Powassan Virus Infection

Powassan virus infection was confirmed serologically in three children with a clinical diagnosis of encephalitis (two) or meningoencephalitis (one). One child had a paresis and recovered but is left with spasticity in one leg; the other two children had no apparent sequelae. Only one patient had a history of a recent tick bite. Powassan hemagglutination-inhibiting and neutralizing antibodies were detected early after onset of illness. Complement-fixing antibody appeared later. Two of the patients also had virological evidence of subsequent, but closely spaced, echovirus infection. A synergistic effect may be of importance in encephalitis associated with Powassan infection. The absence of tick bites in two of the children suggests that other ways of transmission of Powassan virus must be considered.