West Nile Virus

A 72-year-old presents in August with sudden weakness in both legs and no reflexes — the CT is clear, the MRI shows anterior horn cell inflammation. It looks like polio, but polio has been eliminated in the US.

West Nile virus (WNV) is a flavivirus transmitted by Culex mosquitoes, with birds as the natural reservoir and amplifying host. Humans are generally dead-end hosts for mosquito transmission, though rare non-mosquito transmission can occur through blood transfusion, organ transplant, pregnancy, or breastfeeding. WNV is the most common mosquito-borne illness in the continental United States, with peak incidence in late summer and early fall. Approximately 80% of infections are asymptomatic. About 20% develop West Nile fever: acute onset of fever, headache, body aches, fatigue, occasionally a maculopapular rash on the trunk and lymphadenopathy. This is self-limiting, typically resolving in days to weeks, though fatigue may persist for months. Less than 1% develop neuroinvasive disease, which includes three presentations: meningitis (fever, headache, nuchal rigidity), encephalitis (altered mental status, seizures, movement disorders, cranial nerve palsies), and acute flaccid paralysis (AFP — asymmetric limb weakness resembling poliomyelitis, caused by anterior horn cell destruction). AFP may be permanent. Risk factors for neuroinvasive disease: age over 60 and immunosuppression. Diagnosis: serum or CSF IgM antibody detection (present by day 3–8 of illness; IgM may persist for months, limiting its ability to distinguish recent from past infection). CSF in meningitis/encephalitis: lymphocytic pleocytosis, elevated protein. There is no specific antiviral treatment and no human vaccine. Management is entirely supportive: IV fluids, pain management, airway protection for encephalitis, and respiratory support for AFP (which can involve respiratory muscles). Prevention: DEET or picaridin repellent, eliminate standing water around homes, wear long sleeves at dusk and dawn (Culex mosquitoes feed primarily at night, unlike day-biting Aedes), install window screens. The US blood supply has been screened for WNV since 2003 via nucleic acid testing. Nursing: monitor for neurologic deterioration (level of consciousness, motor function, respiratory status including forced vital capacity), falls prevention for patients with weakness or ataxia, aspiration precautions for those with altered consciousness, rehabilitation referrals for AFP (physical and occupational therapy), and emotional support for potentially permanent disability.

Key Distinctions

Acute flaccid paralysis (AFP) in summer or fall with mosquito exposure in an endemic area should raise suspicion for West Nile — it resembles polio but polio has been eliminated in the US. Culex mosquitoes bite at NIGHT (dusk to dawn), unlike Aedes (dengue, Zika) which bite during the DAY — prevention strategies and timing differ. There is no treatment, no antiviral, and no human vaccine — nursing management is entirely supportive and rehabilitative. Most infections are asymptomatic; neuroinvasive disease is rare but can be devastating and permanent, especially in the elderly. Rare non-mosquito transmission (blood transfusion, organ transplant, vertical) is why the US blood supply is screened.

Clinical Pearl

Summer paralysis in an older adult — think West Nile before Guillain-Barré. The mosquito season, the age, and the anterior horn cell pattern tell the story.