Quantification of West Nile virus in the saliva of Culex species collected from the southern United States.

Culex quinquefasciatus, Cx. restuans, Cx. pipiens complex, and Cx. nigripalpus were collected as larvae or egg rafts from the southern USA. Adult female mosquitoes were intrathoracically inoculated with approximately 1,000 plaque-forming units of West Nile virus (WNV) and saliva was collected from them 5 days later. The amount of infectious WNV in the saliva samples was quantified by plaque assay and WNV RNA was detected by reverse transcriptase polymerase chain reaction (RT-PCR). More than 90% of the mosquitoes had either infectious virus or viral RNA in their saliva. The RT-PCR assay detected a greater percent of samples with WNV RNA than the plaque assay detected infectious virus. Pairwise comparisons revealed 6 significant differences between the 7 groups surveyed. The Cx. nigripalpus secreted lower mean amounts of WNV than 3 other groups, and a difference was found between early- and late-season Cx. quinquefasciatus collected in Louisiana.