Culex modestus mosquitoes are considered to be a potential “bridge” vector, being able to transmit pathogens between birds as well as from birds to mammals, including humans. In Belgium, this mosquito species was considered absent until the finding of one larva in 2018 and subsequent evidence of a large population in 2019 to 2020 described here.

Arthropod-borne viruses contribute significantly to global mortality and morbidity in humans and animals. These viruses are mainly transmitted between susceptible vertebrate hosts by hematophagous arthropod vectors, especially mosquitoes. Recently, there has been substantial attention for a novel group of viruses, referred to as insect-specific viruses (ISVs) which are exclusively maintained in mosquito populations. Recent discoveries of novel insect-specific viruses over the past years generated a great interest not only in their potential use as vaccine and diagnostic platforms but also as novel biological control agents due to their ability to modulate arbovirus transmission. While arboviruses infect both vertebrate and invertebrate hosts, the replication of insect-specific viruses is restricted in vertebrates at multiple stages of virus replication. The vertebrate restriction factors include the genetic elements of ISVs (structural and non-structural genes and the untranslated terminal regions), vertebrate host factors (agonists and antagonists), and the temperature-dependent microenvironment. A better understanding of these bottlenecks is thus warranted. In this review, we explore these factors and the complex interplay between ISVs and their hosts contributing to this host restriction phenomenon.

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.