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West Nile Virus

West Nile Virus


West Nile virus is a mosquito-borne viral illness that can manifest with varying seriousness, ranging from no symptoms or mild flu-like symptoms to brain damage and death.


West Nile virus (WNV) is a mosquito-borne flavivirus belonging to the Japanese encephalitis serocomplex, which includes St. Louis encephalitis, Murray Valley encephalitis, and Kunjin virus. Zoonotically maintained, infections occur generally between late summer and early fall in temperate areas, and throughout the year in southern climates. Although typical manifestation of WNV is asymptomatical, the virus can cross the blood-brain barrier and cause severe illness, paralysis, and even death in humans and animals.

The WNV was originally isolated in a feverish woman living in the West Nile District of Uganda during 1937. The virus was ecologically characterized in Egypt during the 1950s and later linked to severe human meningoencephalitis in elderly patients during a 1957 outbreak in Israel. Since 1937, subsequent outbreaks of WNV have been reported in Africa, Asia, Australia, Oceania, Western Europe, and the Middle East.

In the summer of 1999, the first North American cases of WNV occurred in the New York City area. It is still unknown how the WNV reached the continental United States, but it is suspected that the transport of infected birds or the international travel of infected humans may have been to blame. After its arrival in the New Work area,, the virus spread rapidly across the United States, as well as northward into Canada and southward into Mexico. In 2002, a severe outbreak of WNV in the United States killed 284 people and caused 2,944 cases of severe brain damage. During another outbreak a year, 9,858 cases of WNV infection were reported, of which over 2,900 were severe in nature and 262 of which resulted in fatality. As of January 2005, the CDC ArboNET has recorded avian or animal WNV infections in every state except Alaska, Hawaii, and Washington. Additionally, the CDC ArboNET recorded human WNV infections in every state except Alaska, Delaware, Hawaii, Massachusetts, New Hampshire, Rhode Island, Vermont, Washington, and West Virginia. Experts believe that WNV is now firmly established in the Western Hemisphere.

Life Cycle and Transmission

Like most flaviviruses, the WNV is maintained in a natural host-vector-host cycle, where the primary vector is the mosquito. The zoonotic cycle begins with a reservoir host, which is most commonly of avian origin. When a mosquito feeds on the infected bird, the virus is passed to the insect along with the blood meal. The virus then multiplies rapidly within the mosquito's body and salivary glands over the next few days. When the insect feeds on another animal or human, the virus can be transmitted through the bite and cause serious illness.

Most mosquitoes can become infected with the WNV. However, female mosquitoes of the Culex pipiens species are of particular concern, as they live in suburban and urban areas, can survive through the winter, prefer to feed on birds, and frequently bite humans. The Culex pipiens, also known as the house mosquito, is also the most common vector for WNV transmission. Culex restuan, Culex quinquefasciatus, Aedes Albopictus, and Aedes Vexans are also common carriers of the WNV.

Common food sources for mosquitoes, birds represent the primary WNV reservoir species. American crows, in particular, are extremely susceptible to WNV, and have become the virus' primary host population. Indeed, an unusual crow die-off can be used as an excellent indicator for the regional presence of the WNV. The virus has also been identified in more than 250 bird species in the United States, including blue jays, ravens, magpies, sparrows, and starlings. Many in the scientific community believe that the rapid spread of WNV in North America may be due in part to the migratory nature of birds. Infected birds carry the virus with them as they travel in summer and winter, thus acting as reservoirs in their new nesting sites.

Most vertebrates, such as alligators, bats, chipmunks, skunks, squirrels, and rabbits, can also be infected with WNV. Horses, in particular, are commonly infected with WNV. Like humans, the majority of horses suffer either no or mild symptoms, but severe illness and death can and does occur. There are relatively few cases of dogs and cats becoming infected with WNV. Animals of all species exhibiting fever, weakness, poor coordination, spasms, seizures, and/or personality changes may be infected with WNV.

