Differential Virulence and Host-Specific Fitness of Regionally Distinct Human-Derived Powassan Virus Lineage 2 Strains.

Powassan virus (POWV; family Flaviviridae) is a tick-borne encephalitic virus endemic to Canada, the United States, and Russia. In the United States, POWV is transmitted by ixodid ticks, and transmission foci reflect the geographic range of these vectors, primarily Ixodes scapularis. Thus, northeastern and midwestern regions of the United States contain the highest human case burdens and prevalence of infected ticks. Notably, New York (NY) and Minnesota (MN) have a long history of POWV transmission to humans. Over time, genetic divergence has occurred in these regions, giving rise to distinct midwestern and northeastern clades. Despite the established circulation of POWV, increases in reported human cases, and documented genetic distinction, an understanding of strain-specific POWV virulence is limited because of the lack of human isolates. In 2020 and 2021, two POWV strains were isolated from fatal human cases from MN (deer tick virus [DTV] MN-PV320) and NY (DTV NY21-027). Here, we provide the first characterization of geographically distinct, contemporary, human POWV isolates. Comprehensive genetic characterization was completed and phenotypic variability was determined in vitro and in vivo . Although strain fitness was similar in I. scapularis, higher mortality rates were measured in a susceptible POWV mouse model after infection with DTV NY21-027 compared with DTV MN-PV320. Genetic analysis revealed several variable amino acid substitutions, including I2173L in DTV NY21-027, which was selected for in all strains after neurological infection. These data suggest that genetic divergence of POWV strains from regionally distinct transmission foci could contribute to strain-dependent pathogenic potential in humans.