West Nile Virus Overview

We generally think of bugs as a minor annoyance: They spoil our picnics and leave us with red, itchy reminders of their presence. But at least one insect, the mosquito, has a more sinister side. It can transmit dangerous diseases such as malaria and West Nile virus.

Photo courtesy Centers for Disease Control & Prevention,
photographer Jim Gathany
Female mosquito (Anopheles gambiae) feeding

Although West Nile has been a problem on a global scale since at least the 1930s, it first arrived in the United States less than a decade ago. Since the first reported U.S. case in 1999, it has infected thousands and stirred up warnings and educational campaigns to alert the public to the growing threat. But is the threat of West Nile as serious as the hype suggests?

Courtesy Centers for Disease Control
2004 West Nile virus activity in the United States

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In this article, we'll find out how the mosquito transmits West Nile virus, learn the real risks of getting infected and see how health care officials are working to combat the spread of the disease.



What is West Nile Virus?

Photo courtesy USGS
West Nile flavivirus

West Nile virus is a potentially fatal disease that is spread by the bite of infected mosquitoes. In most of North America, it occurs primarily during the late summer and early fall (between July and mid-September). In the winter, the weather is too cold for most species of mosquitoes to survive. But in warmer climates, West Nile can be transmitted year-round.

West Nile virus belongs to a group of disease-causing viruses called flaviviruses, which are spread by ticks and mosquitoes. Other flaviviruses include yellow fever, Japanese encephalitis and dengue. (See Stanford.edu: Flavivirus to learn more about this class of viruses.) In very rare cases, West Nile has also been transmitted through blood transfusions, organ transplants and breastfeeding and has been passed from mother to baby during pregnancy.

Photo courtesy MorgueFile
Crows are one of the bird species especially susceptible to West Nile.

Mosquitoes catch West Nile virus from biting infected birds -- more than 130 different bird species are known to have been infected with West Nile, and more than 40 species of mosquito can carry the West Nile virus. The virus circulates through the mosquito's bloodstream and into its salivary glands. Then, when the mosquito bites a human (or animal), it transmits the virus into their bloodstream.

Once in the human body, the West Nile virus multiplies and may cross the blood-brain barrier. Normally, this barrier prevents bacteria and viruses from getting into the brain. But a few viruses, including Herpes simplex virus, Eastern equine encephalitis and West Nile virus, are able to cross this barrier. Scientists do not yet fully understand the exact mechanism by which this occurs, but it may be due to a substance released during the immune response to these viruses. An immune-cell protein called TLR-3 recognizes the West Nile virus and activates an immune-cell response, leading to the secretion of a substance called TNF-alpha. This substance temporarily breaks down the blood-brain barrier, allowing the West Nile virus to slip through and gain access to the brain and central nervous system. Its entry causes the immune system to launch an inflammatory response, leading to the most serious West Nile symptoms of brain and/or spinal cord inflammation.


What are the Symptoms of West Nile Virus?

Who's at Risk?
Although anyone can get West Nile, people ages 50 and over and anyone with a compromised immune system face the greatest risk of developing severe illness from it. Also, people who work outside and who live in or visit parts of the world where West Nile is currently active are at greater risk of contracting the virus.

Although West Nile gets a lot of press attention each summer, it is actually difficult to contract. The odds of being bitten by an infected mosquito are extremely slim, and the odds of becoming seriously ill following a bite are even slimmer. Approximately 80 percent of those who are bitten by an infected mosquito will have no symptoms at all. Approximately 20 percent will have mild, flu-like symptoms (such as fever, muscle aches, headache and swollen glands). Symptoms usually emerge within three to 14 days after the bite.

Less than 1 percent of people bitten by an infected mosquito come down with the most serious nervous-system conditions related to West Nile:

  • West Nile encephalitis - inflammation of the brain
  • West Nile meningitis - inflammation of the membrane surrounding the brain and the spinal cord
  • West Nile meningoencephalitis - inflammation of the brain and surrounding membrane
  • West Nile poliomyelitis - inflammation of the spinal cord
These conditions occur when the virus is able to penetrate the blood-brain barrier and attack the brain and/or spinal cord. The immune system, in trying to fight off the infection, sends out chemical signals that cause inflammation. This inflammation can interrupt normal functioning of the brain and/or spinal cord.

West Nile encephalitis is a condition that causes inflammation of brain tissue (right).

Symptoms of the most serious forms of West Nile virus can include:

  • high fever
  • stiff neck
  • disorientation
  • convulsions
  • coma
  • numbness
  • paralysis
The neurological effects of these conditions can be permanent. Anywhere from 5 to 15 percent of people with the most severe form of West Nile virus die. West Nile fatality rates are highest among the elderly.

Once someone has had West Nile virus (even the mild version), he or she becomes immune to it. Doctors can identify whether a person has West Nile virus by doing a blood test that detects the presence of antibodies to the virus. Computerized tomography (CT) or magnetic resonance imaging (MRI) can detect swelling in the brain associated with encephalitis.


How is West Nile Virus Treated?

Courtesy COMNAVSURFLANT Maintenance

At present, there is no real treatment for West Nile. People who have milder symptoms typically get over them in a few days without a doctor's care. More serious cases are treated in the hospital with "supportive therapy" -- a regimen of intravenous fluids, respiratory support and careful monitoring.

