Comparative Vector Competence of North American Culex pipiens and Culex quinquefasciatus for African and European Lineage 2 West Nile Viruses

Abstract

West Nile virus (WNV) is a mosquito-borne flavivirus that is phylogenetically separated into distinct lineages. Lineage 1 (L1) and lineage 2 (L2) encompass all WNV isolates associated with human and veterinary disease cases. Although L1 WNV is globally distributed, including North America, L2 WNV only recently emerged out of sub-Saharan Africa into Europe and Russia. The spread of L2 WNV throughout and beyond Europe depends, in part, on availability of competent vectors. The vector competence of mosquitoes within the Culex genus for WNV is well established for L1 WNV but less extensively studied for L2 WNV. Assessing the vector competence of North American Culex mosquitoes for L2 WNV will be critical for predicting the potential for L2 WNV emergence in North America. We address the vector competence of North American Culex pipiens and Culex quinquefasciatus for L2 WNV. Both mosquito species were highly competent for each of the L2 WNV strains assessed, but variation in infection, dissemination, and transmission was observed. An L2 WNV strain (NS10) isolated during the Greek outbreak in 2010 exhibited a reduced capacity to infect Cx. pipiens compared with other L2 WNV strains. In addition, a South African L2 WNV strain (SA89) displayed a significantly shorter extrinsic incubation period in Cx. quinquefasciatus compared with other L2 WNV strains. These results demonstrate that North American Culex mosquito species are competent vectors of African and European L2 WNV and that emergence of L2 WNV is unlikely to be hindered by poor competence of North American vectors.

Figure 1.

Vector competence of Cx. pipiens mosquitoes for L2 West Nile virus (WNV). Mosquitoes were per orally exposed to 8 log₁₀ plaque-forming unit/mL of SA89, UG09, HU04, or NS10. (A) Infection was determined as the number of WNV-positive bodies (n_i) as a function of exposed mosquitoes (n_e). (B) Dissemination was calculated as the number of WNV-positive legs (n_d) as a function of exposed mosquitoes (n_e). (C) Transmission was calculated as the percentage of WNV-exposed mosquitoes (n_e) that were also positive for virus in saliva (n_t). Bars labeled a and b are significantly different.

Figure 2.

Vector competence of Cx. quinquefasciatus mosquitoes for L2 West Nile virus (WNV). Mosquitoes were per orally exposed to 8 log₁₀ plaque-forming unit/mL of SA89, UG09, HU04, or NS10. (A) Infection was determined as the number of WNV-positive bodies (n_i) as a function of exposed mosquitoes (n_e). (B) Dissemination was calculated as the number of WNV-positive legs (n_d) as a function of exposed mosquitoes (n_e). (C) Transmission was calculated as the percentage of WNV-exposed mosquitoes (n_e) that were also positive for virus in saliva (n_t). Bars labeled a and b are significantly different.

Figure 3.

Infection rates of Cx. quinquefasciatus mosquitoes exposed to a 10-fold lower infectious dose (7 log₁₀ WNV [PFU/mL]). Mosquitoes were per orally exposed to 7 log₁₀ PFU/mL of SA89, UG09, HU04, or NS10. Infection is the number of WNV-positive bodies (n_i) over WNV-exposed mosquitoes (n_e). Bars labeled a and b are significantly different. Data are not available (NA) for mosquitoes exposed to UG09 at 14 dpi. PFU = plaque-forming unit; WNV = West Nile virus.