BelgianCulex pipiens pipiensare competent vectors for West Nile virus but not Usutu virus
Authors: Soto Alina , Coninck Lander De , Devlies Ann-Sophie , Celine Van De Wiele , Rosas Ana Lucia Rosales , Wang Lanjiao , Matthijnssens Jelle , Delang Leen 
Published: May 2023

AbstractBackgroundWest Nile virus (WNV) and Usutu virus (USUV) are emerging arboviruses in Europe transmitted byCulexmosquitoes. In Belgium, it is currently unknown whichCulexspecies are competent vectors for WNV or USUV and if these mosquitoes carryWolbachia, an endosymbiotic bacterium that can block arbovirus transmission. The aims of our study were to measure the vector competence of BelgianCulexmosquitoes to WNV and USUV and determine if a naturally acquiredWolbachiainfection can influence virus transmission.Methodology/Principal FindingsWe captured 876 non-engorged femaleCulexmosquitoes from urban and peri-urban sites in Leuven, Belgium. We provided females with an infectious bloodmeal containing WNV lineage 2, USUV European (EU) lineage 3, or USUV African (AF) lineage 3. Blood-fed females (n=154) were incubated for 14 days at 25°C after which the body, head, and saliva were collected to measure infection (IR), dissemination (DR), and transmission (TR) rates, respectively. Mosquito species were identified by qRT-PCR or Sanger sequencing, the presence of infectious virus in mosquitoes was confirmed by plaque assays, and viral genome copies were quantified by qRT-PCR. We found thatCulex pipiens pipienswere able to transmit WNV (11% IR, 40% DR, 100% TR) but not USUV (EU lineage: 13% IR, 0% DR; AF lineage: 16% IR, 17% DR, 0% TR). In contrast,Culex modestuswas able to transmit USUV (AF lineage: 60% IR, 67% DR, 50% TR), but not WNV (0% IR). We found that the presence or absence ofWolbachiawas species-dependent and did not associate with virus transmission.Conclusions/SignificanceThis is the first report that BelgianCulexmosquitoes can transmit both WNV and USUV, forewarning the risk of human transmission. More research is needed to understand the potential influence ofWolbachiaon arbovirus transmission inCulex modestusmosquitoes.Author SummaryWest Nile virus and Usutu virus can cause seasonal epidemics in humans. They are maintained in a transmission cycle between wild birds andCulexmosquitoes, and humans that are bitten by infected mosquitoes can develop life-threatening neurological disease. CertainCulexspecies carry the symbiotic bacteriumWolbachiawhich can block virus transmission in mosquitoes. In Belgium, it is currently unknown whichCulexspecies can transmit West Nile virus and/or Usutu virus, or if they carryWolbachiabacteria. In our study, we captured wild mosquitoes from Belgium and infected them with West Nile virus or Usutu virus. We found that a common European species (Culex pipiens pipiens, the Northern House mosquito) could transmit West Nile virus, whereas a lesser known species (Culex modestus) could transmit Usutu virus.Wolbachiabacteria could be found in almost allCulex pipiens pipiens, but not inCulex modestus, suggesting thatWolbachiaprevalence is species-specific. More research is needed to understand ifWolbachiacan block virus transmission inCulex modestus. This is the first report on the ability ofCulexmosquitoes to transmit West Nile virus and Usutu virus in Belgium, forewarning the risk of transmission to humans.