There is no evidence of WNV transmission from person-to-person through touch, kissing, or other contact. However, there is evidence of WNV transplacental (mother-to-child) transmission, as well as viral transmission through breastfeeding. As such, pregnant mothers should be aware of the presence of WNV in their area and take appropriate precautions. The transmission of WNV has also been evidenced in blood-transfusions and organ transplants; although the current blood supply is now tested for the presence of the WNV. People that are immunocompromised (from disease or chemotherapy, for example) and people aged 50 and older represent the highest risk group for serious WNV infection.

Causes and symptoms

The exact mechanism of WNV-caused illnesses remains unclear. However, it is suspected that the virus enters the host's blood stream and multiples. It can then develop to the point where it crosses the blood brain-barrier, which separates the blood from the central nervous system. When this occurs, the virus can infect the brain, spinal cord, and other vital systems, creating a potentially deadly inflammatory response.


The incubation period for WNV after infection typical ranges between 3 to 14 days. Eighty percent of infected persons will exhibit no clinically apparent symptoms whatsoever. Roughly 20% of infected persons will exhibit a series of mild flu-like symptoms, also known as West Nile Fever. These mild symptoms can persist for 3 to 6 days, possibly weeks, and include:

  • eye pain
  • fever
  • headache
  • loss of appetite
  • lymphadenopathy (abnormal enlargement of the lymph nodes)
  • malaise (nonspecific bodily discomfort)
  • myalgia (nonspecific muscular pain/tenderness)
  • nausea
  • rash (on the neck, torso, and limbs)
  • vomiting

In rare cases, approximately 1 in 150 cases (0.7%), WNV can cross the blood-brain barrier and develop into a severe neuroinvasive disease. Immunocompromised and elderly (>50 years of age) patients are at an increased risk for developing more severe syndromes; a 20-fold increase in incidence among older patients. Symptoms indicating the possible presence of severe West Nile-related syndromes include:

  • severe headache
  • high fever
  • acute muscle weakness
  • neck stiffness
  • convulsions and tremors
  • disorientation and stupor
  • paralysis
  • coma

People exposed to WNV infection, especially the immunocompromised and elderly, should contact their health provider immediately if they develop a severe headache accompanied by high fever.

Typically, severe WNV syndromes manifest as one of three syndromes: West Nile encephalitis (inflammation of the brain); West Nile meningitis (inflammation of the meninges of the brain and spinal cord); or West Nile meningoencephalitis (inflammation of both the brain and the meninges). These three syndromes can cause severe brain damage and even death. Severe WNV disease carries a mortality rate ranging between 3% and 15%, with elderly patients suffering the highest mortality rate. The majority of these deaths are as a result of complication attributable to West Nile meningoencephalitis. Additionally, severe WNV disease can cause acute vision loss due to inflammatory disorders of the eye, such as chorioretinitis, optic neuritis, retinal vasculitis, uveitis, and vitritis. Less frequently, the patient can exhibit acute flaccid paralysis, similar to poliomyelitis (polio ) or Guillain-Barré syndrome, caused by inflammation of the spinal cord and/or damage to the peripheral nerves. In some severe cases, this acute flaccid paralysis can disrupt muscles that control breathing and result in respiratory failure.


A proper diagnosis of WNV infection depends heavily upon clinical presentation, laboratory testing, and patient history. Patients with a known susceptibility to WNV (the elderly and immunocompromised) that exhibit symptoms during the late spring to early fall, or at any time in warmer climates, should be tested for WNV and other arboviral infections. Additionally, health providers should remain constantly aware of the local presence of WNV activity, such as reports of recent animal and/or human cases. Similarities of symptomology between and serological cross-reactivity of WNV and other flaviviruses, may lead to confusion and an incorrect diagnosis. Health providers must use thorough laboratory testing to differentiate WNV antibodies from those of other arboviruses.

Symptomatic WNV infection can be classified as either non-neuroinvasive or neuroinvasive, with each being identified according to certain criteria.


