How is the infection diagnosed?

A diagnosis of West Nile virus infection can be hard to make because the mild symptoms are much like those of many other illnesses. When symptoms become more severe, diagnosis can be easier for the doctor.

Doctors rely on a physical exam, history of symptoms, and certain lab tests to make a diagnosis in most people. Any time an older adult has a sudden onset of encephalitis-like symptoms, West Nile virus should be suspected. This is especially true if West Nile virus is known to have entered a geographic area, and if it is late summer or early fall, when mosquitoes are at their peak in most areas.

When a person is infected with West Nile virus, the body makes certain antibodies to fight the infection. These antibodies can be detected in blood tests or tests of the spinal fluid. This is the best way for a doctor to make the diagnosis. Researchers are working on faster tests to identify the West Nile virus.