Powassan virus (POWV) is a tick‑borne, positive‑sense RNA virus in the genus Flavivirus. Its genome is approximately 11 kilobases and encodes a single polyprotein that is cleaved into three structural proteins—capsid (C), precursor membrane (prM/M) and envelope (E)—and seven nonstructural proteins. The virion is enveloped, with icosahedral symmetry and glycosylated E proteins on its surface.
Genome and Virology
Two genetic lineages of Powassan virus circulate: lineage I, associated with the woodchuck tick Ixodes cookei, and lineage II, often called deer tick virus, associated with Ixodes scapularis. Both lineages share a similar genome organisation and replication strategy. The genomic RNA bears a type I cap at the 5′ end and lacks a poly(A) tail. Upon entry into a host cell, the genome is translated at rough endoplasmic reticulum membranes to produce a polyprotein, which is processed by host signal peptidase and the viral NS2B‑NS3 serine protease into structural and nonstructural components. Replication takes place in membrane invaginations derived from the endoplasmic reticulum, where the NS5 RNA-dependent RNA polymerase synthesises negative-sense and positive-sense strands. Virus assembly involves budding into the endoplasmic reticulum and maturation through the Golgi, during which prM is cleaved to M. Transmission occurs transovarially and transstadially in ticks, ensuring maintenance in tick populations. Powassan virus replicates in a range of vertebrate cells, including neurons, endothelial cells and fibroblasts, contributing to its neurotropism.
Outbreaks and Clinical Insights
POWV is endemic to North America and parts of Russia. In the United States and Canada, cases are associated primarily with wooded habitats where Ixodes ticks feed on small mammals such as woodchucks and white‑footed mice. Human disease is uncommon but has increased in the past two decades, with about 20 to 40 cases reported annually, mainly in the northeastern and upper midwestern states. Infection can occur after a short period of tick attachment, unlike Lyme disease. Clinical presentation ranges from febrile illness to severe meningoencephalitis, with case fatality around 10% and long‑term neurological deficits in up to half of survivors. No specific antiviral therapy or vaccine exists; treatment is supportive. Prevention focuses on avoiding tick bites, using repellents, performing tick checks and managing rodent and deer populations.
Powassan virus highlights the health risks posed by emerging tick-borne flaviviruses, particularly as climate and ecological changes expand the range of tick vectors.
Related Terms: Tick-borne encephalitis virus, West Nile virus, Jamestown Canyon virus, Omsk hemorrhagic fever virus, Flavivirus