Genetic analysis of a rabies virus host shift event reveals within-host viral dynamics in a new host.

Host shift events play an important role in epizootics as adaptation to new hosts can profoundly affect the spread of the disease and the measures needed to control it. During the late 1990s, an epizootic in Turkey resulted in a sustained maintenance of rabies virus (RABV) within the fox population. We used Bayesian inferences to investigate whole genome sequences from fox and dog brain tissues from Turkey to demonstrate that the epizootic occurred in 1997 (±1 year). Furthermore, these data indicated that the epizootic was most likely due to a host shift from locally infected domestic dogs, rather than an incursion of a novel fox or dog RABV. No evidence was observed for genetic adaptation to foxes at consensus sequence level and dN/dS analysis suggested purifying selection. Therefore, the deep sequence data were analysed to investigate the sub-viral population during a host shift event. Viral heterogeneity was measured in all RABV samples; viruses from the early period after the host shift exhibited greater sequence variation in comparison to those from the later stage, and to those not involved in the host shift event, possibly indicating a role in establishing transmission within a new host. The transient increase in variation observed in the new host species may represent virus replication within a new environment, perhaps due to increased replication within the CNS, resulting in a larger population of viruses, or due to the lack of host constraints present in the new host reservoir.