Scientists are working to develop West Nile virus treatments, as well as a vaccine. Several potential vaccines are in clinical trials. Two of the vaccines being tested are chimeric vaccines: They use a combination of West Nile virus genes and the genes of another, closely related virus (yellow fever or dengue). One of these, Acambis' ChimeriVax-West Nile, has shown promise in animal and human studies. Most of the participants who received a low dose of this vaccine developed antibodies to it, giving their bodies the ability to fight off a West Nile infection.

Researchers are also investigating a number of treatments for West Nile. One therapy uses antibodies similar to the ones present in people whose immune system is able to fight off the West Nile virus. So far, this treatment has been successful in studies with mice.


How Can You Avoid Getting West Nile Virus?

Because West Nile is spread by mosquitoes, the best ways to avoid getting it are to protect yourself from mosquito bites and stop mosquitoes from breeding near your home. The CDC recommends that when you go outside, especially during the hours between dusk and dawn, when mosquitoes are most voracious, wear an insect repellant containing 10 to 30 percent of the ingredient DEET. The greater the DEET concentration, the longer the protection will last. On children, parents should use repellants containing only up to 10 percent DEET.

Photo courtesy Amazon.com
Insect repellents containing DEET

The CDC has recently revised its recommendations to also include repellents containing the chemical picaridin (found in several retail insect repellants) and oil of lemon eucalyptus (an ingredient in many natural insect repellents).

If you have to go outside during peak mosquito hours, try to wear long pants and a long-sleeved shirt. Spraying the outside of your clothes with a mosquito repellant provides added protection.

Bad Time to be a Mosquito
In many regions of the United States, public health officials spray (either by helicopter or on the ground) larvicide to kill mosquito larva. They may also spray adulticide to kill full-grown mosquitoes.

Because mosquitoes like to lay their eggs in standing water, empty out any ponds, bird baths, flower pots and inflatable swimming pools you might have in your backyard. Install screens on all the windows and doors of your home to prevent mosquitoes from entering.

If you find a dead bird, contact your local health department immediately. Don't try to touch the bird, because it may have been infected with West Nile.


West Nile Virus Cases

The West Nile virus first emerged in Uganda's West Nile region in 1937. The disease then turned up in Africa, Eastern Europe, West Asia and the Middle East. The first West Nile cases in the United States were discovered during the summer of 1999. In that year, the disease infected 62 people and killed seven in the New York area. By the following year, the West Nile virus had spread up and down the East Coast from Vermont to North Carolina. In 2002, it had reached all the way to Texas. As of 2004, it had spread through all 48 U.S. mainland states, as well as into Canada and throughout Mexico.

Photo courtesy Centers for Disease Control
Worldwide geographic representation of occurrence
of West Nile virus

An estimated 16,000 people in the United States have become ill from West Nile virus since 1999, and more than 600 have died. A far greater number of people have probably been infected without knowing it because they did not have any symptoms. Outside of North America, regular outbreaks also occur in Africa, Europe, and Asia. In 1996-97, for example, an outbreak near Bucharest, Romania, infected more than 500 people and killed about 50. There are no valid statistics on the number of people infected worldwide.

U.S. West Nile Cases*
Year Number of Cases Number of Deaths States with Highest Numbers of West Nile Cases
1999 62 7 New York (62)
2000 18 1 New York (14), New Jersey (4)
2001 66 9 New York (15), New Jersey (12), Florida (12)
2002 4,156 284 Illinois (884), Michigan (614), Ohio (441), Texas (202)
2003 9,862 264 Colorado (2,947), Nebraska (1,942), South Dakota (1,039), Texas (720), North Dakota (617)
2004 2,470 88 California (771), Arizona (391), Colorado (276), Texas (158)
*Source: U.S. Centers for Disease Control (CDC)

To find out more about West Nile virus and related topics, check out the links on the next page.


Lots More Information

Related HowStuffWorks Articles

More Great Links


  • "West Nile Virus: Epidemics Deadly Diseases Throughout History" by Phillip Margulies
  • "West Nile Story" by Dickson Despommier
  • ">West Nile Virus" by Melissa Abramovitz
  • "West Nile Virus" by Jeff Sfakianos


  • Brown, Anthony J. "West Nile Vaccine Making Progress." Reuters Health, May 11, 2005. http://www.nlm.nih.gov/medlineplus/news/fullstory_24599.html
  • CDC, "West Nile Virus." http://www.cdc.gov/ncidod/dvbid/westnile/
  • "CDC Adopts New Repellent Guidance for Upcoming Mosquito Season," CDC Press Release. April 28, 2005. http://www.cdc.gov/od/oc/media/pressrel/r050428.htm
  • "NIAID Begins Clinical Trial of West Nile Virus Vaccine." NIH Press Release, April 18, 2005. http://www.nih.gov/news/pr/apr2005/niaid-18.htm
  • NIAID, "NIAID Research on West Nile Virus." http://www.niaid.nih.gov/factsheets/westnile.htm
  • NBII, "West Nile Virus." http://westnilevirus.nbii.gov/
  • "Official: West Nile Vaccine Promising." Associated Press, May 11, 2005. http://www.cnn.com/2005/HEALTH/conditions/05/11/west.nile.vaccine.ap/
  • "Promising New West Nile Therapy Cures Disease in Mice." NIH Press Release, April 24, 2005. http://www.nih.gov/news/pr/apr2005/niaid-24.htm
  • West Nile Virus. Mayo Clinic. http://www.mayoclinic.com/invoke.cfm?id=DS00438