West Nile

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West Nile


Disease History, Characteristics, and Transmission

Scope and Distribution

Treatment and Prevention

Impacts and Issues

Primary Source Connection



The West Nile virus is a member of the Flaviviridae family. It causes an inflammation of the lining of nerve cells located in the spinal cord (meningitis) and the brain (encephalitis). Originally detected in Africa in the late 1930s, the virus did not spread to North America until 1999. Since that time, its North American prevalence and geographical distribution has increased.

Disease History, Characteristics, and Transmission

West Nile virus was first isolated in 1937 from a woman in the West Nile District of Uganda. The virus took its name from this location. During the 1950s, the ability of the virus to cause meningitis and encephalitis in humans and the resulting health threat of the virus was recognized. A decade later, the virus was linked to the development of encephalitis in horses.

Since the 1930s, the virus has been detected in humans, animals, and birds in Africa, the Middle East, Eastern Europe, and West Asia, but it did not arrive in North America until the end of the twentieth century. Scientists are not certain if West Nile virus spread from Africa to other regions of the world, such as North America, or if the virus was always present in North America, but was only revealed when tests for it were performed. However, the pattern of reported cases in North America is more consistent with the introduction of the virus from overseas and its subsequent spread. Assuming that the virus was, in fact, introduced to the North American continent, the way in which it was transported across the Atlantic Ocean to the East Coast of the United States is still unknown. Bird migration is one theory. Another theory suggests that a mosquito infected with the virus could have arrived in a shipment of goods.

Immediately after its appearance in North America, West Nile became noteworthy. In the summer of 1999, 62 cases of West Nile disease were reported in New York City. Seven people died as a result of the infection. The city experienced another outbreak the following summer, when 21 more cases and two deaths occurred. In the span of 1999–2000, the virus was also detected in other states along the coast of the northeastern United States. These early infections generated a great deal of concern, since they raised the possibility of a looming epidemic. This proved not to be the case, although the geographical range of the virus began to expand.

West Nile is a vector-borne disease. It is spread from infected birds to humans, mainly by mosquitoes (most commonly, the mosquito species Culex pipiens). Robins, jays, and crows are the most common avian reservoirs of the virus. As with other mosquito-transmitted diseases, the virus is acquired by a mosquito when it feeds on the blood of an infected animal or bird. The virus remains in the salivary gland of the mosquito and, when the same mosquito subsequently seeks a blood meal from a person, transfer of the virus can occur. The cases in New York City in 1999, and especially in 2000, were probably caused by mosquitoes that were able to survive the cold winter months by seeking refuge in warm and damp pipes, abandoned tunnels, subway tunnels, or other locations such as root cellars, barns, and caves. In the spring, the mosquitoes re-emerged and a new round of infections began. Not only humans were affected, but in the springs of 2000 and 2001, many crows died from the viral infection. Indeed, for those who monitor the appearance of West Nile disease, bird die-offs in the spring can be a signal that the infection is re-emerging, and that precautions are necessary to avoid human infection.

Research has shown that two populations of Culex pipiens exist in Europe; one seeks its blood meal exclusively from humans and the other from animals. The chance of a mosquito taking a blood meal from an infected bird and then feeding on a human are very low. However, in North America, the mosquito population has adapted to feed on both birds and humans, so the chance that a mosquito will seek blood meals from an animal and then from a human is greater. This is probably why West Nile disease has spread so much faster in North America than in Europe.

When the West Nile virus enters a human host via a mosquito bite, the virus replicates in the blood. Then, in a way that is still not clear, the virus is able to cross the blood brain barrier and enter the brain. Normally, passage into the brain is regulated by this very efficient blood brain barrier. The barrier is so efficient that some drugs are unable to cross it, but the barrier is not able to keep the virus out of the brain. Formation of new virus particles in the brain tissue stimulates an immune response that—along with the infection—can cause inflammation of the brain, a serious condition known as encephalitis.

