Distribution and hybridization of Culex pipiens forms in Greece during the West Nile virus outbreak of 2010.

In 2010, an outbreak of West Nile virus (WNV) infections occurred in the region of Thessaloniki, Central Macedonia, in northern Greece. During this period, Culex pipiens sensu stricto mosquitoes were found infected by WNV lineage 2. Cx. pipiens s.s. presents two distinct biological forms, denoted molestus and pipiens. Hybrids between the two forms may potentiate the accidental transmission of WNV to humans. We have genetically characterized the form composition of Cx. pipiens s.s. samples collected during the outbreak from the region of Thessaloniki, where WNV cases occurred, and from the region Schinias-Marathonas, with no reported cases at the time. Information on bird fauna was also obtained for the two regions. Application of the CQ11FL diagnostic marker revealed a 350 bp variant of the pipiens-specific allele. Sympatric pipiens and molestus populations were detected in Thessaloniki, whereas Schinias-Marathonas presented a more genetically homogenous molestus population. A pattern of asymmetric introgression between molestus and pipiens was also observed in Thessaloniki. The presence of hybrids between molestus and pipiens forms suggests a greater receptivity of the Thessaloniki region for the establishment of WNV zoonotic cycles. However, the Schinias-Marathonas region also displayed characteristics to sustain WNV transmission cycles. These observations highlight the importance of maintaining active surveillance systems in selected regions geographically located within the range of major migratory bird flyways.