The majority of WNV infections are asymptomatic. In approximately 20% of WNV cases, clinically recognizable symptoms can manifest. However, to be clinically classified as non-neuroinvasive West Nile disease, the following must be true:

  • no neuroinvasive symptomology
  • presence of fever without other recognizable cause
  • four-fold or greater increase in serum antibody titer
  • virus isolated from and or deomonstrated in blood, tissue, cerebrospinal fluid (CSF), or other bodily fluid
  • virus-specific immunoglobulin M (IgM antibodies demonstrated in CSF through antibody-capture methods


In the rare cases (0.7%) of West Nile disease, the virus crosses the blood-brain barrier and manifests in severe and life-threatening symptomology. Clinical confirmation of neuroinvasive of neuroinvasive disease requires the presence of a fever and at least one of the following:

  • acutely altered mental status, such as disorientation, stupor, or coma
  • acute central or peripheral neurological difficulties, such as paralysis, nerve palsy, sensory deficits, and abnormal muscle function
  • an increased white blood cell concentration in the CSF coupled with symptoms of meningitis, such as severe headache and neck pain


Currently, there are no treatment modalities for WNV infection. Instead, supportive care is utilized to treat the varying symptoms and syndromes associated with the various West Nile diseases. Although milder symptoms can be treated at home, severe symptoms can require hospitalization. Treatment of severe symptoms may require the use of intravenous infusions, airway and respiratory management and support, and use of preventative measures against secondary infection.


The majority of WNV infections will manifest asymptomatically. West Nile fever offers an excellent prognosis associated with quick recovery and no adverse side-effects. The majority of symptoms will resolve within a few days or weeks of manifestation.

However, the prognosis is not a positive for patients suffering the more severe syndromes attributable to WNV infection. Symptoms of West Nile encephalitis, West Nile meningitis, and West Nile meningoencephalitis can last for several weeks, as well as cause severe and permanent neurological damage. Inflammation can interfere with the brain and central nervous system and result in death, especially amongst the elderly population. Patients with West Nile poliomyelitis may suffer prolonged muscle weakness and loss of motor control. Long-term rehabilitation is typically required and a full recovery is not assured. If the poliomyelitis affects muscles used for breathing, death from respiratory failure may result.


Although there is a vaccine used for horses and exotic birds in zoos, there is no WNV vaccine for humans at the current time. Several pharmaceutical companies, however, have WNV vaccines in development.

Prevention techniques of WNV typically coincide with avoidance measures against mosquito bites; the primary source of the virus. These include the use of insect repellant (with 5% to 20% DEET) on exposed body parts, wearing loose-fitting clothes over the limbs and torso while outdoors, using mosquito coils and/or citronella candles outdoors, and limiting outdoor activities during peak biting periods and/or in areas with high mosquito density. While camping outdoors, knockdown spray or bed netting with pyrethrum is suggested. Mosquito eradication programs have been instituted in most major cities. Public health authorities can utilize United States Environmental Protection Agency-approved "adulticidies" in areas suspected of the presence of WNV.

The Culex pipiens mosquito is the primary vector of WNV transmission and is also commonly live and feed in urban areas. Special precautions should be taken to reduce exposure to these potentially infected insects. Screen doors and enclosed porches can help keep mosquitoes from coming into the house. It should be noted that studies have shown that mosquito control devices such as "bug zappers" and CO2-baited traps do not significantly reduce the risk of being bitten.

Removing potential mosquito breeding areas from near the home and from the neighbourhood can further reduce the risk of bites. Any container which can collect half an inch of standing water can become a potential breeding site in as little as five days. Old tires, empty plant pots, and empty trashcans should be removed, while water sources like ponds or birdbaths should be cleaned regularly. Standing water on any property should be drained, such as from clogged eves. Swimming pools and hot tubs should be properly covered and chlorinated to prevent mosquitoes breeding in them.


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Peterson L, and J. Roehrig. "West Nile Virus: A Reemerging Gobal Pathogen." Emerging Infectious Diseases 7(July-August 2001): 611-614.

Rappole J., S. Derrickson, and Z. Hubalek. "Migratory Birds and Spread of West Nile Virus in the Western Hemisphere." Emerging Infectious Diseases 6 (July-August 2000): 319-328.

