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Emergency and Acute Medicine – West Nile Virus
West Nile Virus is a mosquito-borne viral illness caused by an RNA virus from the Flaviviridae family. It is transmitted primarily by infected Culex mosquito during late summer and early fall. Wild birds serve as the main reservoir, and humans become incidental hosts through mosquito bites. Less commonly, transmission can occur via blood transfusion, organ transplantation, or occupational exposure. Since its introduction to the Western Hemisphere in 1999, the virus has become endemic in many regions. After recovery, immunity is generally lifelong, and recurrence is rare.
The clinical presentation of West Nile virus infection varies widely. Approximately 80% of infected individuals are asymptomatic, while about 20% develop a mild, self-limited febrile illness resembling a viral syndrome. A small proportion—roughly 1 in 150 patients—develops neuroinvasive disease, such as meningitis or encephalitis. The incubation period is typically 2–6 days but may extend up to 2–3 weeks, especially in immunocompromised individuals. Severe disease carries a mortality rate of around 7%, with higher risk in elderly patients and those with weakened immune systems.
Patients with mild disease usually present with fever, malaise, headache, anorexia, and sometimes gastrointestinal symptoms such as nausea or diarrhea. These symptoms typically resolve within a week, although fatigue and weakness may persist for several weeks. In more severe cases, neurologic involvement dominates the clinical picture. Patients may develop altered mental status, confusion, seizures, or focal neurologic deficits. A characteristic feature is profound muscle weakness or flaccid paralysis, which can resemble poliomyelitis due to involvement of anterior horn cells. Cranial nerve abnormalities, bulbar dysfunction, and movement disorders may also occur. A transient maculopapular rash may appear on the trunk and extremities.
Diagnosis relies primarily on serologic testing, with the most sensitive method being detection of IgM antibodies using MAC-ELISA in serum or cerebrospinal fluid (CSF). IgM antibodies are usually detectable within the first week of illness and may persist for months. CSF analysis in neuroinvasive disease typically shows lymphocytic pleocytosis, elevated protein, and normal glucose. Imaging such as CT is often normal, while MRI may reveal nonspecific signs of central nervous system inflammation.
Management of West Nile virus is primarily supportive, as there is currently no specific antiviral therapy or vaccine available. Initial stabilization includes airway, breathing, and circulation support, along with seizure precautions if indicated. Treatment consists of intravenous fluids for dehydration, antipyretics for fever, and analgesics for pain. In patients presenting with suspected meningitis or encephalitis, empiric antibiotics and Acyclovir may be initiated until other treatable causes, particularly herpes simplex virus infection, are excluded. No medications—including interferon, ribavirin, or corticosteroids—have proven benefit in controlled studies.
Disposition depends on disease severity. Patients with neurologic involvement, dehydration, advanced age, or immunocompromise require hospital admission, often with neurologic monitoring. Those with mild illness who can tolerate oral intake and have no signs of central nervous system involvement may be discharged with close follow-up. Long-term sequelae such as fatigue, memory impairment, weakness, and headache may persist for weeks to months, and follow-up with a neurologist is often recommended in severe cases.
A key clinical pearl is to always consider other causes of encephalitis, particularly herpes simplex virus, since it is treatable and requires early intervention.
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