Powassan is a blood-borne virus spread by ticks. The disease is endemic to the United States, Canada, and Russia. In the United States, Powassan infection is most common in the northeastern states and the Great Lakes region. Infections tend to occur when ticks are most active: in the late spring, early summer, and mid-fall. Powassan cases are rare, but cases-per-year have increased sharply in the last decade.

Powassan virus belongs to the flavivirus family, which means it is closely related to mosquito-borne pathogens such as dengue virus, Zika virus, yellow fever virus, West Nile virus, and other serious pathogens. Powassan virus is not related to Lyme disease, which is caused by a bacterium.

Powassan is called a “neuroinvasive” disease because the virus can infect the brain (causing encephalitis) and the membranes around the brain and spinal cord (causing meningitis). Most people with the disease experience fever, headache, vomiting, and weakness/fatigue. In severe cases, people may start experiencing neurological side effects such as confusion, loss of coordination, difficulty speaking, memory problems, and seizures.

There is no vaccine to prevent Powassan virus infection. Hospitalized patients often receive breathing support, fluids, and treatments to reduce swelling of the brain.