A mosquito-transmitted virus causes most cases of West Nile infection. Most people infected with West Nile virus either don't develop signs or symptoms or have only minor ones, such as a fever and mild headache. However, some people develop a life-threatening illness that includes inflammation of the spinal cord or brain.
Mild signs and symptoms of a West Nile virus infection generally go away on their own. But severe signs and symptoms — such as a severe headache, fever, disorientation or sudden weakness — need immediate attention.
Exposure to mosquitoes where West Nile virus exists increases your risk of getting infected. You can lower your risk by protecting yourself from mosquitoes by using mosquito repellent and wearing clothing that covers your skin.
Most people infected with the West Nile virus have no signs or symptoms.
About 20% of people develop a mild infection called West Nile fever. Common signs and symptoms include:
In less than 1% of infected people, the virus causes a serious nervous system (neurological) infection. This may include inflammation of the brain (encephalitis) or of the membranes surrounding the brain and spinal cord (meningitis).
Signs and symptoms of neurological infections include:
Signs and symptoms of West Nile fever usually last a few days. But signs and symptoms of encephalitis or meningitis can linger for weeks or months. Certain neurological effects, such as muscle weakness, can be permanent.
Mild symptoms of West Nile fever usually resolve on their own. Seek medical attention right away if you have signs or symptoms of serious infection, such as severe headaches, a stiff neck, disorientation or confusion. A serious infection generally needs hospitalization.
West Nile virus generally spreads to humans and animals from bites of infected mosquitoes. Mosquitoes get infected and carry the virus after biting infected birds. You can't get infected from casual contact with an infected person or animal.
Most West Nile virus infections happen during warm weather, when mosquitoes are active. The incubation period — the period between when you're bitten by an infected mosquito and the appearance of signs and symptoms of the illness — generally ranges from four to 10 days.
West Nile virus has appeared in Africa, Asia, Europe and the Middle East. It appeared in the United States in the summer of 1999, and since then has been reported in every state — except Hawaii and Alaska — as well as in Canada.
In a few cases, West Nile virus might have spread through other routes, including organ transplants and blood transfusions. However, blood donors are screened for the virus, greatly reducing the risk of infection from blood transfusions.
There also have been reports of some transmission of the virus from mother to child during pregnancy or breastfeeding or exposure to the virus in a lab, but these are rare.
Most cases of West Nile virus in the United States occur June through September. Cases have been reported in all 48 lower states.
Even if you're infected, your risk of developing a serious West Nile virus-related illness is very small. Less than 1% of people who are infected become severely ill. And most people who do become sick recover fully. You're more likely to develop a severe or fatal infection based on:
Your best bet for preventing West Nile virus and other mosquito-borne illnesses is to avoid exposure to mosquitoes and remove standing water, where mosquitoes breed.
To reduce your exposure to mosquitoes:
Besides performing a physical exam, your doctor can confirm the presence of West Nile virus or a West Nile-related illness, such as meningitis or encephalitis, by performing one of the following tests:
Most people recover from West Nile virus without treatment. Most people who are severely ill need supportive therapy in a hospital with intravenous fluids and pain medication.
For mild cases, over-the-counter pain relievers can help ease mild headaches and muscle aches. Use caution when giving aspirin to children or teenagers. Children and teenagers recovering from chickenpox or flu-like symptoms should never take aspirin. This is because aspirin has been linked to Reye's syndrome, a rare but potentially life-threatening condition, in such children.
Scientists are investigating interferon therapy — a type of immune cell therapy — as a treatment for encephalitis caused by West Nile virus. Some research shows that people who receive interferon recover better than those who don't receive the drug, but further study is needed.
If you have signs and symptoms of infection of the brain or spinal cord — high fever, severe headache, stiff neck, confusion or sudden muscle weakness — see your doctor right away or go to an urgent care center.
Here's some information to help you get ready for your appointment.
Take with you a list of the following:
Take a family member or friend along, if possible, to help you remember the information you're given.
For West Nile virus, some basic questions to ask your doctor include:
Don't hesitate to ask other questions.
Your doctor is likely to ask you several questions, such as:
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