Horizontal and vertical transmission of West Nile virus genotype NY99 by Culex salinarius and genotypes NY99 and WN02 by Culex tarsalis.

Culex tarsalis is a superior horizontal and vertical vector of West Nile virus (WNV) compared with Culex salinarius. Culex salinarius transmitted WNV genotype NY99 (CT 2741-99 strain) horizontally to suckling mice at significantly lower rates than Cx. tarsalis on Days 8, 9, 10, and 12 post-infection, and Cx. salinarius transmitted WNV genotype NY99 to offspring at a lower vertical transmission infection rate than Cx. tarsalis. Culex tarsalis transmitted WNV genotypes NY99 and WN02 (CT S0084-08 strain) with equal efficiency. Daily percent horizontal transmission of genotype NY99 by Cx. tarsalis-infected per os and by intra-thoracic infection was not significantly different from daily transmission of genotype WN02 from Days 5-23 and Days 2-9 post-infection, respectively. Our findings do not support the previously published hypothesis that genotype NY99 was replaced in the New World by WN02 because of a shorter extrinsic incubation of WN02.