After the acute infection period, birds persistently infected with West Nile virus (family Flaviviridae, genus Flavivirus, WNV) occasionally shed virus into the bloodstream, but these virions normally are inactivated by neutralizing antibody. The current work tested the hypothesis that these host neutralizing antibodies protect mosquito vectors from WNV infection and reevaluated the minimum WNV infectious dose necessary to infect Culex tarsalis Coquillett. To determine whether host antibodies protect mosquitoes from infection, Cx. tarsalis and Culex stigmatosoma Dyar were fed bloodmeals containing avian blood, WNV, and sera with or without WNV-specific neutralizing antibodies. When viral particles were completely bound by antibody, mosquitoes were protected from infection; however, when incompletely bound, WNV titers as low as 10^2.3 plaque-forming units (pfu)/ml resulted in 5% infection. These data indicated that avian antibodies were protective to mosquito vectors and were not dissociated during digestion. Because recrudescent viremias may not attain the same magnitude as initial acute viremias, Cx. tarsalis vector competence was reevaluated focusing on the fate of low-titered bloodmeals. Females were evaluated for vector competence after ingesting bloodmeals containing 10^2.2, 10^3.4, 10^4.5,10^5.5, or 10^6.5 WNV pfu/ml. Infection increased with bloodmeal titer, with 1% of the mosquitoes ingesting 10^3.4 pfu/ml and 45% of the mosquitoes ingesting 10^6.5 pfu/ml developing disseminated infections. The incomplete neutralization of recrudescent virus may be sufficient to infect a low proportion of competent blood-feeding Culex mosquitoes and perhaps allow persistently infected birds to provide a mechanism for arbovirus overwintering.