Should I consider Powassan virus (POWV) in my patient with acute encephalitis?

POWV is a neuroinvasive arbovirus transmitted by Ixodes ticks (similar to Lyme disease). It has been reported in Canada, Russia and, increasingly, in northeastern states and Great lakes region of United States1. Although many cases occur during warmer months, nearly 30% of cases reported from New York occurred during November, December and January 2.  Rather remarkably, POWV may be transmitted within 15 minutes of a tick bite in contrast to the agent of Lyme disease which typically takes at least 48 hours2!

The most common clinical presentation is a febrile illness associated with encephalitis, meningoencephalitis or aseptic meningitis.  Seizures, focal deficits, aphasia, and dysarthria have been reported2. Typically, cerebrospinal fluid shows lymphocytic pleocytosis, normal glucose, and normal or mildly elevated protein.  Electroencephalography (EEG) reveals generalized slow wave activity and MRI of the brain may suggest microvascular ischemia or demyelinating disease in the parietal or temporal lobes. Collectively, the clinical presentation may resemble those seen in herpes simplex virus encephalitis. 

POWV disease carries 10% mortality with severe neurological sequelae in 50% of survivors4.  Despite lack of effective treatment, POWV should be considered in the differential diagnosis of acute encephalitis in endemic areas.



  1. CDC.  
  2. El Khoury MC, Camargo JF, White JL. Potential role of deer tick virus in Powassan encephalitis cases in Lyme disease-endemic areas of New York, USA. Emerg Infect Dis 2013;19:1928-33.
  3. CDC.
  4. Hermance ME, Thanagamai S. Tick saliva enhances Powassan virus transmission to the host, influencing its dissemination and the course of disease. J Virol 2015; 89:7852-60.
Should I consider Powassan virus (POWV) in my patient with acute encephalitis?

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