West Nile virus is typically transmitted by mosquitoes. It first appeared in the US in 1999. It has been found most often in Africa, Asia, the Middle East, and Europe. Most infections with this virus cause no illness at all. However, about 1/5 of people infected with the virus suffer flu-like symptoms including:
About 1/150 people infected with the virus develop neurologic symptoms including:
West Nile infection is caused by a virus. Most cases occur after a bite from an infected mosquito. The mosquito picks up the disease from biting an infected bird. It then passes the virus on when it bites a person, horse, dog, or some other animal. An increase in dead birds may signal an increased risk for the transmission of this virus.
West Nile virus may also be passed through blood transfusions or organ transplants. Infected donors may not have any symptoms. Tests to screen blood for this virus may be used.
In one case, West Nile virus was passed through breast milk. Experts are studying this possible route. Breastfeeding women who feel ill or suspect that they have West Nile should talk with their doctors.
Being bitten by an infected mosquito poses the greatest threat. A small risk is associated with the following:
Risk factors for a more severe case of the disease include:
Most people who become infected with West Nile virus have no symptoms. About 20% develop a mild condition called West Nile fever. It lasts about 3-6 days. One in 150 people develop a serious neurologic disease. It may last for weeks. Some effects, such as fatigue, memory loss, difficulty walking, or muscle weakness may be permanent. About 12% of hospitalized patients do not survive.
The majority of cases develop in late summer and early fall.
Seek medical care immediately if you develop any symptoms.
Symptoms of West Nile fever include the following:
Symptoms of serious neurologic disease include the following:
The doctor will ask about your symptoms and medical history. A physical exam will be done. He or she may inquire about:
Some symptoms of this disease may be due to other conditions. Tests help determine the cause of the symptoms. Tests may include those listed here:
No definitive treatment exists for West Nile infection. The treatment given is supportive. Severe cases it may need a machine to help with breathing. Care includes IV fluids and preventing other infections. Two drugs are being studied to see if they can shorten the length of symptoms or decrease the disease’s severity. These drugs are:
The best preventive measure is to avoid mosquito bites. Tips to do so include the following:
Other prevention tips include the following:
Researchers are developing a vaccine for people at high risk.
Centers for Disease Control and Prevention
National Institute of Allergy and Infectious Diseases
US Food and Drug Administration
Public Health Agenncy of Canada
Centers for Disease Control and Prevention website. Available at: http://www.cdc.gov .
Glass JD, Samuels O, Rich MM. Poliomyelitis due to West Nile virus [letter]. N Engl J Med . 2002;347:1280-1281.
Petersen LR, Marfin AA. West Nile virus: a primer for the clinician. Ann Intern Med . 2002;137:173-179.
Petersen LR, Roehrig JT, Hughes JM. West Nile virus encephalitis. N Engl J Med . 2002;347:1225-1226.
Last reviewed January 2009 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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