West Nile Virus (WNV) is a vector-borne flavivirus endemic to Europe, Africa, West Asia, and North America. Although it is the leading cause of mosquito-borne illness in the United States, most cases are asymptomatic, and symptomatic individuals will likely only experience subclinical flu-like symptoms, lymphadenopathy, or a petechial rash. In rare instances, it can invade the central nervous system and manifest as encephalitis and/or meningitis, causing tremor, numbness, coma, or an acute flaccid paralysis (AFP) referred to as WNV poliomyelitis-like syndrome.

The present report describes a previously healthy 42-year-old male who presented to the emergency department with acute onset back pain and difficulty walking following a 6-day history of fever without rash, cough, sick contacts, or recollection of bug bites. Physical exam revealed asymmetric weakness and absent knee jerk reflex in the left leg, with normal sensation and no signs of encephalopathy or meningeal irritation. White blood cells and protein were elevated in the CSF, and thoracic MRI indicated a T2 hyperintensity lesion from T9 to L1 primarily affecting the central gray matter. Serological tests confirmed WNV infection.

This case comprises one of exceedingly few reports of WNV complicated by acute flaccid paralysis and contributes to mounting evidence that active WNV infection can result in the death of anterior horn cells. There are several distinguishing factors, including that the spinal cord lesion occurred in the absence of any intracranial involvement or clear signs of meningitis, which are typically accompanying features. Furthermore, little from the patient’s history or presentation suggested an infectious etiology, as nearly all previous cases were preceded by rash, gastrointestinal, or genitourinary symptoms in addition to fever. In conclusion, neuroinvasive WNV may present variably and should be considered as a cause of isolated AFP, especially in the late summer months.