Powassan virus encephalitis, Minnesota, USA

Abstract

Powassan virus (POWV) is a rare tick-borne agent of encephalitis in North America. Historically, confirmed cases occurred mainly in the northeastern United States. Since 2008, confirmed cases in Minnesota and Wisconsin have increased. We report a fatal case of POWV encephalitis in Minnesota. POWV infection should be suspected in tick-exposed patients with viral encephalitis.

Figure 1

A) Noncontrast, sagittal T1-weighted magnetic resonance image of the brain of a 67-year-old woman with suspected Powassan virus encephalitis, obtained 4 days after admission. Image is notable for nonspecific signal changes within the thalami, midbrain, cerebellar vermis, and both cerebellar hemispheres. B) Noncontrast, sagittal T1-weighted magnetic resonance image of the brain obtained 8 days after patient’s admission. Changes include marked interval progression of signal abnormality involving the cerebellum, thalamus, and midbrain. Mass effect within the posterior fossa and crowding of structures at the foramen magnum are also evident. Marked dilatation of the lateral and third ventricles with acute hydrocephalus is apparent.

Figure 2

Geographic distribution of confirmed Powassan virus (POWV) infections (diagnosis made by serology, reverse transcription PCR) and counties with POWV–infected ticks in Minnesota. Data provided by the Minnesota Department of Health.