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.

Clinical Reasoning: A man with rapidly progressive weakness and respiratory failure

“…7 Postmortem pathologic evaluation of patients with POWV infection has revealed neuronal destruction in multiple areas of the CNS, including the cortex, basal ganglia, thalami, brainstem, spinal cord, anterior horn cells, and cerebellum, which may explain the differing patterns of neurologic deficits that have been reported. 6 It is reasonable to assume that our patient's age and history of immunosuppression made him more susceptible to profound disease sequelae, given that these 2 characteristics are associated with increased severity of other flaviviridae infections, such as WNV. 9 The patient's CSF pleocytosis is not unexpected given his low dose of prednisone (5 mg) and the presence of leptomeningeal enhancement, which indicates that he can generate at least a partial immune response.…”

“…POWV infection typically presents with an initial febrile illness that can progress to encephalitis or encephalomyelitis after an incubation period of 1-5 weeks. 6,7 Prior case reports have described both flaccid and spastic paralysis in adults with POWV. [6][7][8] Around 10% of infections are fatal, with over 50% of patients having permanent neurologic deficits, including hemiplegia, ophthalmoplegia, headaches, and memory loss.…”

“…6,7 Prior case reports have described both flaccid and spastic paralysis in adults with POWV. [6][7][8] Around 10% of infections are fatal, with over 50% of patients having permanent neurologic deficits, including hemiplegia, ophthalmoplegia, headaches, and memory loss. 6 CSF analysis typically shows a neutrophil or lymphocyte pleocytosis with elevated protein.…”

“…[6][7][8] Around 10% of infections are fatal, with over 50% of patients having permanent neurologic deficits, including hemiplegia, ophthalmoplegia, headaches, and memory loss. 6 CSF analysis typically shows a neutrophil or lymphocyte pleocytosis with elevated protein. MRI findings commonly demonstrate T2 hyperintensities (with or without enhancement) in the brainstem, cerebellum, deep gray structures, and cortex, though normal brain and spine imaging have been seen in some cases.…”

“…While there have been reports of using high-dose corticosteroids and IV immunoglobulin in patients with POWV encephalitis, their effect on disease outcome is unclear. 6 POWV infection has been increasingly documented over the last decade, whether due to augmented testing for the virus or true disease emergence. A high index of suspicion is needed to diagnose POWV encephalitis.…”