West Nile virus
Medically reviewed on Jan 3, 2018
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 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 — require immediate attention.
Exposure to mosquitoes where West Nile virus exists increases your risk of getting infected. Protect yourself from mosquitoes by using mosquito repellent and wearing clothing that covers your skin to reduce your risk.
Most people infected with the West Nile virus have no signs or symptoms.
Mild infection signs and symptoms
About 20 percent of people develop a mild infection called West Nile fever. Common signs and symptoms include:
- Body aches
- Skin rash
Serious infection signs and symptoms
In less than 1 percent of infected people, the virus causes a serious neurological infection, including inflammation of the brain (encephalitis) and of the membranes surrounding the brain and spinal cord (meningitis).
Signs and symptoms of neurological infections include:
- High fever
- Severe headache
- Stiff neck
- Disorientation or confusion
- Stupor or coma
- Tremors or muscle jerking
- Partial paralysis or muscle weakness
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.
When to see a doctor
Mild symptoms of West Nile fever usually resolve on their own. For signs or symptoms of serious infection, such as severe headaches, a stiff neck, disorientation or confusion, seek medical attention right away. A serious infection generally requires hospitalization.
Typically, West Nile virus spreads to humans and animals via infected mosquitoes. Mosquitoes become infected when they feed on infected birds. You can't get infected from casual contact with an infected person or animal.
Most West Nile virus infections occur during warm weather, when mosquitos 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 — ranges from two to 14 days.
West Nile virus has occurred 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.
Other possible routes of transmission
In a few cases, West Nile virus might have spread through other routes, including organ transplantation and blood transfusion. However, blood donors are screened for the virus, substantially reducing the risk of infection from blood transfusions.
There also have been reports of possible transmission of the virus from mother to child during pregnancy or breast-feeding or exposure to the virus in a lab, but these are rare and not conclusively confirmed.
When a mosquito bites an infected bird, the virus enters the mosquito's bloodstream and eventually moves into its salivary glands. When an infected mosquito bites an animal or a human (host), the virus is passed into the host's bloodstream, where it may cause serious illness.
Most cases of West Nile virus in the United States occur June through September. Cases have been reported in all 48 lower states.
Risk of serious infection
Even if you're infected, your risk of developing a serious West Nile virus-related illness is extremely small — less than 1 percent 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:
- Age. Being older puts you at higher risk.
- Certain medical conditions. Certain diseases, such as cancer, diabetes, hypertension and kidney disease, increase your risk. So does receiving an organ transplant.
Your best bet for preventing West Nile virus and other mosquito-borne illnesses is to avoid exposure to mosquitoes and eliminate standing water, where mosquitoes breed.
- Unclog roof gutters.
- Empty unused swimming pools or empty standing water on pool covers.
- Change water in birdbaths and pet bowls regularly.
- Remove old tires or unused containers that might hold water and serve as a breeding place for mosquitoes.
- Install or repair screens on windows and doors.
To reduce your exposure to mosquitoes:
- Avoid unnecessary outdoor activity when mosquitoes are most prevalent, such as at dawn, dusk and early evening.
- Wear long-sleeved shirts and long pants when outdoors.
- Apply mosquito repellent containing an Environmental Protection Agency-registered insect repellent to your skin and clothing. Choose the concentration based on the hours of protection you need — the higher the percentage (concentration) of the active ingredient, the longer the repellent will work. Follow the directions on the package, paying special attention to recommendations for use on children.
- When outside, cover your infant's stroller or playpen with mosquito netting.
Besides conducting 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:
- Laboratory tests. If you're infected, a blood test may show a rising level of antibodies to the West Nile virus. Antibodies are immune system proteins that attack foreign substances, such as viruses.
- Lumbar puncture (spinal tap). The most common way to diagnose meningitis is to analyze the cerebrospinal fluid surrounding your brain and spinal cord. A needle inserted between the lower vertebrae of your spine is used to extract a sample of fluid for laboratory analysis. The fluid sample may show an elevated white cell count — a signal that your immune system is fighting an infection — and antibodies to the West Nile virus.
- Brain tests. In some cases, an electroencephalography (EEG) — a procedure that measures your brain's activity — or an MRI scan can help detect brain inflammation.
During a lumbar puncture (spinal tap) procedure, you typically lie on your side with your knees drawn up to your chest. Then a needle is inserted into your spinal canal — in your lower back — to collect cerebrospinal fluid for testing.
Most people recover from West Nile virus without treatment. Most severe cases require 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 more study is needed.
Preparing for an appointment
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.
What you can do
Take with you a list of the following:
- Your symptoms, including any that seem unrelated to why you're seeing a doctor
- Key personal information, including recent activities or travel to an area where West Nile virus is prevalent
- All medications, vitamins or other supplements you take, including the doses
- Questions to ask the doctor
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:
- What's likely causing my symptoms?
- What are other possible causes for my symptoms?
- What tests do I need?
- Is my condition likely temporary or chronic?
- What's the best course of action?
- What are the alternatives to the primary approach you're suggesting?
- I have other health conditions. How can I best manage them together?
- Are there restrictions I need to follow?
- Should I see a specialist?
- Are there brochures or other printed material I can have? What websites do you recommend?
Don't hesitate to ask other questions.
What to expect from your doctor
Your doctor is likely to ask you several questions, such as:
- When did your symptoms begin?
- Have your symptoms been continuous or occasional?
- How severe are your symptoms?
- What, if anything, seems to improve your symptoms?
- What, if anything, appears to worsen your symptoms?