- Patients with encephalitis require close monitoring for the development of elevated intracranial pressure, seizures, and inability to protect their airway.
- Patients with severe meningeal symptoms may need pain control for headaches and antiemetic therapy and rehydration for associated nausea and vomiting.
Powassan virus (POWV) is a flavivirus spread by ticks that causes encephalitis. It was discovered in 1958 in Ontario, Canada, and between 1958 and 1998, only about 25 cases were reported. This may have been due to under-recognition, limited testing, and because the POWV lineage I is transmitted by tick species rarely encountered by humans, Ixodes cookei (American castor bean tick) and Ixodes marxi (squirrel tick). Ixodes spinipalpis and Dermacentor andersoni (Rocky Mountain wood tick) may also spread Powassan virus. In 1996, a second lineage of POWV was discovered and named deer tick virus (DTV), as it is transmitted by the more common vector of human disease, Ixodes scapularis. DTV, also known as POWV lineage II, has been reported as a cause of encephalitis. It is a Flavivirus antigenically related to POWV, with 84% of its RNA sequence being identical to POWV. Ixodes scapularis can also transmit Lyme disease, babesiosis, Borrelia miyamotoi infection, and anaplasmosis.
Cases have been reported in the United States (primarily the northeastern states and Great Lakes region), Canada, and Russia. In the United States, cases of Powassan virus human disease have been increasing, from 1 in 2004 to 49 in 2023. In 2024 (as of June 25), there were 15 cases in 8 states.
Multiple mammals act as reservoirs for the virus. Transmission of infection usually occurs from June to September.
Asymptomatic infection is thought to be common. The incubation period is 8-34 days. Few patients recall a tick bite due to the small size of Ixodes ticks. Symptomatic patients present with fever, headache, nonspecific upper respiratory symptoms, gastrointestinal complaints (including vomiting), fatigue, and weakness. If infection progresses to meningoencephalitis, then altered mental status, confusion, seizures (especially in children), speech problems, movement disorders, cranial nerve palsies, and hemiplegia may ensue.
Typical cerebrospinal fluid (CSF) analysis findings are similar to those seen in tick-borne encephalitis: lymphocytic pleocytosis (usually < 500 WBC/mm3) and normal-to-high protein levels. MRI of the brain may reveal abnormalities in the basal ganglia, cerebellum, thalamus, brain stem, cerebral cortex, and temporal lobes.
Diagnosis is made by measuring CSF or serum for POWV-specific IgM antibodies or detecting a fourfold rise of virus-specific IgG in paired acute and convalescent sera. Immunoassays for POWV-specific IgM antibodies are available commercially, through American state public health laboratories, and through the US Centers for Disease Control and Prevention (CDC).
The case fatality rate is estimated at 10%. One-third to one-half of survivors have permanent neurologic symptoms (recurring headaches, memory problems, and muscle wasting).
