Determining the Pathogenesis and Enzootic Transmission of Usutu Virus

Usutu virus (USUV) is an emerging zoonotic virus within the Flaviviridae family that can cause neurological disease in humans and wild birds. USUV is maintained in an enzootic cycle between wild birds, primarily passerine species, and ornithophilic mosquitoes, predominantly Culex spp. mosquitoes. Since its first isolation in 1959 in South Africa, USUV has spread throughout sub-Saharan Africa and Europe. Its emergence into Europe was marked by large die-offs, or epizootics, of the Eurasian blackbird (Turdus merula), as well as an increase in human cases. This dissertation sought to understand whether USUV has evolved to become more pathogenic in humans or transmissible in birds. We compared the pathogenesis of five different USUV isolates, four recent isolates: Spain 2009, Netherlands 2016, Senegal 2003, Uganda 2012, and South Africa 1959, in an interferon α/β receptor knockout (Ifnar-/-) mouse model. We observed significant mortality, high viral levels in serum and tissues in all USUV strains except for the Netherlands 2016 strain. Eighteen non-synonymous mutations were identified throughout the genome of Netherlands 2016 strain compared to the other USUV isolates. To further understand USUV infection in wild birds, we developed a physiologically relevant model of infection using juvenile chickens. In juvenile chickens, we found that the European strains were characterized by more pathogenesis and higher viral titers in tissues compared to the African strains. This work established the first viremic bird model of USUV infection. Passerine birds have been suggested to be important for USUV maintenance, however a species competent for transmission has not been identified. We first determined that wild-caught house sparrows (Passer domesticus) and Culex quinquefasciatus mosquitoes were susceptible to Netherlands 2016 and Uganda 2012 USUV strains. Following an infectious feed to assess enzootic transmission, house sparrows were able to transmit both USUV strains to Cx. quinquefasciatus mosquitoes, with the Netherlands 2016 strain being more infectious compared to the Uganda 2012 strain. The collection of these chapters provides great insights on the pathogenesis of distinct USUV strains, disease presentation in birds, and enzootic transmssion of USUV. Additionally, they indicate that USUV emergence in the United States is entirely feasible.