Determinants of Usutu virus bird-to-mosquito transmission

Project Summary West Nile virus (WNV), St. Louis encephalitis virus (SLEV), and Usutu virus (USUV) are closely-related mosquito-borne viruses that cause neuroinvasive disease in humans. All three viruses are maintained in overlapping bird/mosquito cycles, with spillover to humans. The introduction of WNV into the U.S. in 1999 quickly displaced the previously-circulating SLEV due to cross-reactive immune responses in birds. However, a novel genotype of SLEV recently emerged in the U.S., causing an outbreak in 2015, and we have found that WNV immune responses in birds do not neutralize the novel SLEV genotype in vitro. USUV recently emerged in Europe, where it has been introduced at least three times from Africa. We have found that House sparrows (Passer domesticus) and Culex quinquefasciatus mosquitoes, which are competent for WNV and SLEV, are also susceptible to USUV. The long-term goal of this project is to understand the factors that influence novel flavivirus emergence in order to predict outbreaks in humans. The objectives of this study are to identify competent North American avian and mosquito species for USUV and to determine the role of WNV immunity in birds on SLEV and USUV emergence. The hypothesis is that the novel SLEV genotype and USUV have established or have the potential to establish transmission cycles in North America due to the competence of local birds and mosquitoes and the evasion of WNV immunity in birds, thus leading to an increased risk of neuroinvasive disease in humans. Two specific aims will address this hypothesis: 1) Determine whether North American passerines and mosquitoes are competent for USUV; and 2) Assess the influence of previous exposure to WNV on SLEV and USUV infection in birds. In the first aim, we will trap three species of wild birds to determine the susceptibility of local birds to USUV. We will quantify the infectious threshold of USUV for bird-to-mosquito transmission by feeding mosquitoes on infected birds and then measure infectious virus in the expectorate of infected mosquitoes. In the second aim, the effect of WNV pre-exposure on subsequent infection by the novel SLEV genotype and USUV will be evaluated in birds. Sera from inoculated birds will be tested for in vitro neutralizing activity against all three viruses to determine the capacity for diagnostically identifying flavivirus infections using serology. The research proposed here is innovative because it investigates a novel SLEV genotype and USUV, which are neglected, emerging viruses, using a novel avian experimental model. Upon successful completion of the proposed research, the anticipated contribution of this work will be the identification of USUV reservoir and vector species and factors that influence enzootic transmission of SLEV and USUV. This contribution is expected to be significant because understanding the transmission cycle of emerging flaviviruses will lead to the ability to predict disease in humans.