Powassan virus disease is caused by a tickborne flavivirus (Orthoflavivirus powassanense) naturally found in North America and Russia, named after the town of Powassan, Ontario, where a fatal pediatric encephalitis case was identified [1]. It is an RNA virus with two lineages, including the deer tick virus lineage, and is the only tick-borne flavivirus endemic to North America [1].
Disease overview
Evidence-backed information EN 5/7 applicable sections · ZH 5/7 applicable sections Updated Oct 23, 2026
Powassan virus disease is a tickborne flavivirus infection endemic to North America and Russia that can cause severe encephalitis with frequent neurologic sequelae [1]. It was first recognized in Ontario and remains the only tick-borne flavivirus with confirmed human pathogenicity in North America [1]. Human illness has been reported in the United States, Canada, and Russia [1].
Read the full clinical and epidemiological profile 6
Infection may produce fever, headache, nausea, occasional confusion, and weakness beginning one to three weeks after exposure [1]. Severe Powassan encephalitis can develop, often with nonspecific features that mimic acute disseminated encephalomyelitis and complicate diagnosis [1][2]. Long-term neurologic sequelae are common among affected persons [1].
Powassan virus is the only tick-borne flavivirus endemic in North America, with human illnesses reported in the United States, Canada, and Russia [1]. The first U.S. case was documented in New Jersey in 1970 and the first Russian case in 1978 [1]. Lineage I is associated with the Great Lakes region and lineage II with the northeastern United States [1].
The virus is transmitted through the bite of infected Ixodes ticks, with humans able to acquire infection within 15 to 30 minutes of tick attachment [1]. Lineage I is spread by Ixodes cookei and lineage II (deer tick virus) by Ixodes scapularis [1]. It is an increasingly recognized Ixodes-transmitted cause of severe encephalitis [2].
No approved human vaccine or antiviral drug exists for Powassan virus disease, so avoidance of tick bites is the primary precaution [1]. Prevention focuses on reducing exposure to Ixodes tick habitats and prompt tick removal to lower infection risk [1].
- 1 Wikipedia contributors. Powassan virus - Wikipedia [Internet]. Wikipedia. cited 23 Oct 2026. Available from: https://en.wikipedia.org/wiki/Powassan_virus
- 2 Piantadosi 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
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