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Scientists don't know how West Nile virus (WNV) got to the United States. Theories range from the sinister, such as smuggled birds, to the mundane, such as a sick person coming into the country.
No one really knows how WNV got to the United States. One thing is for certain. It's here to stay.
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2006 West Nile Virus Activity in the United States
(Reported to CDC as of May 1, 2007) |
WNV is a potentially serious illness. Experts believe WNV is established as a seasonal epidemic in North America that flares up in the summer and continues into the fall. The WNV first appeared in New York in 1999. The Centers for Disease Control and Prevention (CDC) made the link between the WNV found in birds in New York and the ongoing human encephalitis outbreak in the area. This was after identifying a West Nile-like virus in a tissue sample from a New York City man who died from encephalitis, which is inflammation of the brain. Since 1999, the case counts in the United States grew from 62 disease cases to 3,000 cases in 2005 with 119 deaths.
There is no direct person to person transmission, however WNV can be transmitted through blood transfusion or organ transplantation from infected donors (see below map). People get the virus only from the bite of a mosquito, which became infected by feeding on birds that have the virus. Elderly people, young children and people with weakened immune systems are most vulnerable to the disease.
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2006 West Nile Virus
Viremic Blood Donor
Activity in the United States
(Reported to CDC as of May 1, 2007) |
Not everyone bitten by an infected mosquito becomes sick. About 80 percent of people infected with WNV will not show any symptoms. The remaining 20 percent usually have mild symptoms, including fever, headache, and body aches, nausea, vomiting, and sometimes swollen lymph glands or a skin rash on the chest, stomach and back. Symptoms typically last a few days and don't require medical intervention.
About one in 150 people infected with WNV develop severe illness, with symptoms of high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis. These symptoms may last several weeks, and neurological effects may be permanent. Although illness from West Nile is usually mild, serious illness and death are possible, particularly for anyone older than 50. Less than 1 percent of people who are bitten by mosquitoes develop any symptoms of the disease and relatively few mosquitoes actually carry WNV.
When a person goes to see a health care provider and has symptoms of WNV, a specimen may be sent to a laboratory to find out if the person has the disease. The lab checks for antibodies to the virus (the body's response to infection). The results are sent to the doctor. WNV does not have a specific treatment, so the diagnosis will not necessarily change the way the person is treated, but a diagnosis will let the doctor know that no more investigation is needed as to what is causing the symptoms. Severe cases might require hospitalization, intravenous fluids (through the vein), airway management, respiratory support (ventilator), prevention of secondary infections (pneumonia, urinary tract, etc.) and good nursing care.
The West Nile virus, which is commonly found in Africa and western Asia, is closely related to St. Louis encephalitis virus. In fact, St. Louis encephalitis was initially blamed for making 14 people in New York sick. St. Louis encephalitis is the most common human disease caused by infected mosquitoes in the United States.
The West Nile virus, St. Louis encephalitis and similar diseases such as Japanese encephalitis are considered arboviruses (short for arthropod-borne virus). Each is spread from a bite of a mosquito, which often becomes infected by feeding on infected birds.
Wildlife experts knew the disease affecting New York birds was not encephalitis because the birds were dying. St. Louis encephalitis generally does not kill the birds that carry it. Initial testing of people who had symptoms of St. Louis encephalitis were consistent with that disease, because St. Louis encephalitis and West Nile are genetically similar, according to the CDC. Further testing, however, was compatible with West Nile.
If you find a dead bird, do not handle it. Call your local health department to have it carried away.
What can I do to prevent WNV?
Don't let mosquitoes bite you. The CDC recommends the following:
- When you are outdoors, use insect repellents containing DEET (N, N-diethyl-meta-toluamide) or picardin (KBR 3023). Follow the label's directions. Do not use repellents with DEET on infants less than 2 months old.
- Oil of lemon eucalyptus [active ingredient: p-menthane 3,8-diol (PMD)], a plant-based repellent, is also registered with EPA. When oil of lemon eucalyptus was tested against mosquitoes found in the US it provided protection similar to repellents with low concentrations of DEET. According to the label, oil of lemon eucalyptus products should not be used on children younger than 3.
- Products containing permethrin, a highly effective insecticide and repellent, are recommended for use on clothing, shoes, bed nets and camping gear. Permethrin-treated clothing repels and kills ticks, mosquitoes, and other arthropods and retains this effect after repeated laundering. Some commercial products are available pretreated with permethrin.
- Mosquitoes are most active at dusk and dawn. Stay indoors during these times, or use insect repellent and wear long sleeves and pants, if possible. Light-colored clothing can help you see mosquitoes that land on you.
- Make sure you have good screens on your windows and doors to keep out mosquitoes.
- Get rid of mosquito breeding sites by emptying standing water from flowerpots, buckets and barrels. Change the water in pet dishes, and replace the water in birdbaths weekly. Drill drainage holes in tire swings so water drains out. Keep children's wading pools empty and on their sides when they aren't being used.
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This article was reviewed and updated June 2007.
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