Clinical Evaluation & Disease

Diagnosis & Reporting

Powassan virus disease should be considered in any person with a febrile or acute neurologic illness who has had recent likely exposure to ticks (primarily Ixodes cookei, Ix. marxi, or Ix. scapularis) in endemic areas.

In addition to other common causes of encephalitis and aseptic meningitis (e.g., herpes simplex virus and enteroviruses), other arboviruses (e.g., West Nile, St. Louis encephalitis, La Crosse, Jamestown Canyon, eastern equine encephalitis viruses) should also be considered in the differential etiology of suspected Powassan virus disease.

Powassan virus disease is a nationally notifiable condition.  All cases should be reported to local public health authorities in a timely manner. Reporting can assist local, state, and national authorities to implement control measures to reduce future infections.

Clinical Signs & Symptoms

The incubation period for Powassan virus disease ranges from 1–4 weeks. Initial symptoms include fever, headache, vomiting, and generalized weakness. The disease can progress to encephalitis, meningoencephalitis, or aseptic meningitis. Symptoms of encephalitis may include altered mental status, seizures, speech problems (aphasia, dysarthria), paresis or paralysis, movement disorders, and cranial nerve palsies.

Clinical Evaluation

Cerebrospinal fluid (CSF) findings include lymphocytic pleocytosis in the majority of patients with Powassan virus meningitis or encephalitis; neutrophils can predominate early in the disease. CSF protein is generally normal or mildly elevated, while glucose concentration is normal. Electroencephalography (EEG) in patients with Powassan virus encephalitis reveals generalized slow wave activity and results can resemble those seen in herpes simplex virus encephalitis. Magnetic resonance imaging (MRI) of the brain in patients with Powassan virus encephalitis can show hyperintensities in the superficial and deep white matter.


Approximately 10% of Powassan virus neuroinvasive disease cases are fatal and about half of survivors have long-lasting neurologic deficits, such as headaches, muscle weakness, focal paralysis, or cognitive difficulties.