title: West Nile Virus: High Transmission Rate in North-Western European Mosquitoes Indicates Its Epidemic Potential and Warrants Increased Surveillance.
abstract: BACKGROUND
  West Nile virus (WNV) is a highly pathogenic flavivirus transmitted by Culex spp. mosquitoes. In North America (NA), lineage 1 WNV caused the largest outbreak of neuroinvasive disease to date, while a novel pathogenic lineage 2 strain circulates in southern Europe. To estimate WNV lineage 2 epidemic potential it is paramount to know if mosquitoes from currently WNV-free areas can support further spread of this epidemic.
  
  
  RESULTS
  We assessed WNV vector competence of Culex pipiens mosquitoes originating from north-western Europe (NWE) in direct comparison with those from NA. We exposed mosquitoes to infectious blood meals of lineage 1 or 2 WNV and determined the infection and transmission rates. We explored reasons for vector competence differences by comparing intrathoracic injection versus blood meal infection, and we investigated the influence of temperature. We found that NWE mosquitoes are highly competent for both WNV lineages, with transmission rates up to 25%. Compared to NA mosquitoes, transmission rates for lineage 2 WNV were significantly elevated in NWE mosquitoes due to better virus dissemination from the midgut and a shorter extrinsic incubation time. WNV infection rates further increased with temperature increase.
  
  
  CONCLUSIONS
  Our study provides experimental evidence to indicate markedly different risk levels between both continents for lineage 2 WNV transmission and suggests a degree of genotype-genotype specificity in the interaction between virus and vector. Our experiments with varying temperatures explain the current localized WNV activity in southern Europe, yet imply further epidemic spread throughout NWE during periods with favourable climatic conditions. This emphasizes the need for intensified surveillance of virus activity in current WNV disease-free regions and warrants increased awareness in clinics throughout Europe.
authors.affiliation: Laboratory of Virology, Wageningen University, Wageningen, The Netherlands.
authors.forename: Jelke J
authors.initials: JJ
authors.lastname: Fros
authors.affiliation: Laboratory of Virology, Wageningen University, Wageningen, The Netherlands.
authors.forename: Corinne
authors.initials: C
authors.lastname: Geertsema
authors.affiliation: Laboratory of Entomology, Wageningen University, Wageningen, The Netherlands.
authors.forename: Chantal B
authors.initials: CB
authors.lastname: Vogels
authors.affiliation: Laboratory of Geo-information Science and Remote Sensing, Wageningen University, Wageningen, The Netherlands.
authors.forename: Peter P
authors.initials: PP
authors.lastname: Roosjen
authors.affiliation: Institut Pasteur, Department of Virology, Arboviruses and Insect Vectors, Paris, France.
authors.forename: Anna-Bella
authors.initials: AB
authors.lastname: Failloux
authors.affiliation: Laboratory of Virology, Wageningen University, Wageningen, The Netherlands.
authors.forename: Just M
authors.initials: JM
authors.lastname: Vlak
authors.affiliation: Laboratory of Entomology, Wageningen University, Wageningen, The Netherlands.
authors.forename: Constantianus J
authors.initials: CJ
authors.lastname: Koenraadt
authors.affiliation: Laboratory of Entomology, Wageningen University, Wageningen, The Netherlands.
authors.forename: Willem
authors.initials: W
authors.lastname: Takken
authors.affiliation: Laboratory of Virology, Wageningen University, Wageningen, The Netherlands.
authors.forename: Gorben P
authors.initials: GP
authors.lastname: Pijlman
mesh_terms: D000818:Animals; D002460:Cell Line; D003465:Culex / Q000821:virology*; D058872:Epidemics; D005060:Europe / Q000453:epidemiology; D006801:Humans; D011159:Population Surveillance; D012463:Saliva / Q000821:virology; D013696:Temperature; D014901:West Nile Fever / Q000453:epidemiology* / Q000821:virology*; D014902:West Nile virus / Q000145:classification / Q000302:isolation & purification*