples that were collected before 6th post onset days of illness were tested for DENV RNA and serotyped by real time RT-PCR based methods. Representative samples of each serotype were processed for virus isolation and envelope (E) gene sequencing. Among the 472 samples tested for DENV serotypes from Nashik, DENV-1 was observed in 36.2%, DENV-2 in 12.9%, DENV-3 in 35.4%, DENV-4 in 8.0%, and multiple serotypes in 7.4% of the samples respectively. In Pune region, among the 109 samples tested for DENV serotypes, DENV-1 was observed in 27.5%, DENV-2 in 11.0%, DENV-3 in 52.3%, DENV-4 in 4.6%, and multiple serotypes in 4.6% of the samples respectively. Comparison of serotype distribution from 2009 to 2017 from the Pune region revealed the emergence of DENV-3 as the dominant serotype followed by DENV-1 in 2017. In the Nashik region, both DENV-1 and DENV-3 were predominant in 2017. Phylogenetic analyses revealed co-circulation of American African (AM/AF) and Asian genotypes of DENV-1. DENV-1 Asian genotype was detected for the first time in the region. No genotype changes were observed for DENV-2 (cosmopolitan genotype), DENV-3 (genotype III) and DENV-4 (genotype I). For DENV-3, a unique amino acid substitution (I380T) was observed in the domain III of E protein of 2017 isolates and was not observed in earlier DENV-3 genotype III isolates. To conclude, the results suggest the emergence of DENV-1 with circulation of both Asian and AM/AF genotypes and DENV-3 with unique amino acid substitutions in Pune and Nashik regions. The study underscores the need for continuous molecular monitoring at a large scale to detect the changes in DENV serotypes/genotypes that might have implications for earlier prediction of dengue outbreaks and designing dengue vaccines and predicting its efficacy.