Powassan Virus: An Emerging Arbovirus of Public Health Concern in North America

Abstract

Powassan virus (POWV, Flaviviridae) is the only North American member of the tick-borne encephalitis serogroup of flaviviruses. It is transmitted to small- and medium-sized mammals by Ixodes scapularis, Ixodes cookei, and several other Ixodes tick species. Humans become infected with POWV during spillover transmission from the natural transmission cycles. In humans, POWV is the causative agent of a severe neuroinvasive illness with 50% of survivors displaying long-term neurological sequelae. POWV was recognized as a human pathogen in 1958 when a young boy died of severe encephalitis in Powassan, Ontario, and POWV was isolated from the brain autopsy of this case. Two distinct genetic lineages of POWV are now recognized: POWV (lineage I) and deer tick virus (lineage II). Since the index case in 1958, over 100 human cases of POWV have been reported, with an apparent rise in disease incidence in the past 16 years. This recent increase in cases may represent a true emergence of POWV in regions where the tick vector species are prevalent, or it could represent an increase in POWV surveillance and diagnosis. In the past 5 years, both basic and applied research for POWV disease has intensified, including phylogenetic studies, field surveillance, case studies, and animal model development. This review provides an overview of POWV, including the epidemiology, transmission, clinical disease, and diagnosis of POWV infection. Recent research developments and future priorities with regard to the disease are emphasized.

Keywords: Ixodes scapularis; Powassan virus; deer tick virus; encephalitis; tick-borne flavivirus.

FIG. 1.

Life cycle and feeding stages of Ixodes scapularis. I. scapularis is a three-host tick that requires approximately 2 years to complete its life cycle. I. scapularis larvae typically feed in the late summer months (Year 0), promptly molting and overwintering as unfed nymphs. In May to July of the following year (Year 1), the nymphs feed and molt to adults. Adult I. scapularis begin questing from approximately October of Year 1 to April of Year 2, with replete females laying eggs in the late spring (Year 2) that hatch by August, completing the 2-year life cycle.

FIG. 2.

Timeline of POWV infection, symptoms, and diagnostic testing. Shown is the chronology of POWV infection in relation to the clinical signs and symptoms of disease. During the early, viremic phase of disease, POWV can be diagnosed by virus isolation or by detection of specific nucleic acid/viral antigen. At later stages of disease, POWV-specific IgM and IgG antibodies can be detected. POWV, Powassan virus.