Powassan encephalitis

Definition

A tick-borne zoonotic disease caused by the Powassan virus (POWV), a flavivirus transmitted primarily by ixodid ticks (Ixodes scapularis, I. cookei, and related species). The virus circulates in enzootic cycles involving small mammals and ticks, with humans as incidental dead-end hosts. Infection can result in severe neuroinvasive disease including encephalitis, meningitis, or meningoencephalitis, and may co-occur with Lyme disease due to shared vector species. Incidence has increased in recent decades with range expansion of competent tick vectors.

Etymology

Named for Powassan, Ontario, where the virus was first isolated in 1958 from the brain of a fatal pediatric case.

Example

In the northeastern United States, Powassan encephalitis cases have risen concurrently with Ixodes scapularis range expansion, with transmission occurring when infected nymphal or adult ticks feed for as little as 15 minutes.

Synonyms

• POWV encephalitis
• deer tick virus encephalitis

Related Terms

• Arbovirus
• flavivirus
• Zoonosis
• vector-borne disease
• Ixodes
• Tick-borne encephalitis
• Lyme disease
• neuroinvasive disease
• enzootic cycle

Usage Notes

The term refers specifically to the clinical disease in humans; specialists distinguish the virus (POWV or deer tick virus) from the resulting pathology. Two genetic lineages exist, historically called Powassan virus (lineage I, associated with I. cookei and woodchucks) and deer tick virus (lineage II, associated with I. scapularis and white-footed mice), though both now fall under POWV. Co-infection with Borrelia burgdorferi (Lyme disease) is epidemiologically significant but does not imply synergistic pathology is established.