West Nile virus: Viral dynamics within the avian host and host-targeted transmission control

Abstract

West Nile virus (WNV) is the leading cause of arboviral disease in the continental United States. There are no vaccines nor targeted treatments available for human use, so reducing WNV risk primarily relies on mosquito control efforts, but efficacy depends on municipal management. To develop effective control strategies, it is critical to understand relationships within the WNV enzootic cycle. To do so, we investigated viral dynamics within the house sparrow (Passer domesticus), described the development of a reservoir-targeted vaccine, and evaluated multiple formulations of ivermectin (IVM)-treated birdfeed as WNV transmission control.In chapter 2, we study WNV dynamics within the house sparrow to determine whether viral populations undergo bottlenecks within blood and upon dispersal to secondary sites. We infected house sparrows with fitness-neutral molecularly barcoded WNV (bcWNV), evaluated host competency, and analyzed barcode distribution within infected samples. We found that these birds were highly susceptible to bcWNV infection and that barcode diversity correlated strongly with total count, regardless of sample type. These results indicate that genetic bottlenecks are unlikely to influence viral population structure in the blood of the amplifying avian host, but could limit infection to secondary sites such as tissues. In chapter 3, we designed and evaluated a probiotic vaccine in wild birds as a novel reservoir-targeting WNV control strategy. A probiotic platform was selected as recombinant bacteria can be genetically modified in the laboratory, delivered orally to induce a potent mucosal immune response, lyophilized for environmental stability, and bound to palatable baits for distribution in the field. The resultant construct described here, which expressed truncated WNV non-structural protein 1 (NS1) and dendritic-cell targeting peptide, did not protect house sparrows against challenge, but it did induce NS1-recognizing antibodies in the sera of vaccinated birds, which has not been accomplished via oral delivery to passerine birds before. Though the probiotic vaccine did not prevent disease, it did induce development of WNV-recognizing antibodies via oral inoculation for the first time in passerine birds, indicating a promising path forward for future orally-delivered vaccine development. In Chapter 4, we evaluated the safety, efficacy, and utility of ivermectin (IVM)-treated bird feed as a vector control strategy by assessing its impact on multiple bird species and mosquitoes. IVM is an endectocidal drug that selectively affects invertebrates while remaining safe at high concentrations in mammals and birds. It can be delivered to Culex (Cx.) tarsalis via blood meals from birds fed IVM-treated bird feed, resulting in death upon ingestion. Here, we determined a safe and efficacious dose and formulation by evaluating several different doses in chickens, pigeons, house sparrows, and zebra finches, and feeding their blood to both laboratory-reared and field caught Cx. tarsalis to proceed to use in future field trials. Our work with bcWNV in house sparrows demonstrated that these birds can maintain highly diverse populations of neutral-fitness virus during viremia. In developing and evaluating an oral probiotic WNV vaccine construct, we demonstrated that this platform and route of administration can induce an antibody response against a specific WNV antigen in a wild bird. We also completed extensive safety and efficacy studies in several avian species and Cx. tarsalis mosquitoes to demonstrate the utility of IVM-treated feed. Taken together, the work presented here contributes to existing literature regarding the diversity of WNV infection in birds and can also inform future development of WNV transmission control strategies.