Ecology of Powassan Virus in the United States

Abstract

Zoonotic viruses threaten the lives of millions of people annually, exacerbated by climate change, human encroachment into wildlife habitats, and habitat destruction. The Powassan virus (POWV) is a rare tick-borne virus that can cause severe neurological damage and death, and the incidence of the associated disease (Powassan virus disease) is increasing in the eastern United States. The mechanisms by which POWV is maintained in nature and transmitted to humans are complex and only partly understood. This review provides an overview of what is known about the vector species, vector-host transmission dynamics, and environmental and human-driven factors that may be aiding the spread of both the vector and virus.

Keywords: Powassan virus; deer tick virus; tick-borne virus; ticks.

Figure 1

Timeline of major POWV milestones.

Figure 2

The distribution of Powassan virus cases and I. scapularis. (A) The first initial case of POWV in New Jersey, US in 1970. (B) Human cases of POWV from 2004–2021 (as of 29 October 2021), reported by the CDC and ArboNet. (C) The distribution of I. scapularis based on data from the CDC. (D) The potential range expansion of POWV cases based on the distribution of I. scapularis. Red states indicate current POWV presence, yellow states designate I. scapularis distribution, and arrows show the potential direction of spread for POWV.

Figure 3

The number of human POWV cases reported from 1958–2021. From 1958–2005, only 20 cases were reported. From 2006–2020, an average of 14 cases a year were recorded [13,16]. Data for 2021 is represented up to 29 October 2021.

Figure 4

Transmission dynamics of POWV in I. cookei and I. scapularis. The maintenance of POWV-LB is purported to be between groundhogs and I. cookei; however additional mechanisms of transmission may exist, similar to DTV. I. scapularis may maintain DTV in nature through (1) cofeeding on a host, irrespective of host viremia, (2) vertical transmission, (3) transstadial transmission, (4) venereal transmission, and (5) a sylvatic cycle with an unknown reservoir. Illustrations were created using Biorender (biorender.com, accessed on 30 September 2021).

Figure 5

Life cycle of I. scapularis. (1) Female tick lays eggs in the spring. (2) The larvae hatch in the fall and feed on small mammals and birds. (3) The larvae overwinter and molt into nymphs. The nymph’s quest on medium to large mammals. (4) The nymphs molt into adults, and the adults quest on large mammals. (5) The adults overwinter, and the females lay eggs in spring and then die. Illustrations were created using Biorender (biorender.com, accessed on 30 September 2021).