Approximately 80% of infected individuals present no symptoms. Many individuals, however, can exhibit symptoms of West Nile that include: the development of fever, headache, muscle aches throughout the body (particularly in the back), loss of appetite, nausea with vomiting, diarrhea, swelling of the lymph nodes, and a skin rash. The infection tends to clear within a few weeks with no or mild complications.

In fewer than 1% of people who are infected with the virus, the infection becomes more serious. Inflammation of the nerve lining in the brain (encephalitis) and spinal cord (meningitis) develops. When meningitis or encephalitis develops, symptoms include a high fever, severe headache, stiff neck, mental disorientation, uncontrolled muscle spasms, loss of coordination, paralysis, and convulsions. A person can lapse into a coma and die. Survivors can be left with permanent damage, such as paralysis on one side of the body, similar to the paralysis seen in cases of polio. In severe cases, the paralysis affects the muscles used for breathing, and mechanical breathing assistance may be necessary.

The prospect that a serious disease can be acquired from a mosquito bite is alarming to many, since mosquitoes are often very common during spring and summer in North America and, despite precautions, can be hard to avoid. Fortunately, at least for now, the incidence of the virus in North American mosquito populations is very low. Scientists who have sampled mosquitoes for the presence of the virus have determined that typically only about 1% of mosquitoes harbor the virus, even in an area that is a known hotspot of the disease. The risk of a person contracting West Nile disease is small, and can be minimized still further by taking some common-sense precautions.

A number of factors increase the risk of contracting West Nile disease. The time of year is one factor. In more northern climates, late spring to early fall is the peak season for mosquitoes and the risk of contracting the disease is higher during those seasons. In southern regions that are warmer year-round, the risk is more constant.

Another risk factor is geography. Certain areas of the United States and Canada have greater mosquito populations that other areas, and therefore are areas of higher risk. For example, in 2006, Texas—which has coastline on the Gulf of Mexico—reported 330 cases of West Nile disease, while the drier and more inland state of New Mexico reported only eight cases. More locally, areas that have more stagnant water are more apt to be a breeding ground for mosquitoes.

A third risk factor is occupation. Someone whose job or recreational activities takes them outdoors is more at risk of exposure to mosquitoes than someone who spends more time indoors. Finally, people whose immune systems is not functioning efficiently—such as the elderly, the sick, and transplant patients whose immune systems have been deliberately supressed—are at higher risk, since they are less able to fight a viral infection.

Scope and Distribution

West Nile disease has occurred in Europe, Africa, the Middle East, parts of Asia, and North America. Outbreaks have occurred in all these regions, most recently in the United States and Canada from 1999–2003. The geographical distribution of the virus in North America has been steadily increasing since its appearance on the continent in 1999. By the summer of 2001, dead birds that tested positive for the virus were found in Toronto, Ontario (Canada), northern Florida, and Milwaukee, Wisconsin. A year later, over 300 cases and at least 14 deaths were reported and the virus was recovered from dead birds in more western states. By August 2002, West Nile virus was reported in 41 states and by 2003 only the states of Alaska, Hawaii, Washington, Oregon, and Maine had not reported cases of the disease. By 2006, the disease had spread to the states of Washington and Oregon, and into Mexico. As of early 2007, the hotspots for the disease are California, Illinois, Louisiana, Nebraska, South Dakota, and Texas.


ENCEPHALITIS: A type of acute brain inflammation, most often due to infection by a virus.

MENINGITIS: Meningitis is an inflammation of the meninges—the three layers of protective membranes that line the spinal cord and the brain. Meningitis can occur when there is an infection near the brain or spinal cord, such as a respiratory infection in the sinuses, the mastoids, or the cavities around the ear. Disease organisms can also travel to the meninges through the bloodstream. The first signs may be a severe headache and neck stiffness followed by fever, vomiting, a rash, and, then, convulsions leading to loss of consciousness. Meningitis generally involves two types: nonbacterial meningitis, which is often called aseptic meningitis, and bacterial meningitis, which is referred to as purulent meningitis.

