This patient, who has a fever and acute flaccid paralysis of a lower extremity after outdoor activity, should be screened for the West Nile virus (WNV) antibody. Transmission is most often from an infected mosquito, and infections often occur during the summer and fall when mosquito populations are largest. Focal motor weakness is a common finding in West Nile neuroinvasive disease (WNND), either combined with meningoencephalitis or as an isolated myelitis. In its most severe form, infection of the anterior horn cells can cause a symmetric or asymmetric flaccid paralysis, analogous to that seen with polio in the prevaccination era. Respiratory failure may occur when diaphragmatic nerves are involved. The diagnosis of WNND can be confirmed through identification of the IgM antibody in the cerebrospinal fluid (CSF). Positivity for serum antibodies suggests infection but is less specific because WNV IgM remains detectable more than 1 year after acute infection. Furthermore, cross-reactivity occurs with other flaviviruses (such as Saint Louis encephalitis virus or Japanese encephalitis virus), which may produce a false-positive antibody result after natural infection or vaccination. WNV polymerase chain reaction of the CSF is of limited diagnostic utility in most patients because when symptoms become apparent, the virus has cleared the bloodstream and spinal fluid.
Eastern equine encephalitis virus infection is also caused by transmission from an infected mosquito. However, it predominantly occurs along the Atlantic and Gulf states, and encephalitic illness is characterized by fever, headache, vomiting, diarrhea, and convulsions.
La Crosse encephalitis virus infection is also transmitted by mosquito but typically occurs in the Eastern and central United States. Motor weakness is not associated with this infection, and symptoms are generally mild.
Powassan virus is spread by ticks rather than mosquitos, and infections are limited to the northeastern and north central United States. Symptoms include fever, headache, weakness, and other neurologic complications.