The Powassan virus is a Flavivirus, related to West Nile virus, and is spread primarily by Ixodes scapularis in the United States. Disease is seen most often in the Northeast and upper Midwestern states. It may be asymptomatic or symptoms may occur after an incubation period of one week to one month after the tick bite. Symptoms may include:
- Loss of coordination
- Speech difficulties
- Seizures may be seen in meningitis and encephalitis
The CSF will have a lymphocytic pleocytosis of less than 500 white blood cells/mm3. Half of symptomatic cases have residual disease, such as chronic headaches, memory problems, and muscle wasting. Of those cases diagnosed with encephalitis, 10% are fatal. Diagnosis is mostly by PCR because cell culture is difficult and seldom productive. Diagnosis can also be made by testing neutralizing antibodies in acute and convalescent sera.