Clinical and Post-acute Features of Powassan Virus Encephalitis in a High-volume Northeastern Hospital System
George Harrold1, Erik Klontz2, Karan Singh1, Isaac Solomon4, Jacob Lemieux3, Shibani Mukerji1, John Branda2
1Neurology, 2Pathology, 3Medicine, Massachusetts General Hospital, 4Pathology, Brigham and Women's Hospital
Objective:

To describe the clinical presentation, diagnostic features, and outcomes of Powassan virus encephalitis.

Background:

Powassan virus is an emerging cause of tick-borne encephalitis with high morbidity and mortality. The clinical spectrum of this disease remains incompletely characterized.

Design/Methods:

We performed a retrospective chart review of all adults with positive Powassan virus testing and a compatible neuroinvasive syndrome between 2016 and 2025 in a large, northeastern United States hospital system.

Results:

Thirty-eight patients met inclusion criteria. The median age was 67.5 years (range 34–86) and 15 (39%) were female. Presentation to care occurred in all months except February, with a median symptom duration of 4 days. Seventeen patients (45%) had a history of hematologic malignancy or immunosuppression. Encephalopathy was present in 34 (89%), ataxia in 19 (50%), and brain MRI abnormalities in 34 (89%), with cerebellar involvement in 25 (66%). CSF pleocytosis was present in 34/37 (92%), with a median nucleated cell count of 80 (range 1–938). Positive Powassan virus testing included serum IgM in 18 (47%), CSF IgM in 24 (63%), CSF metagenomic next-generation sequencing in 8 (21%), and CSF PCR in 4 (11%). All patients with positive CSF PCR had hypogammaglobulinemia. In-hospital mortality was 21% (8 patients), and 1-year mortality was 32% (12 patients). Nine patients (24%) developed parkinsonism, and 3 (33%) of those improved with levodopa. EMG/NCS performed in 8 patients showed radiculopathies and/or neuropathies in 5 patients.

Conclusions:

This study represents the largest known series of Powassan virus encephalitis. The disease is associated with high morbidity and mortality. Encephalopathy and ataxia are common, and cerebellar MRI findings may aid diagnosis. CSF profiles vary. IgM-based testing is standard, PCR-based testing may add value in immunocompromised patients.  Post-infectious parkinsonism and peripheral nervous system involvement are underrecognized. Clinicians should consider Powassan virus in patients with encephalitis and exposure to endemic areas.

10.1212/WNL.0000000000217283
Disclaimer: Abstracts were not reviewed by Neurology® and do not reflect the views of Neurology® editors or staff.