VECTOR: Any agent, living or otherwise, that carries and transmits parasites and diseases. Also, an organism or chemical used to transport a gene into a new host cell.

The number of cases have been increasing as well. In 2002, 4,155 cases and 284 deaths were reported to the U.S. Centers for Disease Control and Prevention (CDC). The next year, 9,862 cases and 264 fatalities were reported. About 30% of these cases involved meningitis or encephalitis and required extensive hospital care. From 2004–2006, 9,758 cases and 380 deaths were reported. During these last three years, the number of cases and number of deaths increased each year.

In Canada in 2001, the virus was uncovered in dead birds and mosquitoes in the province of Ontario. In 2002, 10 deaths occurred among the 416 cases reported to Health Canada, and the disease had spread to the province of Quebec. The following year, the number of cases increased to 1,494 and 14 deaths were reported. By 2003, the virus had been detected all across the country from British Columbia to Nova Scotia and as far north as the Yukon. In 2005, the last full year for which data is available, there were 239 cases and 12 deaths in Canada.

While birds are involved in the transmission of West Nile disease to people, dogs and cats also can be infected with the virus. This has caused fears that many pet owners could be at risk of the disease. While it does mean that a pet owner could acquire the virus after a mosquito has bitten a cat or dog, there is no evidence of a direct transmission from either animal to people.

Squirrels may also be susceptible to infection with West Nile virus. While there is no evidence that the virus is transmitted to someone from a squirrel or from handling a squirrel carcass, the presence of a dead squirrel could be an indication that West Nile disease is present in an area. Sensible precautions, including the use of gloves when disposing of a squirrel carcass, will prevent infection.

There may also be a genetic component to West Nile virus susceptibility. Researchers have found that an alteration in a gene called CCR5, which affects the functioning of T cells (important immune system cells), can produce more serious symptoms of the disease. Several studies have found that the proportion of people possessing the gene mutation is much higher in those with West Nile disease than in the general population. Curiously, the gene mutation helps protect people infected with the human immunodeficiency virus (HIV) from developing acquired immunodeficiency syndrome (AIDS, also cited as acquired immune deficiency syndrome).

Treatment and Prevention

West Nile disease is almost always acquired from a mosquito bite. While some species of ticks can harbor the virus, no tick-borne disease has been reported in a human. Furthermore, West Nile disease is not contagious— routine person-to-person transmission does not occur. In 2002, the CDC reported that it is possible to transmit West Nile virus via transfusion of virus-contaminated blood, transplant of a contaminated organ, and breast milk. Infection of a fetus by its mother prior to birth may also be possible. However, infections that do not involve mosquito bites are thought to be rare.

In Canada, all donated blood is screened yearround. Blood banks in the United States screen donated blood during peak infection periods. In addition, Britain's National Blood Service screens donated blood for the virus if the donor is known to have visited the United States or Canada in the previous month.

While there is no human vaccine effective against West Nile virus, a vaccine for horses is available. The vaccine has been used by some zoos to vaccinate birds; however, whether this strategy worked cannot be gauged until the birds are exposed to the virus. The equine vaccine, which contains weakened but intact West Nile virus, has not been studied in humans, and people should not use it. Veterinary vaccines are not subject to the same regulatory approvals as are human vaccines, and their safety for humans cannot be assured.

Prevention of infection focuses on minimizing the opportunity for contact with mosquitoes. Sensible precautions include using insect repellent sprays or creams that contain DEET (meta-dimethyl toluamide), wearing protective clothing such as long-sleeve shirts and long pants when outdoors, avoiding areas of stagnant water that can be breeding grounds for mosquitoes, and removing any objects that could contain stagnant water— birdbaths, clogged roof gutters, unused swimming pools, and disused tires—from a backyard. In addition, avoiding outdoor activity during the early morning and evening, when mosquitoes are most active, is a wise precaution.

