Clinical Features
Mild Infection
Most WNV infections are mild and often clinically unapparent.
- Approximately 20% of those infected develop a mild illness (West Nile fever).
- The incubation period is thought to range from 3 to 14 days.
- Symptoms generally last 3 to 6 days.
Reports from earlier outbreaks describe the mild form of WNV infection as a febrile illness of sudden onset often accompanied by
- malaise
- headache
- nausea
- rash
- vomiting
- anorexia
- myalgia
- lymphadenopathy
- eye pain
The full clinical spectrum of West Nile fever has not been determined in the United States.
Severe Infection
Approximately 1 in 150 infections will result in severe neurological disease.
- The most significant risk factor for developing severe neurological disease is advanced age.
- Encephalitis is more commonly reported than meningitis.
In recent outbreaks, symptoms occurring among patients hospitalized with severe disease include:
- fever
- gastrointestinal symptoms
- weakness
- change in mental status
- A minority of patients with severe disease developed a maculopapular or morbilliform rash involving the neck, trunk, arms, or legs.
- Several patients experienced severe muscle weakness and flaccid paralysis.
- Neurological presentations included:
- ataxia and extrapyramidal signs
- optic neuritis
- cranial nerve abnormalities
- polyradiculitis
- myelitis
- seizures
Although not observed in recent outbreaks, myocarditis, pancreatitis, and fulminant hepatitis have been described.
Clinical Suspicion
Diagnosis of WNV infection is based on a high index of clinical suspicion and obtaining specific laboratory tests.
- WNV, or other arboviral diseases such as St. Louis encephalitis, should be strongly considered in adults >50 years who develop unexplained encephalitis or meningitis in summer or early fall.
- The local presence of WNV enzootic activity or other human cases should further raise suspicion.
- Obtaining a recent travel history is also important.
Note: Severe neurological disease due to WNV infection has occurred in patients of all ages. Year-round transmission is possible in some areas. Therefore, WNV should be considered in all persons with unexplained encephalitis and meningitis.
