What Is West Nile Virus?
West Nile virus is a Flaviviridae virus transmitted to humans by mosquito bites. Virus symptoms range from none to severe: encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal cord). The neurologic disease the virus causes is termed West Nile encephalitis (WNE). WNE currently is endemic in Asia, Africa, and the Middle East. Since 1999, the disease has been detected in many states (see map below) in the U.S. The disease is considered to be endemic now in the U.S.; in 2013, 39,567 individuals had been diagnosed with the disease. From 2013-2015, about 2,000 per year are detected with new West Nile infections in 47 states in the U.S.
West Nile virus was discovered in 1937 in the West Nile district of Uganda. Although wild birds are the preferred hosts for the virus and are likely the hosts that spread the disease from country to country, the West Nile virus can infect other mammals such as horses and dogs, for example. The virus is transferred from animals or birds to humans by mosquitoes. Since the virus was first detected in the United States in 1999, every year since then there has been an outbreak in the U.S. of West Nile virus (for example, outbreaks have occurred in California, Arizona, Illinois, Massachusetts, Oregon, Pennsylvania, Wisconsin, and Texas); the virus has been detected in 47 U.S. states and in Canada.
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What Causes West Nile Virus and West Nile Encephalitis?
West Nile virus is transmitted to humans by the bite of an infected mosquito. Mosquitoes become infected by biting birds (or other animals like horses or dogs) that harbor the virus; thus, mosquitoes are the vectors of West Nile encephalitis (WNE). The virus is not spread from person to person nor is it spread from infected birds to humans without a mosquito bite. The virus has now been found in 111 bird species and about a dozen mammals.
- How West Nile virus entered New York in 1999 is not entirely clear. The most likely explanation is that the virus was introduced by an imported infected bird or by an infected human returning from a country where West Nile virus is common. Before the 1999 New York outbreak, West Nile encephalitis had been identified previously only in Africa, Asia, the Middle East, and only rarely in Europe.
- Most cases of West Nile occur during the warm weather months when mosquitoes are most active. Nonetheless, the mild climate in southern U.S. states is expected to sustain the mosquitoes beyond those months.
What Are the Risk Factors for West Nile Virus and West Nile Encephalitis?
The major risk factor for West Nile virus and West Nile encephalitis is being exposed to mosquitoes that may be carrying the virus. Such individuals are those who spend time outdoors and have skin exposed for mosquitoes to bite (for example, campers, hikers, those engaged in outdoor working conditions). Individuals who are is 50 years of age or older and individuals who have a weakened immune systems (cancer patients, diabetics, for example) are at higher risk for both infection and encephalitis.
What Are the Symptoms and Signs West Nile Virus and West Nile Encephalitis?
Signs and symptoms of the West Nile virus infection range from no symptoms at all to a rapidly fatal brain infection. The incubation period ranges from two to 14 days although two to six days is the most common range. In areas where the virus is common, people are more likely to show no symptoms of the infection or have only a mild, flu-like illness rather than a severe brain infection. The following describes symptoms and their frequency of occurrence:
- According to the CDC, the majority of people (70%-80%) who become infected show no symptoms and recover completely.
- When symptoms develop, West Nile virus infection typically begins with the abrupt onset of fever, chills, muscle aches, headache, and flu-like symptoms. Headache is particularly common and may be severe. The person may have sensitivity to light with pain behind the eyes, and some patients may also develop vomiting, diarrhea, or a rash. Although fatigue and weakness may last for weeks or even months, most patients recover completely. The set of symptoms described here may occur in about 20% of infected patients.
- In others, particularly the elderly, the disease can progress to cause encephalitis or meningitis. These patients may show neurological changes such as disorientation, tremors, seizures, and develop other symptoms such as headache, high fever, and neck stiffness. Some of the neurological effects will become permanent, and about 10% of people who develop severe neurological infections will die. People with certain medical conditions (cancer, diabetes, hypertension, and kidney disease) are at higher risk for serious West Nile virus infections.
When Should I Call the Doctor About West Nile Virus Infection?
West Nile virus is transmitted by mosquitoes mainly during the summer months, and infections generally appear between the months of May to October. Rarely, transmission via blood transfusions, organ transplants, and mother-to-fetus or mother-to-infant from breastfeeding has been reported.
- People who have been bitten by a mosquito in the geographic area where West Nile virus is known to appear and who experience signs or symptoms of serious illness should see their doctor immediately.
- Most people with mild symptoms of low-grade fever and muscle aches do not have West Nile virus and will not require specific diagnostic testing.
Anyone who has symptoms of severe illness such as mental-status changes, high fever, neck stiffness, sensitivity to light, or confusion should go to an urgent-care center or hospital emergency department immediately. The West Nile encephalitis that occurred during the initial 1999 New York outbreak was especially notable for its severe muscle weakness. This is another important warning symptom.
How Is West Nile Virus and West Nile Encephalitis Diagnosed?
Diagnosing West Nile virus infections is generally done through a combination of observing signs and symptoms along with specialized molecular biologic testing for the virus itself.
- Only people with severe symptoms will require further testing. There is no cure for West Nile and therefore little to gain by widespread testing of people with mild symptoms.
