Pathogenicity and virulence of Powassan virus.

Powassan viruses (POWV) are emergent tick-borne Flaviviruses that cause severe encephalitis and long-term neurologic sequelae in patients. POWVs are rapidly spread in the saliva of Ixodes tick species during a 15-minute tick bite. In recent years, there has been an increased incidence of severe POWV disease in the Northeastern United States and seroprevalence studies suggest that POWV is underreported. Concurrently, the geographic range of vector tick species has and continues to expand, suggesting that POWV incidence in the US will only increase. Despite the expanding geographic range of vector ticks and increased incidence of POWV disease in recent years, there are currently no licenced vaccines or therapeutics for treating POWV disease. Here, we review the current literature and understanding of POWV with an emphasis on POWV epidemiology, pathogenesis, virulence factors and host immune responses. We also highlight gaps in the current POWV knowledge and directions for future POWV research efforts.