Powassan virus

• Tick-borne viral infection that can cause encephalitis
• Closely related to deer tick virus
• Named for Pawassan, Ontario, but endemic in Eurasia and North America

Background

Microbiology

• Single-stranded positive-sense RNA virus in the genus Flavivirus, family Flaviviridae
• Two genotypes: lineage 1 and lineage 2

Epidemiology

• Transmitted by a number of Ixodes and Dermacentor ticks
  • Ixodes cookei, Ixodes scapularis, Ixodes marxi, Ixodes spinipalpus, Dermacentor andersoni, and Dermacentor variabilis
• Transmission only requires 15 minutes of tick attachment

Clinical Manifestations

• Incubation period 1 to 3 weeks
• Fever, headache, confusion, weakness, encephalopathy, focal neurological deficits, and vomiting
• Progresses to severe encephalitis

Diagnosis

• Serology with hemagglutination assay (HI) for IgM + IgG
  • Fourfold or greater increase or decrease in titre between the acute phase and convalescent phase serum is considered to be diagnostic
  • Cross-reacts with other flaviviruses, including dengue
• Molecular testing available at NML

Management

• Supportive

Prognosis

• Mortality is 10%, with survivors having permanent neurologic sequelae