POW virus has been isolated from four species of tick in North America—I. cookei, Ixodes marxi, Ixodes spinipalpus, and Dermacentor andersoni—with I. cookei the most commonly involved. It is transmitted among several species of small wild mammals but is most often associated with the woodchuck. Infection rates can also be high in red and gray squirrels, Eastern chipmunks, porcupines, deer mice, voles, snowshoe hares, striped skunks, and raccoons. The virus can survive through winter in dormant ticks at various stages in their life cycle and be transmitted to mammals in the spring when the tick becomes active again. Studies in the former U.S.S.R. detected POW virus in mosquitoes and antibodies to the virus in a variety of bird species. Whether this is important in maintenance of the virus or for transmission to people is not known.

Complications of POW encephalitis include secondary bacterial infections of the respiratory and urinary tracts, neuronal injury and defects, impairment of intelligence, and psychiatric disturbances. Extrapyramidal features (especially dystonia and occasionally parkinsonism), weakness, seizure disorders, and death may occur.

The incubation period is 7 to 14 days and onset is sudden, with headache, fever, disorientation, and convulsions. Prodromal symptoms include sore throat, sleepiness, headache, and disorientation. Encephalitis is characterized by vomiting, prolonged fever or fever of variable length, respiratory distress, and lethargy. Seizures are common, and patients may then become semicomatose with some paralysis, hypotonia, spasticity, or ophthalmoplegia.