"Investigations of viral genotype displacement events using in vivo superinfection and co-infection fitness assays for a vertebrate RNA virus"
Viral superinfection occurs when a host is infected with multiple, genetically distinct viruses as a result of sequential exposures to each virus. While intra-specific superinfection has been observed for a number of human viruses, publications describing controlled, laboratory experiments investigating viral superinfection in vivo are rare. We have developed a novel in vivo superinfection assay using a natural host-pathogen system: Infectious Hematopoietic Necrosis Virus (IHNV) infection of juvenile rainbow trout. A member of the rhabdovirus family, IHNV causes acute disease and significant mortality in juvenile salmon and trout species. We have undertaken controlled experiments to define the viral dynamics of superinfection of rainbow trout. The frequency of superinfection and the viral load for each isolate in mixed infections is quantified using genotype-specific RT-qPCR assays. We have begun to define a temporal window within which superinfection can occur and our results demonstrate a clear restriction of superinfection with increased time between primary and secondary exposures. Additionally, during superinfection, we observe a significant decrease in the ability of the secondary virus to replicate within the host.
Ongoing epidemic monitoring of IHNV in the field has documented three major viral genotype displacement events within the Pacific Northwest habitat of rainbow and steelhead trout. Viral displacement events are characterized by the replacement of a dominant viral genotype in a specific region with a novel genotype which, in turn, dominates the viral population for a period of time ranging from years to decades. Using our novel assays for investigating in-host viral fitness in co-infection and superinfection competitions, we will determine whether competitive fitness is correlated with displacement events for three genotype pairs involved in those events. This research will further our understanding of the importance of viral fitness in epidemic disease and whether phenotypic traits such as virulence, in-host growth kinetics, or innate immune suppression may be correlated with dominance in a viral population. Although the evolution of these pathogenic traits has severe consequences for host populations, there have been few laboratory investigations of the selective pressures involved. Thus, our novel systems for studying in vivo viral fitness will provide insight into the correlates of viral dominance and provide a foundation for studies on the impact of superinfection on clinical disease.