What are the symptoms?

About 80% of people who are infected with West Nile virus have no symptoms.1 When symptoms do appear, they develop 3 to 14 days after infection and include fever, tiredness, headache, and body aches, occasionally with skin rash and swollen lymph nodes. Symptoms usually last 3 to 6 days in people who have a mild infection. With more severe infections, symptoms can last from weeks to months.

On rare occasions the infection affects the brain or spinal cord, sometimes causing encephalitis, meningitis, or myelitis. Symptoms may include headache, high fever, stiff neck, reduced attention to surroundings, disorientation, tremors, convulsions, muscle weakness or paralysis, and coma.

In rare cases, West Nile virus can cause death.

How is West Nile virus diagnosed?

If a health professional thinks you may have West Nile virus, he or she will take a medical history, asking questions such as when you were bitten by a mosquito and what symptoms you have.

If you have symptoms of West Nile encephalitis, your blood will be tested for antibodies to the virus-specifically, immunoglobulin M (IgM) and immunoglobulin G (IgG) antibodies-a sign that you have been infected. If the first test shows West Nile virus infection, another test may be done 2 weeks later to look for increasing antibodies.

Your health professional may do a lumbar puncture, also called a spinal tap, to look for antibodies and signs of infection in the cerebral spinal fluid, which surrounds the brain and spinal cord. If antibodies are found, a test will be done to detect the virus’s genetic material.

A magnetic resonance imaging (MRI) test may be done to identify encephalitis.

How is it treated?

No treatment is needed for mild West Nile virus infection, and no specific treatment is available for severe infection. Treatment involves supportive care in a hospital to help the body fight the illness on its own. Supportive care is often used when no treatment exists for an illness.

Supportive care can include intravenous (IV) fluids, help with breathing (using a ventilator), and prevention of secondary infections, such as pneumonia.