WNV-specific IgM, total IgG, IgG1, and IgG2a levels were determined by an ELISA using purified recombinant E protein as previously described [55] . The neutralization titer of serum antibody was determined by using a previously described plaque reduction neutralization assay [40] . Briefly, sera samples from mock or WN-TX infected mice were diluted in DMEM followed by incubation at 56uC for 30 minutes to inactivate virus and complement factors. Sera were further diluted in two-fold increments and incubated with 100 PFU of WN-TX at 37uC for 1 hour. Standard plaque assays were performed on BHK21 cells and the dilution at which 50% of plaques were neutralized was determined by comparing the number of plaques formed from WNV-infected sera samples to mock infected sera samples.