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West Nile Virus: Symptoms, Transmission, Prevention, and CDC Guidance

West Nile Virus (WNV) is the leading cause of mosquito-borne disease in the continental US. Most infections are asymptomatic; severe neuroinvasive disease occurs in <1%.

Transmission Cycle

  • Primary vectors: Culex species mosquitoes
  • Reservoir hosts: Birds (corvids, raptors highly susceptible)
  • Dead-end hosts: Humans, horses (insufficient viremia for mosquito transmission)
  • Season: Late spring → fall; peak August–September
  • Geography: All 48 contiguous states; highest in Plains, Mountain, Southern states

Clinical Spectrum

PresentationProportionFeatures
Asymptomatic~80%No symptoms
West Nile Fever~20%Fever, headache, myalgia, arthralgia, rash, fatigue, GI symptoms; weeks–months recovery
Neuroinvasive Disease<1%Encephalitis, meningitis, acute flaccid paralysis (AFP); 10% case-fatality; long-term sequelae common

Diagnosis

  • Serology: IgM antibody in serum/CSF (gold standard); cross-reactivity with other flaviviruses
  • PCR: CSF/blood early in illness; low sensitivity
  • Reporting: Nationally notifiable; report to state/local health department

Treatment

  • Supportive care only: No specific antiviral
  • Neuroinvasive: Hospitalization, IV fluids, respiratory support, seizure management, rehab
  • Investigational: IVIG, corticosteroids, interferon (insufficient evidence)

Prevention (No Human Vaccine Available)

  1. Insect repellent: EPA-registered (DEET, picaridin, IR3535, oil of lemon eucalyptus, PMD, 2-undecanone)
  2. Protective clothing: Long sleeves/pants; permethrin-treated gear
  3. Mosquito-proof home: Screens, AC, eliminate standing water (weekly: tires, buckets, birdbaths, gutters, pool covers)
  4. Peak hours caution: Dusk to dawn (Culex feeding)
  5. Community control: Larviciding, adulticiding, surveillance programs

Special Populations

  • Immunocompromised: Higher neuroinvasive risk
  • Pregnancy: Rare vertical transmission; standard precautions
  • Blood/organ donation: Screening implemented 2003

Seek medical care for severe headache, high fever, neck stiffness, confusion, muscle weakness, or paralysis during WNV season.

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