Simultaneous co-circulation of two genotypes of dengue virus serotype 3 causing a large outbreak in Sri Lanka in year 2023.

As many other countries, Sri Lanka experienced a marked rise in the number of dengue cases in 2023, with an unusual pattern of disease epidemiology. This rise coincided with the emergence of dengue virus (DENV) serotype 3 in Sri Lanka as the predominant serotype after 2009. Interestingly, a discrepancy between NS1 rapid antigen test positivity and quantitative real time PCR positivity was observed, with 50% of NS1 positive samples being negative by molecular diagnostics. Following sequencing of the DENV-3 strains in 2023, we identified two DENV-3 genotypes (I and III) co-circulating. While DENV-3 genotype III was detected by the modified CDC DENV-3 primers, genotype I evaded detection due to key mutations at forward and reverse primer binding sites. The co-circulation of multiple genotypes associated with an increase in cases highlights the importance of continuous surveillance of DENVs to identify mutations resulting in non-detection by diagnostics and differences in virulence. One-sentence summary line:Co-circulation of two genotypes of the dengue virus serotype-3 (I and III) were responsible for the large dengue outbreak in Sri Lanka in 2023, with genotype I not been detected by certain PCR primers.