DEET-containing insect repellents should not be used on infants or young children. For these youngsters and those who prefer not to be exposed to DEET, the CDC recommends oil of lemon eucalyptus. It is an efficient repellent, but does not retain its potency as long as DEET does.

Impacts and Issues

West Nile disease has quickly become a significant public health threat in North America. The possibilities of large outbreaks and the potential seriousness of the infection—one of every 150 people who contract the disease develops meningitis or encephalitis—has created near-panic in the public. Agencies, such as the CDC, have devoted significant effort to informing people about the disease and publicizing the common-sense preventative measures that can help protect people.

The economic consequences of West Nile disease can be great. For example, in 2002, it was estimated that about $200 million in health care costs were associated with the disease in the United States. There are other costs as well. For example, national, state, and local agencies have surveillance programs to monitor mosquito, bird, and even human populations for the virus, and they also conduct initiatives to increase awareness of the disease among the public and health care providers.

Spraying of mosquito breeding sites is a proven way of reducing the mosquito population. However, those opposed to such spraying are concerned that the possible environmental degradation from the chemical spray is a greater danger than any cases of West Nile that might develop. Those in favor of spraying maintain that the increasing spread of West Nile disease and the increasing number of deaths argues for intervention and control programs, including spraying.

Primary Source Connection

As part of an effort to combat West Nile Virus, the United States Food and Drug Administration (FDA) publishes information articles designed to alert the public to potential dangers and to provide concrete steps to reduce risk. The recommendations were formulated by the Centers for Disease Control and Prevention (CDC).


According to the National Institute for Occupational Safety and Health: “Workers at risk of exposure to WNV (West Nile Virus) include those working outdoors when mosquitoes are biting. Outdoor workers at risk include farmers, foresters, landscapers, groundskeepers and gardeners, painters, roofers, pavers, construction workers, laborers, mechanics, and other outdoor workers. Entomologists and other field workers are also at risk while conducting surveillance and other research outdoors.”

“Although WNV is most often transmitted by the bite of infected mosquitoes, the virus can also be transmitted through contact with infected animals, their blood, or other tissues. Thus laboratory, field, and clinical workers who handle tissues or fluids infected with WNV or who perform necropsies are at risk of WNV exposure.”

SOURCE: National Institute for Occupational Safety and Health

This press release announced a new test designed to expedite diagnosis of West Nile virus along with a list of preventative steps recommended by the FDA in 2000— and that are current as of March 2007—to avoid risk of acquiring West Nile disease, especially until definitive treatment or vaccines are developed and tested.

First Test Approved to Help Detect West Nile Virus

The Food and Drug Administration has cleared the first test that will help physicians diagnose cases of potentially deadly West Nile virus earlier than with current methods.

The West Nile Virus IgM Capture ELISA test is intended to be used in people who have symptoms of viral encephalitis or meningitis, which are serious inflammatory conditions of the brain or spinal cord that may occur in people infected with the virus.

“The rapid review and approval of this blood test, which uses antibody levels to identify persons who were recently exposed to West Nile virus, reflects FDA's commitment to making safe and effective medical products available promptly,” says FDA Commissioner Mark B. McClellan, MD, Ph.D. The new test works by detecting the levels of IgM, a particular type of antibody to West Nile virus, in blood serum. It is manufactured by PanBio Ltd. of Windsor, Australia.

West Nile virus is a mosquito-borne virus first detected in the United States in 1999. While it often causes a mild infection that clears without further treatment, some people, especially those over 50, develop severe infections resulting in neurological disease and even death. The virus is most prevalent during peak mosquito season, beginning in July and ending in October.

By 2002, West Nile virus had spread to most of the continental United States. The CDC reported this season's first human case of West Nile virus in the United States in early July. As of early August, three deaths in Texas and Alabama had been attributed to the virus. The CDC says that West Nile virus activity detected in humans began a “significant uptick” in early August 2003.

