Establishment of Culex modestus in Belgium and a glance into the virome of Belgian mosquito species

Culex modestus is a mosquito species with a relevant role in the transmission of West Nile virus and Usutu virus. Its presence has been reported across Europe, yet it is absent in Belgium. Field mosquitoes in the city of Leuven and surroundings were collected in the summer of 2019 and 2020. Species identification was performed by morphological features and partial sequences of the mitochondrial cytochrome oxidase subunit 1 (COI) gene. In order to examine the genetic structure of the Cx. modestus population found in Leuven, mitochondrial sequences of Cx. modestus mosquitoes collected in other 8 countries across Europe were retrieved from GenBank and confronted to the COI sequences from Belgian mosquitoes to construct a haplotype network. Hereby, we confirmed the new establishment of Cx. modestus in the surroundings of Leuven, Belgium. Haplotype network analysis showed that the Belgian population is rather diverse, suggesting that it must have been establish in Belgium for some time. The Belgian population was most closely related to Cx. modestus populations from the UK and Germany. The NetoVIR protocol was used to study the virome of 8 pools of mosquitoes. At least 33 eukaryotic viral species were identified. Nine (near-)complete genomes belonging to 6 viral species were identified, named Culex totivirus Leu1, Leu2 and Leu3, Alphamesonivirus Leu4, Iflavirus Leu5 and Leu6, Negevirus Leu7 and Leu8, and Rhabdovirus Leu9, all of which were closely related to known viruses. In conclusion, with the introduction of Cx. modestus in Belgium, the evaluation of its potential role in the transmission of arboviruses that could cause disease in animals and humans is necessary. Importance for non-specialistCulex modestus is a mosquito species that plays a role in nature as a bridge vector, being able to transmit pathogens between birds, as well as from birds to mammals, including humans. In Belgium, this species was considered absent. Field mosquitoes were collected in urban, peri-urban and wetland areas in the summer of 2019 and 2020 in Leuven, and morphological and molecular methods were performed to confirm the presence of Cx. modestus in this region. The ability of mosquitoes to transmit pathogens can depend on a lot of factors, one of them being the natural virus composition in their bodies. For this purpose, we aimed to identify the whole virus group harbored by Belgian mosquitoes. This could provide more insight for mosquito and, therefore, disease control. Besides, the introduction of Cx. modestus may increase the risk of disease transmission. It is advisable to implement mosquito surveillance programs monitoring this species.