A case study of Powassan virus encephalitis in an immunocompromised host.

Background: Powassan virus (POWV) is a tick-borne, neurotropic arbovirus endemic to Canada and the United States and is transmitted by Ixodes ticks, which also carry Lyme disease. It is an increasingly recognized cause of neuro-invasive infection in humans, for which treatment is supportive. Prognosis in POWV encephalitis is variable, ranging from complete recovery to death. As with other arboviral encephalitides, the mainstay of diagnosis is serological testing. However, molecular diagnostics play an increasingly important role, particularly in the case of immunocompromised hosts. Case Summary: A 47-year-old woman with rheumatoid arthritis taking methotrexate experienced a tick bite, followed by target-shaped rash. She was clinically diagnosed with Lyme disease and started on doxycycline therapy. She then developed headache, ataxia, dysarthria, and diplopia. An extensive infectious workup was notable for positive Powassan virus IgM confirmed with neutralizing antibody testing several weeks later. CSF Powassan virus PCR was negative. She was treated empirically for neuroborreliosis and later with pulse-dose steroids and intravenous immunoglobulin (IVIG). She ultimately recovered after a 10-day hospitalization and 5 days of inpatient rehabilitation. Conclusion: Physicians should be aware of POWV, an underrecognized cause of encephalitis, and of the complexities in diagnostic testing. Notably, cross-reactive serologies are common in arboviral infection. Serological testing remains first line in diagnosis but requires confirmatory testing typically performed at reference laboratories.