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Powassan Virus

Powassan virus (POWV) is a tickborne flavivirus that resides in rodents such as squirrels and woodchucks. It is transmitted by Ixodes scapularis, I. cookei, and I. marxi ticks. Ixoces scapularis is the same tick that transmits Lyme disease, babesiosis and anaplasmosis. Transmission of POWV can occur as quickly as 15 minutes after tick attachment.

A median of 7 cases of POWV disease (range 1–12) are reported annually in the United States. Cases occur predominantly in the Northeast and Great Lakes regions from June to September. Incubation time varies from 8 to 34 days. Symptoms of POWV infection include fever, weakness, paralysis, somnolence, headache and seizures. Long-term sequelae are common and include hemiplegia, headaches, memory impairment, and persistent ophthalmoplegia. Approximately 10% of reported cases are fatal.

POWV disease should be considered whenever a patient in a tick-endemic area presents with symptoms of encephalitis and tests negative for the most common arboviruses. Testing for POWV infection is not usually included in arbovirus encephalitis panels and must be requested specifically.

Powassan virus can be challenging to diagnose, because direct detection of the virus in blood or CSF with PCR is possible only during the prodromal phase of illness. Patients do not often seek clinical attention during this phase. Once the illness enters the encephalitic phase, viremia is often undetectable, and diagnosis is dependent on antibody testing. Anti-POWV IgM antibodies are detectable during the first days or weeks of encephalitis. False positive IgM results may be caused by cross-reactivity with antibodies directed against other flaviviruses. A positive serum or CSF test for Powassan virus IgM antibodies should be confirmed with a test for neutralizing antibodies, such as the plaque reduction neutralization test.


Tutolo JW, et al. Powassan Virus Disease in an infant-Connecticut, 2016. MMWR Weekly April 21, 2017;66(15):408-409.

Ebel GD, Kramer LD. Short report: duration of tick attachment required for transmission of powassan virus by deer ticks. Am J Trop Med Hyg 2004;71:268–71.

Anderson JF, Armstrong PM. Prevalence and genetic characterization of Powassan virus strains infecting Ixodes scapularis in Connecticut. Am J Trop Med Hyg 2012;87:754–9.

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