Targeted surveillance reveals native and invasive mosquito species infected with Usutu virus

Background: The emergence of Usutu virus (USUV) in Europe was first reported in Austria, 2001, and the virus has since spread to many European countries. Initial outbreaks are marked by a mass die-off of European blackbirds (Turdus merula) and other bird species. During outbreaks, the virus has been detected in pools of Culex pipiens mosquitoes, and these mosquitoes are probably the most important enzootic vectors. Beginning in 2017, a second wave of blackbird deaths associated with USUV was observed in eastern Austria; the affected areas expanded to the Austrian federal states of Styria in the south and to Upper Austria in the west in 2018. We sampled the potential vector population at selected sites of bird deaths in 2018 in order to identify infected mosquitoes. Results: We detected USUV RNA in 16 out of 19 pools of Cx. pipiens/Cx. torrentium mosquitoes at sites of USUV-linked blackbird mortality in Linz and Graz, Austria. A disseminated virus infection was detected in individuals from selected pools, suggesting that Cx. pipiens form pipiens was the principal vector. In addition to a high rate of infected Cx. pipiens collected from Graz, a disseminated virus infection was detected in a pool of Aedes japonicus japonicus. Conclusions: We show herein that naturally-infected mosquitoes at foci of USUV activity are primarily Cx. pipiens form pipiens. In addition, we report the first natural infection of Ae. j. japonicus with USUV, suggesting that it may be involved in the epizootic transmission of USUV in Europe. Ae. j. japonicus is an invasive mosquito whose range is expanding in Europe.