Impact of West Nile Virus Dose and Incubation Period on Vector Competence of Culex nigripalpus (Diptera: Culicidae)

Female Culex nigripalpus were fed blood containing a low dose (6.3 +/- 0.01 logs plaque-forming units (PFU)/mL) or high dose (7.3 +/- 0.1 logs PFU/mL) of West Nile virus (WNV) and maintained at 28 degrees C for incubation periods (IPs) of 6 or 12 days. Vector competence was measured using rates of infection (% with WNV-positive bodies), dissemination (% infected with WNV-positive legs), and transmission (% infected with WNV-positive saliva). Infection rates were not influenced by dose or IP. Dissemination rates were significantly higher at the high dose, and this was dependent on IP. Despite 100% infection and 90% dissemination in the most permissive treatment of high dose and 12 days, only 11% transmission was observed. Virus titers in body and leg tissues were significantly lower at the low dose and the titers were not influenced by IP. We show that not all mosquitoes with infections and/or disseminated infections transmit WNV under the conditions of this test. Therefore, characterizing the transmission ability of a vector population using infection or dissemination as indicators of transmission may provide inaccurate information. The complex relationships between infection, dissemination, and transmission must be evaluated under a variety of biological and environmental conditions to begin to assess the epidemiological risk of natural mosquito populations.