Emerging Cases of Powassan Virus Encephalitis in New England: Clinical Presentation, Imaging, and Review of the Literature

Clin Infect Dis. 2016 Mar 15;62(6):707-713. doi: 10.1093/cid/civ1005. Epub 2015 Dec 13.


Background: Powassan virus (POWV) is a rarely diagnosed cause of encephalitis in the United States. In the Northeast, it is transmitted by Ixodes scapularis, the same vector that transmits Lyme disease. The prevalence of POWV among animal hosts and vectors has been increasing. We present 8 cases of POWV encephalitis from Massachusetts and New Hampshire in 2013-2015.

Methods: We abstracted clinical and epidemiological information for patients with POWV encephalitis diagnosed at 2 hospitals in Massachusetts from 2013 to 2015. We compared their brain imaging with those in published findings from Powassan and other viral encephalitides.

Results: The patients ranged in age from 21 to 82 years, were, for the most part, previously healthy, and presented with syndromes of fever, headache, and altered consciousness. Infections occurred from May to September and were often associated with known tick exposures. In all patients, cerebrospinal fluid analyses showed pleocytosis with elevated protein. In 7 of 8 patients, brain magnetic resonance imaging demonstrated deep foci of increased T2/fluid-attenuation inversion recovery signal intensity.

Conclusions: We describe 8 cases of POWV encephalitis in Massachusetts and New Hampshire in 2013-2015. Prior to this, there had been only 2 cases of POWV encephalitis identified in Massachusetts. These cases may represent emergence of this virus in a region where its vector, I. scapularis, is known to be prevalent or may represent the emerging diagnosis of an underappreciated pathogen. We recommend testing for POWV in patients who present with encephalitis in the spring to fall in New England.

Keywords: New England; Powassan; deer tick virus; encephalitis.

Publication types

  • Case Reports
  • Review

MeSH terms

  • Acyclovir / therapeutic use
  • Adult
  • Aged
  • Aged, 80 and over
  • Animals
  • Antibodies, Viral / cerebrospinal fluid
  • Antiviral Agents / therapeutic use
  • Brain / diagnostic imaging
  • Brain / pathology
  • Brain / virology
  • Encephalitis Viruses, Tick-Borne* / drug effects
  • Encephalitis Viruses, Tick-Borne* / immunology
  • Encephalitis Viruses, Tick-Borne* / pathogenicity
  • Encephalitis, Tick-Borne / diagnosis
  • Encephalitis, Tick-Borne / diagnostic imaging*
  • Encephalitis, Tick-Borne / epidemiology*
  • Encephalitis, Tick-Borne / virology
  • Female
  • Flavivirus* / drug effects
  • Flavivirus* / immunology
  • Flavivirus* / pathogenicity
  • Humans
  • Ixodes / virology
  • Magnetic Resonance Imaging
  • Male
  • Massachusetts / epidemiology
  • Meningitis, Bacterial / drug therapy
  • Middle Aged
  • New Hampshire / epidemiology
  • Prevalence
  • Seasons
  • United States / epidemiology
  • Young Adult


  • Antibodies, Viral
  • Antiviral Agents
  • Acyclovir