West Nile virus

West Nile virus is a single-stranded, positive-sense RNA arbovirus in the Flavivirus genus (same family as Dengue, Zika, and Yellow Fever viruses). It is maintained in nature through a bird-mosquito-bird cycle, with Culex species mosquitoes as the primary vector; humans and horses are incidental, dead-end hosts (no significant human-to-human transmission except via blood transfusion, organ transplant, or transplacentally).

Pathophysiology: After mosquito inoculation, the virus replicates locally and disseminates hematogenously, potentially crossing the blood-brain barrier to infect neurons, particularly in the brainstem and anterior horn of the spinal cord.

Clinical presentation: ~80% of infections are asymptomatic. Symptomatic cases typically present with West Nile fever—an acute febrile illness with headache, myalgias, and sometimes a maculopapular rash. Less than 1% develop neuroinvasive disease: meningitis, encephalitis, or acute flaccid paralysis (asymmetric, resembling poliomyelitis due to anterior horn cell destruction). Risk factors for severe/neuroinvasive disease include age >60 and immunosuppression.

Diagnosis: Serology (IgM ELISA) on serum or CSF is the primary diagnostic tool; CSF typically shows lymphocytic pleocytosis. PCR can be used but viremia is often brief and low-titer by the time of presentation. MRI may show T2/FLAIR hyperintensities in the thalamus, basal ganglia, and brainstem.

Treatment: Supportive care only; no specific antiviral therapy or approved vaccine exists for humans (a veterinary vaccine exists for horses).

Board relevance: Classic high-yield associations include flaccid paralysis resembling polio, elderly patients with encephalitis in summer/fall (mosquito season), and the differential diagnosis of viral encephalitis (along with HSV, which characteristically involves the temporal lobes).