A faster extrinsic incubation period could explain Usutu leading West Nile in temperate Europe

Usutu virus (USUV) is a mosquito‑borne flavivirus that has recently expanded northwards in Europe and become endemic in the UK. USUV emergence often precedes the closely related West Nile virus (WNV), potentially reflecting differences in epidemiological parameters. One key parameter is the extrinsic incubation period (EIP), the time taken for a mosquito to become infectious after a blood meal. Here we present the first quantitative estimate of the temperature dependent EIP for USUV in the vector Culex pipiens molestus . We were able to quantify the shortening of the USUV EIP from 68.06 days (95% CrI: 38.62 to 141.04) at 17°C to 12.35 days (95% CrI: 8.10 to 17.09) at 25°C. This was achieved by re-analysing laboratory data with a bespoke Bayesian model that accounted for important features of the experimental design. Under UK summer temperatures, the median EIP of USUV is 37.75% (95% CrI: 3.38 to 55.68) shorter than that of WNV, and the potential transmission season of USUV is both longer and geographically more extensive. Under RCP8.5 climate projections, WNV transmission suitability could match or exceed current USUV levels between 2055 and 2065, highlighting the future threat to the UK from emerging mosquito borne pathogens. Our findings support USUV as a precursor for WNV in Europe and provide a robust characterisation of a key epidemiological parameter of USUV, enabling accurate modelling of its transmission dynamics.