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West Nile Virus

Clinical Information

Information for Physicians

Transmission & Symptoms

People of all ages can get sick from West Nile virus infection, although the chance of becoming seriously ill is low. Persons over the age of 50 are generally at a higher risk for serious illness. The majority of people that are infected with West Nile virus have no symptoms or only a mild illness. Approximately 20 percent of infected people will have "flu-like" symptoms such as fever, headache, body aches, swollen glands and muscle weakness. Symptoms can last for days or weeks. In rare cases, West Nile virus can cause more serious conditions such as encephalitis (an inflammation of the brain) and/or meningitis (an inflammation of the linings of the brain and spinal cord). Signs and symptoms might include high fever, headache, confusion, disorientation, seizures, muscle weakness, and coma.

Symptoms generally occur three to 15 days following the bite of an infected mosquito. Symptoms may include high fever, headache, stiff neck, muscle weakness or paralysis, seizures, confusion and/or disorientation. Of the most serious encephalitis cases, approximately 3-15% may be fatal.

West Nile virus is not spread by person-to-person contact or directly from animals to people.

In Arizona, mosquito-borne viruses are most likely to be spread from April through October during the peak of mosquito activity.

West Nile virus activity can occur anywhere in the State.

Risk Assessment & Treatment

You do not need to be tested unless you develop West Nile virus symptoms. There is no specific treatment for West Nile virus infection. As with other viral illnesses, supportive care and symptomatic therapy are given.


There is no vaccine currently available to protect people against West Nile virus infection. However, research is underway to develop a vaccine for people. There is a vaccine available for horses. Contact your local veterinarian for more information.