West Nile Virus

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.