West Nile virus (WNV) is an infection that is most commonly spread by a mosquito bite. In rare cases, WNV can lead to serious complications, or death.
This infection is found worldwide. In the United States, it tends to occur during late summer and early fall.
West Nile is caused by a virus. The virus is passed through the bite of an infected mosquito. The virus can also be passed through a blood transfusion with infected blood, but this is rare.
The greatest risk factors for WNV are spending time in areas where mosquitoes are present and not using insect repellent. The risk of complications with WNV is highest in people:
- Over 50 years old
- With a weakened immune system
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Most people with WNV have no symptoms. About 20% of the people who become infected with WNV develop flu-like symptoms such as:
- Fever and chills
- Muscle pain
- Nausea and vomiting
- Swollen lymph nodes
Symptoms may appear within 2-6 days and can last from a few days to several weeks.
A small number of people with WNV will develop serious, neurological symptoms. These symptoms may include:
- High fever
- Stiff neck
- Vision loss
- Severe muscle weakness
These serious symptoms need immediate medical care.
In addition to taking your medical history and doing a physical exam, your doctor will ask you:
- What kind of symptoms you are experiencing
- Where you have been living or traveling to
- Whether you have been exposed to mosquitoes
A blood test is commonly used to confirm the diagnosis of WNV. Depending on the symptoms that you have, your doctor may order other tests, such as:
Treatment for WNV focuses on supportive care. Depending on your symptoms your doctor may recommend:
- Pain relievers
- Antiemetics to reduce nausea and vomiting
If you are dehydrated, you may need to drink an oral rehydration solution or have fluid delivered through an IV.
Treatment for severe symptoms may include:
The best preventive measure is to avoid mosquito bites. Prevention steps include:
- Avoid outdoor activities at dawn or dusk.
- Wear long pants and long-sleeve shirts when outdoors.
- Use an insect repellent with DEET.
- Repair screens to prevent mosquitoes from entering the house.
- Use proper mosquito netting at night. Look for netting treated with insecticide.
- Remove standing water to prevent mosquito breeding. Empty items like bird baths and gutters around your house.
Mosquitoes pick up the WNV by biting infected birds. If you see a dead bird, call the public health department. Do not touch the dead bird unless you are wearing disposable gloves.
West Nile virus infection. EBSCO DynaMed website. Available at:
http://www.ebscohost.com/dynamed. Updated February 1, 2013. Accessed May 21, 2013.
West Nile virus: What you need to know. Centers for Disease Control and Prevention website. Available at:
http://www.cdc.gov/westnile/index.html. Updated September 12, 2012. Accessed May 21, 2013.
10/1/2013 DynaMed Systematic Literature Surveillance
: Reimer LJ, Thomsen EK, et al. Insecticidal bed nets and filariasis transmission in Papua New Guinea. N Eng J Med. 2013;369(8):745-753.
Last reviewed December 2014 by David L. Horn, MD, FACP
Please be aware that this information is provided to supplement the care provided by your physician. It is neither intended nor implied to be a substitute for professional medical advice. CALL YOUR HEALTHCARE PROVIDER IMMEDIATELY IF YOU THINK YOU MAY HAVE A MEDICAL EMERGENCY. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with any questions you may have regarding a medical condition.
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