Most WNV infections
are mild and often clinically unapparent.
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
- eye pain
The full clinical
spectrum of West Nile fever has not been determined in the United States.
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:
- gastrointestinal symptoms
- 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.
and extrapyramidal signs
- cranial nerve
Although not observed
in recent outbreaks, myocarditis, pancreatitis, and fulminant hepatitis
have been described.
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.