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.