The new diagnostic test is a significant breakthrough in the detection of West Nile virus. However, it's important to know that it is not a donor screening test, but is one of several tools used by the physician to determine if the patient is infected. Results from the IgM Capture ELISA must be confirmed with other laboratory tests as part of a comprehensive evaluation. The test is designed to be used in cases when someone has symptoms of West Nile encephalitis or meningitis—headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis.

In addition to its usefulness for diagnosing individuals with the infection, the test has the potential to help monitor the scope and spread of the disease. The FDA has established guidance and procedures to avoid collection and use of blood that might be at risk for transmitting West Nile virus. The agency is cooperating with the country's blood organizations, both in the laboratory and in epidemiological investigations of the virus. In August 2002, prior to any actual report of transmission, the FDA alerted the blood industry to be vigilant in excluding symptomatic donors and then later that year provided guidance to blood establishments on procedures to protect the blood supply. The FDA updated this guidance in May 2003, based on experience with the 2002 outbreak.

Additionally, the agency is working with manufacturers to expedite development of necessary medical products, such as screening tests and additional diagnostic methods. Experimental donor screening tests have been put into place and have been available nationwide since July 1, 2003. These tests add a measure of safety and will prevent contaminated blood from entering the nation's blood supply.

Other federal efforts are ongoing to combat West Nile virus. The National Institutes of Health (NIH) is supporting ongoing research at universities and companies nationwide aimed at developing the public health tools to help fight the infection. Currently the NIH is funding four areas of research for West Nile virus: diagnosis, prevention, therapy, and basic research that look at the virus as it replicates in animals, humans, and mosquitoes.

In the area of prevention, the NIH is supporting three different approaches to vaccines, including a live vaccine made by mixing West Nile virus with the already established yellow fever vaccine Through its grants and contracts, the NIH is the largest supporter of infectious disease research in the United States.

For now, the CDC, the FDA and the NIH all agree that the most important message about the virus is that people need to be prepared and take the steps necessary to prevent mosquito bites and avoid exposure, especially until treatment or vaccines are available to add additional layers of protection.

Reduce the Risk of West Nile Virus

  1. Avoid mosquito bites
    • Cover up. Wear long-sleeved shirts, long pants, and socks sprayed with repellent while outdoors.
    • Avoid mosquitoes, which often bite between dusk and dawn.
    • Limit time outdoors during these hours.
    • Spray insect repellent containing DEET (look for N, N-diethyl-m-toluamide) on exposed skin outdoors.
    • Spray clothing with repellents containing DEET or permethrin.
    • Don't spray repellent on skin under clothing.
    • Don't use permethrin on skin.
    • Use repellent carefully.
    • Don't put repellent on kids’ hands because it may get into their mouths or eyes.
  2. Mosquito-proof your home
    • Install or fix window and door screens.
    • Drain standing water, where mosquitoes like to breed.
    • Look around every week for possible mosquito breeding places.
    • Empty water from buckets, cans, pool covers, flowerpots, and other items.
    • Throw away or cover up stored tires and other items not being used.
    • Clean outdoor pet water bowls weekly.
    • Check to see if rain gutters are clogged.
  3. Help your community
    • Dead birds help health departments track West Nile virus.
    • Check with your local or state health department to find out their policy for reporting dead birds.


See AlsoArthropod-borne Disease; Climate Change and Infectious Disease; Emerging Infectious Diseases; Encephalitis; Meningitis, Viral; Mosquito-borne Diseases; Vector-borne Disease.



Sfakianos, Jeffrey N., and David Heymann. West Nile Virus. London: Chelsea House, 2005.

White, Dennis J., and Dale L. Morse. West Nile Virus: Detection, Surveillance, and Control. New York: New York Academy of Sciences, 2002.


Boyer, Jere, Thomas File, and William Franks. “West Nile Virus: The First Pandemic of the Twenty-first Century.” Ohio Journal of Science 102(2002):98–102.

Web Sites

Centers for Disease Control and Prevention. “West Nile Virus.” March 6, 2007. <http://www.cdc.gov/ncidod/dvbid/westnile/index.htm> (accessed March 23, 2007).

Brian Hoyle

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