Powassan virus (POWV) is a Flavivirus transmitted by ticks, found in North America and in the Russian Far East.

[9] Cross-neutralization occurs among flaviviruses due to the conservation of the envelope protein; this is what contributes to the fact that the two lineages are "serologically indistinguishable.

[10] According to the last data,[13] evolutionary rate of Powassan virus is 3.3 × 10−5 nucleotide substitution per site per year (95% HPD, 2.0 × 10−5–4.7 × 10−5), which is significantly lower than values reported in the previous studies.

[9] I. scapularis is an important vector for the deer tick virus, which plays a vital role in maintaining the POWV.

[7] In Canada and the Northeastern United States Ixodes cookei is the predominant species, while I. scapularis is a significant vector in Minnesota and Wisconsin.

[18] In North America, the lineages of the POWV are maintained in three main enzootic cycles[9] involving three different tick species and their respective small to medium-sized woodland mammals.

[6] Powassan encephalitis has symptoms compatible with acute disseminated encephalomyelitis, oftentimes making it difficult to diagnose.

If not treated, symptoms could extend to meningoencephalitis, which may include: seizures, aphasia, cranial nerve palsies, paresis and altered mental status.

[7][12] Currently, the best ways to treat POWV illnesses include medications to reduce brain swelling, respiratory support and intravenous fluids.

[7] A rare case of a five-month-old Connecticut infant boy contracting Powassan virus infection was published in 2017.

He survived with normal motor and verbal development on follow-up at the age of 10 months, but a head MRI showed severely abnormal brain conditions, including scarring (gliosis) and softening (encephalomalacia) in the thalamus and basal ganglia on both sides, and volume loss and early mineralization in the left basal ganglia.

[23] In May 2022, the Connecticut Department of Public Health confirmed that a man aged in his 50s contracted Powassan virus in the state during March and was hospitalised with severe neurological symptoms, though he was later discharged from hospital to recover at home.

[24] In May 2023, the Maine Center for Disease Control and Prevention reported that a 58-year-old adult male from Sagadahoc County died from Powassan virus.

[25] On June 3, 2024, artist, author, and musician MaryAnn Harris, spouse and long-time collaborator of Charles de Lint, died after a long struggle with Powassan virus, at the age of 71.

[2] Diagnostic criteria as set by the CDC are: resides in an endemic area, reported tick exposure, and presented with fever, altered mental status, seizures and focal neurological deficits and blood, tissue or cerebrospinal fluid (CSF) are positive on Powassan IgM or Powassan PRNT tests.