First Report of Dengue Virus in Sierra Leone: Implications for Arbovirus Surveillance and Control

Abstract Dengue is the leading mosquito-borne viral cause of human illness and death. More than four billion people globally live at risk of Dengue Virus (DENV) infection and most infections are asymptomatic or present with a non-specific febrile illness. In this report the first laboratory-confirmed dengue in Sierra Leone was identified one month after the launch of the national Syndromic Sentinel Surveillance Strategy (4S). A febrile adult tested RT-PCR–positive; we generated a near-complete genome assigned to DENV-2 genotype II, lineage F.1.1. Phylogenetically, the Sierra Leone genome formed a well-supported sister lineage with a 2024 USA genome, both nested within, but clearly diverged from, Indian sequences (2021–2022) and distinct from the Réunion clade. The degree of divergence is incompatible with a recent or direct import from South Asia lineages. It is more consistent with diversification in an undersampled Indian-Ocean/South Asia network or via unsampled intermediates outside Asia. No canonical NS4B resistance substitutions were detected. With a single Sierra Leone genome, the source and extent of local transmission remain unresolved. These findings underscore the benefits of integrating differential diagnostics and genomics into routine febrile-illness care and sustaining regional arboviral surveillance.