Powassan virus (Orthoflavivirus powassanense) is a tick-borne RNA flavivirus named after Powassan, Ontario, where it was first identified in a five-year-old boy who died of encephalitis in 1958. It is the only tick-borne flavivirus endemic in North America known to cause human disease and includes two genetic lineages, with lineage II (deer tick virus) representing the probable ancestral lineage [1].
Disease overview
Powassan virus disease
波瓦桑病毒病
Evidence-backed informationEN 5/7 applicable sections · ZH 5/7 applicable sectionsUpdated Sep 3, 2026
Powassan virus disease is a tick-borne flaviviral infection that can cause severe encephalitis and long-term neurological sequelae. The virus is maintained in nature in North America and Russia, where it circulates between Ixodes tick vectors and vertebrate hosts. No approved human vaccine or antiviral therapy exists, so prevention depends on avoiding tick bites [1].
Read the full clinical and epidemiological profile6
Infection is rarely diagnosed but, when symptomatic, typically presents 1 to 3 weeks after exposure with fever, headache, nausea, occasional confusion, and weakness, and can progress to severe Powassan encephalitis with neurologic sequelae. The clinical, laboratory, and imaging features are nonspecific and may resemble acute disseminated encephalomyelitis, which complicates diagnosis and requires specialized serologic testing available at only a few laboratories [1].
Powassan virus is endemic across parts of North America, including the Great Lakes region and the northeastern United States, and has also been reported in Canada and Russia. Human illnesses caused by both lineage I and lineage II have been confirmed in North America, with the first U.S. case recorded in 1970 in New Jersey and the first Russian case in 1978 [1].
The virus is transmitted to humans through the bite of infected Ixodes ticks, with lineage I associated with Ixodes cookei in the Great Lakes region and lineage II (deer tick virus) associated with Ixodes scapularis in the northeastern United States. Human infection can occur within 15 to 30 minutes of tick attachment, underscoring the efficiency of transmission [1].
Because no human vaccine or antiviral drug is approved, prevention of Powassan virus disease relies on avoiding tick bites, including personal protective measures and tick-bite precaution strategies in endemic areas [1].
- 1Wikipedia contributors. Powassan virus - Wikipedia [Internet]. Wikipedia. cited 4 Sept 2026.Available from: https://en.wikipedia.org/wiki/Powassan_virus
- 2Piantadosi A et al. Powassan Virus Encephalitis. Infect Dis Clin North Am. 2022 Sep. PMID: 36116842. doi: 10.1016/j.idc.2022.03.003.PubMed: https://pubmed.ncbi.nlm.nih.gov/36116842/
- A84.8
Data access
Page dataset index with source links and update metadata.
Official sourcesAuthority, cadence, notes1
Public Health Ontario IDTO Monthly
Ontario, Canada
Ontario-level current-year preliminary monthly case counts from Public Health Ontario's IDTO report.