ABSTRACT
Isolates and/or antibodies to Powassan (POW) virus have been documented most extensively in mammals but also in birds, amphibians, and reptiles. There is no evidence currently available to suggest strain variation among POW virus isolates and few studies to determine potential strain variation have been published. Human infections with POW virus, both symptomatic and asymptomatic, have been encountered in the U.S.S.R. POW virus has been demonstrated in the US and Canada and its occurrence in Mexico has been suggested. North American studies have clearly implicated mammals, particularly of the order Ro-dentia and Carnivora, as the vertebrate hosts of POW virus. Transstadial transmission of POW virus allows for maintenance of virus throughout the tick life cycle. POW virus has been isolated from ticks of the genera Dermacentor, Haemaphysalis, and Ixodes, all of which are hard ticks belonging to the family Ixodidae. The suggestion has been made that POW virus enters the U.S.S.R. in migratory birds from Asia.
