Assessing vector competence of mosquitoes from northeastern France to West Nile virus and Usutu virus

West Nile virus (WNV) and Usutu virus (USUV) are two arthropod-borne viruses that circulate in mainland France. Assessing vector competence has only been conducted so far with mosquitoes from southern France while an increasingly active circulation of WNV and USUV has been reported in the last years. The main vectors are mosquitoes of the Culex genus and the common mosquito Culex pipiens . Here, we measure the vector competence of five mosquito species ( Aedes rusticus, Aedes albopictus, Anopheles plumbeus, Culex pipiens and Culiseta longiareolata ) present in northeastern France. Field-collected populations were exposed to artificial infectious blood meal containing WNV or USUV and examined at different days post-infection. We show that (i) Cx. pipiens transmitted WNV and USUV, (ii) Ae. rusticus only WNV, and (iii) unexpectedly, Ae. albopictus transmitted both WNV and USUV. Less surprising, An. plumbeus was not competent for both viruses. Combined with data on distribution and population dynamics, these assessments of vector competence will help in developing a risk map and implementing appropriate prevention and control measures. Author summary West Nile virus (WNV) and Usutu virus (USUV) are on the rise in Europe and in France. WNV is reported in France as early as the 1960s in the Camargue and USUV more recently, in 2015 in eastern France. The re-emergence of WNV infections in the Camargue is associated with an expansion towards the North which is also favorable to maintain a viral transmission cycle. USUV frequently co-circulates with WNV sharing the same mosquito vectors. Culex pipiens , able to feed on birds and humans, is considered to be the main vector in France. Our study is the first to investigate the vector competence to WNV and USUV of five different mosquito species collected in northeastern France. We ascertain that French Cx. pipiens mosquitoes are competent to both WNV and USUV. More surprisingly, the mosquito Aedes albopictus from northeastern France was able to transmit WNV and USUV. Based on our result, we propose that surveillance of mosquitoes combined with viral detections must be implemented in northeastern France to allow early viral detection and timely intervention to prevent outbreaks of these two neurological diseases. ### Competing Interest Statement The authors have declared no competing interest.