West Nile virus is an infection that usually affects birds but humans and other animals can become infected, usually after being bitten by a mosquito that previously bit an infected bird.
In the United States, West Nile virus is a seasonal epidemic that lasts from summer through fall, peaking around mid-August to early September.
Most of the time, people who are infected with West Nile virus do not develop symptoms and do not know they have been infected with the virus. Of those who do become ill, symptoms occur within two to 15 days following infection.
It is believed once a person is infected with the West Nile virus, they will have lifelong immunity and cannot become infected with the West Nile virus again.
About 25% of people infected with West Nile virus develop West Nile fever with symptoms that last from a few days to several weeks. Symptoms of West Nile fever include:
- Fever
- Headache
- Tiredness
- Back pain
- Muscle aches
- Decreased appetite
- Rash
- Nausea
- Vomiting
- Diarrhea
- Abdominal pain
- Prolonged fatigue
Fewer than one percent of those infected with West Nile virus develop severe neuroinvasive disease, in which the virus travels through the blood and infects the brain and spinal cord. Symptoms of severe neuroinvasive disease may last for several weeks and can cause long-lasting neurological abnormalities. Symptoms of severe neuroinvasive disease include:
- Headache
- Fever
- Disorientation
- Convulsions
- Muscle weakness
- Coma
- Paralysis
People at higher risk for developing severe disease include:
- People over age 50
- People with certain types of advanced cancer
- People who take immunosuppressive medication after organ transplantation
How Do You Get West Nile Virus?
The West Nile virus is a virus typically found in birds that is transmitted to people through the bite of a mosquito carrying the infection.
- While mosquito bites are the main way people get the virus, the virus can be transmitted from a blood transfusion or organ transplant, but the risk of acquiring West Nile virus from blood transfusion is very low because all blood donations are screened for the virus.
How Is West Nile Virus Diagnosed?
West Nile virus is diagnosed with a patient history of symptoms and a physical examination.
- A blood test for antibodies to West Nile virus is usually done to confirm a diagnosis. In some cases, antibody tests may be negative if a person is tested within the first week of illness and the test may need to be done again at a later date.
- Cerebrospinal fluid (the fluid that surrounds the brain) may also be tested if neuroinvasive disease caused by West Nile virus is suspected.
What Is the Treatment for West Nile Virus?
There is no specific treatment for West Nile virus. Treatment is supportive and aimed at relief of symptoms. West Nile fever generally goes away on its own.
Treatment for patients with severe meningeal symptoms may include:
- Pain medication for headaches
- Antinausea medication and fluids for nausea and vomiting
Severe disease may require hospitalization for monitoring and support.
What Are Complications of West Nile Virus?
About one percent of people who are infected with West Nile virus develop severe neuroinvasive disease. Fewer than 40% of patients with severe disease will be fully recovered at one year.
Complications of severe neuroinvasive disease include:
- Long-term neurologic abnormalities
- Muscle weakness
- Loss of concentration
- Confusion
- Lightheadedness
- Paralysis (only about one-third of patients with paralysis will fully recover)
- Death, in about 10% of cases
How Do You Prevent West Nile Virus?
There is currently no vaccine available to prevent West Nile virus infection. West Nile virus may be prevented in some cases with basic measures to prevent mosquito bites:
- Prevent mosquito bites
- Use insect repellents containing DEET, picaridin, oil of lemon eucalyptus, 2-undecanone, para-menthane-diol, or IR3535 when outdoors
- Wear long pants and long sleeves to minimize skin exposure
- Applying permethrin to clothing (not skin)
- Drain areas of standing water (e.g., buckets, bird baths, gutters, wading pools, and pet dishes), where mosquitos frequently lay eggs
- Ensure all windows and doors have good quality screens to help keep mosquito exposure to a minimum
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