The role of Culex pipiens mosquitoes in transmission of West Nile virus in Europe

West Nile virus (WNV) is maintained in a natural transmission cycle between mosquito vectors and bird hosts. However, mosquitoes can also transmit WNV to mammals, such as humans and horses, which may result in disease. In Europe, such cases of WNV disease are yearly recurring in southern and central Europe, but have not been detected in northern Europe. The absence of WNV outbreaks in northern Europe may be explained by lowered vector competence of the main vector for WNV: the northern house mosquito, <em>Culex </em>(<em>Cx.</em>) <em>pipiens</em>. The aim of this thesis was, therefore, to investigate the role of <em>Cx. pipiens </em>mosquitoes in transmission of WNV in Europe, in order to understand differences in WNV circulation between northern and southern Europe. The species <em>Cx. pipiens </em>consists of two biotypes, <em>pipiens </em>and <em>molestus</em>, which can form hybrids. Both biotypes and their hybrids are morphologically identical but differ in their behaviour, which may have consequences for WNV transmission. In this thesis, the <em>Cx. pipiens </em>biotype composition was investigated in The Netherlands, and more broadly at the European scale. These studies revealed that both biotypes and their hybrids are present throughout The Netherlands, and that there is a shift towards relatively more biotype <em>pipiens </em>mosquitoes at northern latitudes in Europe. The next step was to assess vector competence of these northern European biotypes and their hybrids, and to make a direct comparison with vector competence of a southern European population. Both biotypes and their hybrids originating from The Netherlands are competent vectors for WNV. Interestingly, no differences in vector competence were found between a Dutch and Italian biotype <em>pipiens </em>population. However, both studies revealed that low temperatures of 18 °C are an important limiting factor for WNV transmission. A more in-depth study on the effects of WNV on the host-seeking behaviour of biotype <em>pipiens </em>mosquitoes revealed that WNV infection reduces the host-seeking response towards host odour, but that other fitness parameters (e.g. flight activity, blood-feeding, and survival) are not affected. When results from the biotype composition and vector competence studies were included in a newly developed <em>R</em>0 model, it becomes clear that biotype <em>pipiens </em>is the main contributor to WNV transmission. The results presented in this thesis show that the <em>Cx. pipiens </em>biotypes and their hybrids are present throughout The Netherlands, and that they can transmit WNV under favourable climatic conditions. The absence of WNV outbreaks in northern Europe can most likely be explained by low temperature which has a negative effect on mosquito abundance, vector competence, and the duration of the infectious period. When considering the outcomes of this thesis in the light of climate change, northern European countries such as The Netherlands should be prepared for future WNV transmission.