- Confirmatory diagnosis of West Nile virus infection is generally done by a DNA test called polymerase chain reaction (PCR) or viral culture of fluid removed from around the spinal cord (lumbar puncture procedure). A doctor sends both blood samples and spinal fluid samples, obtained by lumbar puncture (also called a spinal tap), to a specialized laboratory for these tests. This testing helps exclude dengue fever, equine encephalitis, Lyme disease, and other infections from WNE.
Are There Home Remedies for West Nile Virus?
Home care for people who suspect they may have become infected with West Nile virus is limited to relief of symptoms. There is no specific treatment for the virus.
Mild illness does not require therapy other than medications to reduce fever and pain. Avoid aspirin use in children because it presents a risk for a fatal condition known as Reye's syndrome.
Severe symptoms require hospital treatment.
What Is the Treatment for West Nile Encephalitis and West Nile Virus?
There is no known effective antiviral treatment or vaccine to prevent West Nile virus.
- Milder illnesses do not require treatment.
- In severe cases of West Nile virus, intensive supportive therapy is indicated. This includes hospitalization, IV fluids and nutrition, airway management (some people may need a tube placed in their airway to keep the airway open), ventilatory support (some people may need a machine to help them breathe), prevention of secondary infections, as well as good nursing care.
What Is the Follow-up for West Nile Virus?
Anyone who has developed West Nile encephalitis should follow up with a doctor regularly. The most severely affected people may take as long as six months to a year to recover. Some may have permanent nervous system problems and may require rehabilitation training.
How Can I Prevent West Nile Encephalitis and West Nile Virus?
The most effective way to reduce the risk of contracting West Nile virus is to avoid contact with mosquitoes during the months of April to October. Mosquitoes are active in the early morning, from dawn until 10 a.m., and in the later afternoon and early evening. Limit outdoor activities or take the following precautions:
- If outside during dawn and evening hours, when mosquitoes are most active, wear protective clothing such as long-sleeved shirts, long pants, and socks.
- Use an insect repellent containing 10%-30% DEET solution. Children should not use a DEET repellent stronger than 10% strength. Other precautions when using DEET include avoiding use on infants or pregnant women, avoiding prolonged or excessive use, storing DEET out of the reach of children, and preventing children from applying it themselves. Adults should apply DEET products to children following manufacturer's instructions.
- Spray clothing with repellents containing permethrin or DEET because mosquitoes may try to bite through thin clothing. Do not apply repellents containing permethrin directly to exposed skin. If clothing is sprayed, there is no need to spray repellent containing DEET on the skin under the clothing. For details on pesticide application, check the National Pesticide Information Center.
- Citronella, frequently used in outdoor candles, is a good deterrent against mosquitoes; however, it does not provide long-term protection.
- Limiting mosquito habitats and breeding grounds may also help limit human exposure. Mosquitoes lay their eggs in standing water, so dispose of all tin cans, plastic containers, rubber tires, or any other possible water-holding containers from nearby property. Mosquito larvacides may be used. Drain water from pool covers. Make sure roof gutters drain properly and clean gutters regularly.
- Make sure doors and windows have tightly fitted screens without holes.
- Birds who have died from West Nile virus pose no health risk. The disease is only transmitted by a mosquito bite. Mosquitoes become infected by biting birds that harbor the virus. Report dead birds (especially crows and blue jays) to local health officials so they can be tested and the virus tracked. Touching dead birds is not recommended; if a dead bird must be removed or handled, wear gloves.
- Development of a vaccine: Since the virus first appeared in New York, researchers began looking for a vaccine. There are several vaccines available for horses, but these vaccines are not approved for use in humans. Research is ongoing; currently, there is no vaccine available for humans against WNE.
What Is the Prognosis for West Nile Virus and West Nile Encephalitis?
The prognosis of West Nile virus is directly related to the severity of the illness and the age of the person with the infection.
- Those with mild infection recover fully with no permanent disability.
- Death occurs in about 10%-12% of people with West Nile encephalitis (WNE), but only about one of 150-250 people who are infected will develop WNE. The elderly are at highest risk for death. Younger people recover much more quickly and are much less likely to show signs and symptoms of severe illness. In some patients, symptoms possibly due to WNV persisted up to eight years after infection. Data from 2016 indicate the incidence of serious and even fatal West Nile virus may be significantly underestimated.
West Nile Virus and West Nile Encephalitis Pictures
The Culex mosquito, common to the Eastern United States, is the primary vector responsible for infecting humans with West Nile virus. Prevention of West Nile virus infection is primarily directed at reducing the mosquito population from May to October and by taking precautions to limit human exposure during these months of high mosquito activity; SOURCE: CDC Click to view larger image.
Cumulative data for West Nile virus, Jan. 14, 2015; SOURCE: CDC Click to view larger image.
Reviewed on 8/30/2021
Medically reviewed by Robert Cox, MD; American Board of Internal Medicine with subspecialty in Infectious Disease
Salinas, Jess D. "West Nile Virus." Medscape.com. July 30, 2014. <http://emedicine.medscape.com/
United States. Centers for Disease Control and Prevention. "West Nile Virus." Jan. 30, 2015. <http://www.cdc.gov/ncidod/dvbid/westnile/index.htm>.
United States. Centers for Disease Control and Prevention. "West Nile Virus: FAQ: General Questions About West Nile Virus." Nov. 22, 2013. <http://www.cdc.gov/westnile/faq/genQuestions.html#vaccine>.