About one in five (20%) of those who are infected with WNV will experience symptoms. These usually are described as flu-like, with fever, gastrointestinal problems, headache, and body ache as well as potentially skin rash and swollen lymph glands. These symptoms typically last for only a few days and have no long-term health impact.
Less than 1% (about 1 in 150) of those infected will develop more serious disease, in which there is brain involvement. This can lead to life-threatening encephalitis (inflammation of the brain) and/or meningitis (inflammation of the lining of the brain and spinal cord). Symptoms include high fever, extreme muscle weakness, neck stiffness, stupor, disorientation, coma, tremors, and convulsions. The fatality rate ranges from 3-15% and is highest among the elderly. Individuals with compromised immune systems also seem to be at increased risk of severe disease.
The clinical presentation of paralysis in some individuals is reminiscent of that seen in the past with the polio virus. However, WNV is not in the same family of viruses. There is no explanation for this as of yet.