Cross-Protection between West Nile Virus and Emerging Flaviviruses in Wild Birds.

West Nile virus (WNV), St. Louis encephalitis virus (SLEV), and Usutu virus (USUV) are zoonotic flaviviruses that cause neuroinvasive disease in humans and are maintained in overlapping avian-mosquito transmission cycles. West Nile virus and SLEV cocirculate in the United States, and WNV and USUV cocirculate in Europe. Cross-reactivity of immune responses against closely related flaviviruses is well documented. In birds, prior infection with WNV provides strong protection against SLEV genotype II and V infection, which may explain the decrease in SLEV circulation in the United States after WNV emergence in 1999. However, in 2015, a new SLEV genotype (III) emerged in the United States, suggesting that WNV immunity in birds may not provide cross-protection against this SLEV genotype. Here, we tested whether prior WNV infection protects birds against infection with SLEV genotype III, as well as USUV. First, we established a house sparrow (Passer domesticus) model of infection for SLEV genotype III. We then inoculated house sparrows with WNV and, 4 weeks later, challenged WNV-immune birds with SLEV genotype III or USUV. All birds were completely protected against secondary challenge, with no viremia detected. Low levels of cross-neutralizing antibodies against SLEV and USUV were found in the blood prior to secondary challenge. However, two naturally WNV-exposed house sparrows did develop SLEV genotype III and USUV viremia after inoculation. These results indicate that experimental WNV infection may protect birds against infection with SLEV genotype III and USUV; however, additional studies to investigate the role of avian immunity in flavivirus emergence are necessary.