Most people infected with WNV are asymptomatic; however, about 20–30% will have fever, head and body aches, swollen lymph nodes, and/or a rash. The incubation period is 2–14 days after the mosquito bite.
Less than 1% develop a neuroinvasive disease. The incidence of neuroinvasive WNV disease increases with age. WNV meningitis is typical of other viral meningitis cases, and symptoms are mild, including fever, headache, and neck stiffness. WNV encephalitis has symptoms of fever with altered mental status, focal neurological deficits, or tremors similar to Parkinson's disease. Older adults are more likely to develop encephalitis or meningoencephalitis and have higher case-fatality rates.
WNV can also cause flaccid paralysis, reminiscent of polio, with damage to the anterior horn cells leading to limb paresis. It can progress to respiratory paralysis, and patients may require ventilator support. Guillain-Barre, cardiac arrhythmias, hepatitis, pancreatitis, orchitis, rhabdomyolysis, and eye diseases have been reported rarely.
In 2024, there were 2,445 human cases with 165 confirmed deaths. This was a slight decrease from 2023 in which there were 182 confirmed deaths. However, this is still increased from 2022 at 93 deaths.
There is no antiviral treatment and no vaccine for WNV.
