Abstract
Several flaviviruses outbreaks, such as Dengue, West Nile and Usutu viruses, have become more worrying due to extensive circulation, a greater associated mortality in animals, and frequent neuroinvasive infections in humans. Because of the lack of protective vaccines, targeting the mosquito vectors with insecticides is often the only affordable measure to control transmission. Several studies evidenced that insecticides exposure can modulate vector-pathogen interactions and impact the control of disease transmission. As the number of approved insecticides for public health is very limited in France and Europe, the increased risk of flavivirus transmission has become serious enough to warrant evaluation of the efficacy of insecticides. We have investigated the interactions between deltamethrin exposure and infection by Usutu virus in Culex pipiens. We evaluated whether Usutu infection influences the susceptibility of mosquito to deltamethrin. After feeding mosquito with USUV infectious blood, they were exposed to impregnated paper using WHO test tubes 7 and 13 days later. Feeding on infectious blood did not affect deltamethrin susceptibility at 7 days however it reduced the deltamethrin associated mortality at 13 days. This reduction occurs late in the extrinsic incubation period when the virus reaches the mosquito saliva and the transmission to the next host may be possible. We further analyzed the presence of Usutu virus in mosquito head and salivary glands and no difference were found between dead or surviving females suggesting. These findings indicated that feeding on blood containing USUV rather than the infection itself may have induced physiological changes that reduced deltamethrin susceptibility. Further research is ongoing to investigate the mechanisms behind the protective effect of infection against the insecticide effect and we discussed the impact on the risk to human and animal health.