Emerging Cases of Powassan Virus Encephalitis in New England: Clinical Presentation, Imaging, and Review of the Literature

Abstract

Background: Powassan virus (POWV) is a rarely diagnosed cause of encephalitis in the United States. In the Northeast, it is transmitted by Ixodes scapularis, the same vector that transmits Lyme disease. The prevalence of POWV among animal hosts and vectors has been increasing. We present 8 cases of POWV encephalitis from Massachusetts and New Hampshire in 2013-2015.

Methods: We abstracted clinical and epidemiological information for patients with POWV encephalitis diagnosed at 2 hospitals in Massachusetts from 2013 to 2015. We compared their brain imaging with those in published findings from Powassan and other viral encephalitides.

Results: The patients ranged in age from 21 to 82 years, were, for the most part, previously healthy, and presented with syndromes of fever, headache, and altered consciousness. Infections occurred from May to September and were often associated with known tick exposures. In all patients, cerebrospinal fluid analyses showed pleocytosis with elevated protein. In 7 of 8 patients, brain magnetic resonance imaging demonstrated deep foci of increased T2/fluid-attenuation inversion recovery signal intensity.

Conclusions: We describe 8 cases of POWV encephalitis in Massachusetts and New Hampshire in 2013-2015. Prior to this, there had been only 2 cases of POWV encephalitis identified in Massachusetts. These cases may represent emergence of this virus in a region where its vector, I. scapularis, is known to be prevalent or may represent the emerging diagnosis of an underappreciated pathogen. We recommend testing for POWV in patients who present with encephalitis in the spring to fall in New England.

Keywords: New England; Powassan; deer tick virus; encephalitis.

Figure 1.

Initial T2/fluid-attenuation inversion recovery magnetic resonance images from patients 1 (A), 2 (B), 3 (C), 4 (D), 5 (E), 6 (F), and 7 (G) show varying degrees of hyperintensity in the bilateral caudate heads, lenticular nuclei, and thalami, with mass effect on the lateral ventricles seen in patients 3, 4, 6, and 7. Patient 3 (C) and patient 6 (F) also demonstrate hyperintensity throughout the cortical ribbon with sulcal effacement, which for patient 3 correlated with reversible diffusion restriction (Figure 3). No abnormal enhancement was found for any of the patients. Patient 8 had normal imaging.

Figure 2.

Selected images from 2 fatal cases of Powassan virus encephalitis. Axial T2/fluid-attenuation inversion recovery (FLAIR) (A), T1 post-contrast (B), and diffusion weighted imaging (C) images obtained from patient 1 ten days after symptom onset demonstrate bilateral cerebellar edema (A), diffuse cerebellar parenchymal and leptomeningeal enhancement (B), and multiple foci of restricted diffusion (C). Susceptibility weighted imaging demonstrated multiethnic areas of blooming artifact consistent with hemorrhage (not shown). Axial T2/FLAIR (D) and sagittal T1 (E) images obtained from patient 7 three days after prior imaging (Figure 1G) show extensive sulcal effacement and ventricular enlargement (D) with cerebellar edema and tonsillar herniation through the foramen magnum (E).

Figure 3.

Initial imaging from patient 3 shows increased signal intensity throughout the cortical ribbon on diffusion weighted imaging (DWI) (A) and decreased signal intensity on apparent diffusion coefficient (ADC) (B), demonstrating restricted diffusion. On follow-up DWI (C) and ADC (D) 9 months later, this had resolved, as had fluid-attenuation inversion recovery hyperintensities (Supplementary Figure 1).

Figure 4.

Axial diffusion weighted imaging (A) and apparent diffusion coefficient (B) imaging from patient 6 demonstrates bilateral midbrain diffusion restriction (arrows).