Huhn G', J. Sejvar, S. Montgomery, and M. Dworkin. "West Nile Virus in the United States: An Update on an Emerging Infectious Disease." American Family Physician 68 (August 2003): 653-672.


Centers for Disease Control and Prevention. 1600 Clifton Rd, Atlanta, GA 30333. (800)311-3435.


Flavivirus An arbovirus that can cause potentially serious diseases, such as dengue, yellow fever, Japanese encaphilitis, and West Nile fever.

Guillain-Barré A disorder in which the body's immune system attacks part of the peripheral nervous system. Weakness, tingling, and abnormal sensations in the arms and upper body can progress until the muscles become totally disabled and the patient is effectively paralyzed.

Meninges A series of membranous layers of connective tissue that protect the central nervous system (brain and spinal cord). Damage or infection to the meninges, such as in meningitis, can cause serious neurological damage and even death.

Zoonotic diseases Diseases caused by infectious agents that can be transmitted between (or are shared by) animals and humans. This can include transmission through the bite of an insect, such as a mosquito.

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West Nile Virus Infection

West Nile virus infection


The West Nile virus is an arbovirus (meaning it is spread by mosquitos, ticks, or other arthropods) that can cause infections in animals and humans; in some cases, the infections can lead to fatal meningitis or encephalitis, which are inflammations of the spinal cord and brain. West Nile virus is considered a seasonal epidemic in North America, and it occurs mainly in the summer, but can continue into the fall. In many cases, it can be a serious illness that generally affects the central nervous system , leading to a variety of symptoms that differ from person to person. It is not contagious by touch, but can be spread by infected mosquitoes, transfusions, transplants, or from mother to child during pregnancy.


West Nile virus infections usually begin with flu-like symptoms. Only approximately one in 150 people infected will develop severe symptoms, including headaches , neck stiffness, disorientation, seizures , fever, numbness, paralysis, and/or muscle weakness. In the worst cases, infection with West Nile virus can lead to death or permanent disability. These cases are usually due to either the age of the patient or the health status. Symptoms generally do not occur in healthy individuals.


The West Nile virus has been observed mainly in temperate regions of Europe and North America, and has also been discovered to be the cause of human illness in the United States. The first known case in the United States was reported by the New York City Department of Health in late August 1999. Careful surveillance identified 59 patients who were hospitalized in New York City due to West Nile virus infections during August and September 1999. The median age of these patients was 71 years (range is five to 95). As of April 2004, only one case has been reported by the Centers for Disease Control. The West Nile virus has been observed in Africa, the Middle East, and west and central Asia. The first case was discovered in 1937 in an adult woman in the West Nile district of Uganda. The virus was characterized in Egypt during the 1950s.

An infection due to the West Nile virus does not produce symptoms in most people. In fact, only 20% of people who are infected will develop symptoms. Of these, the majority will recover and will not become infected again. The West Nile virus can infect males and females with equal frequency. There is no known predilection for people of specific ethnic backgrounds. People over 50 years old are at the highest risk of having serious illness associated with the infection. There is a very low risk of contracting this illness by medical procedures such as transplantation and blood transfusions. Although pregnancy and breast-feeding do not increase the risk of becoming infected with the virus, the risk to the fetus or nursing infant of an affected mother is currently being investigated. Horses, birds, and other animals have also been shown to be susceptible to viral infection.

Causes and symptoms

When a person is infected with West Nile virus, usually via a mosquito bite from a mosquito harboring the virus, it is unlikely that the individual will develop symptoms. Of the infected individuals that develop symptoms, there are either mild or severe clinical manifestations. The majority of infections are mild.

