West Nile Virus Seroreversion and the Influence of Herd Immunity on Disease Risk in a Long-term Study of Free-ranging Birds

The transmission and amplification of West Nile virus (WNV) is driven by the number of susceptible avian hosts. The quantity of susceptible hosts is determined by 1) recruitment of juvenile birds during the breeding season, 2) adult birds previously unexposed to WNV, and 3) adult birds with waning antibody titers. Although rarely tested, avian herd immunity and seroreversion could have a direct impact on Culex infection rate, a proxy for the reproductive number of WNV. In this study, we utilized data from a seven year study investigating WNV transmission ecology in suburban Chicago, Illinois. We tested the hypothesis that herd immunity in the avian community has an inverse relationship with WNV infection in Culex species mosquitoes. This longitudinal study, with repeated blood samples from individual free-ranging birds, revealed a rate of antibody decay such that most birds have undetectable titers after two years. Additionally, the magnitude of the antibody response, measured by the geometric mean antibody titer, was inversely associated with WNV infection in Culex species mosquitoes suggesting that herd immunity depends on a highly protective antibody titer. These results illustrate the need to understand the dynamics of the host immune response in relation to WNV transmission. Utilizing a unique, long-term data-base, this study contributes to understanding the mechanisms of WNV amplification in the urban environment