Characteristics of mild infections include:

  • mild illness, including fever
  • fever and symptoms persist no more than six days, usually lasting only three days
  • symptoms usually develop three to 14 days after exposure, consistent with the incubation period
  • illness can be sudden and accompanied by anorexia (loss of appetite), nausea, headaches, rash, muscle weakness, vomiting, and/or lymphadenopathy (swollen lymph glands)

Characteristics of severe infections include:

  • Severe symptoms can result in neurological disease in approximately one in 150 cases, with the elderly at highest risk.
  • Neurological symptoms include disorientation, seizures, and cranial nerve abnormali>ties.
  • Symptoms include high fever, weakness, significant alterations in behavior, eye problems, and stomach problems.
  • In rare cases, flaccid paralysis along with severe muscle weakness can occur.
  • Illness can be sudden and accompanied by anorexia (loss of appetite), nausea, headaches, rash, muscle weakness, vomiting, and/or lymphadenopathy (swollen lymph glands).


Diagnosis requires clinical observation by an experienced physician as well as positive results from specific laboratory tests. Factors that assist in the diagnosis are recent travel experiences, the season that the symptoms developed, the age of the patient, and whether there are reports of other cases in the same geographical location that the patient was present during the time of exposure. Patients who have encephalitis, meningitis, or symptoms involving the central nervous system, which could lead a physician to suspect the West Nile virus, can be referred to health departments nationwide or the Centers for Disease Control (CDC) for testing. The CDC has confirmed all human cases.

The diagnostic test involves an assay that detects a virus-specific antibody (IgM) in the cerebral spinal fluid from patients. Blood can also be tested. If this test is negative, it is very unlikely that the infection is due to the West Nile virus; the other clinical explanations such as St. Louis encephalitis (SLE) should be considered. There is also a test that measures SLE virus-specific antibodies. Currently, there is a vaccination for horses, but not for humans.

Laboratory findings include normal to elevated white blood cell numbers with anemia (low red cell numbers). A deficiency of sodium in the blood (hyponatremia), which is usually associated with encephalitis, as well as normal glucose and a general increase in proteins can all be observed. A magnetic resonance imaging (MRI) scan can also be helpful, if specific areas of the brain show an abnormality, including the leptomeninges and/or the periventricular areas.

Treatment team

The treatment team might consist of the physician who initially sees the patient, usually a general practitioner, an infectious disease specialist, and neurologist . In severe cases, a complete medical team consisting of emergency room physicians and staff, nurses, and officers from the CDC might be necessary. Due to the risk of an epidemic, it is important for physicians to report these types of infections to the local health department.


There is no cure for West Nile virus infection once the infection occurs. Treatment, therefore, is supportive and palliative. In the more severe cases, recurrent hospitalizations may necessitate life support services. The primary treatment is focused on lessening the symptoms and preventing secondary infections, which could include urinary tract infections and pneumonia in patients that develop severe illness. Intravenous fluids can be helpful during hospitalizations, along with airway management and good nursing care.

Recovery and rehabilitation

Most patients who develop symptoms recover from West Nile virus infections. The symptoms can be no worse than getting the flu. However, older patients and patients with health-related problems (particularly those that affect the immune system) have more difficulty recovering.

Clinical trials

The Warren G. Magnuson Clinical Center is currently recruiting participants for a clinical trial on the West Nile virus. The Patient Recruitment and Public Liaison Office's e-mail address is

The National Institutes of Health is conducting phase II clinical trials to investigate whether an experimental drug, Omr-IgG-amIV, is a safe and effective treatment for West Nile virus-induced infections. This drug contains antibodies that help fight infection and is designed to target the West Nile virus. Another study by the same center has also been initiated to investigate the natural history of infection in patients with, or at risk of developing, West Nile virus-specific encephalitis or myelitis.

A third clinical trial sponsored by the National Institute of Allergy and Infectious Diseases (NIAID) in phase I and II is to test the tolerability of Omr-IgG-am, its efficacy as a vaccine, and its effectiveness in reducing morbidity and mortality (disability and death) in patients with a confirmed diagnosis of the West Nile virus disease. The contact is Walla Dempsey; the e-mail is

Finally, a clinical trial is ongoing to identify healthy individuals who might be eligible for a phase I vaccine clinical trial sponsored by the Vaccine Research Center at the National Institutes of Health. The Patient Recruitment and Public Liaison Office's e-mail address is

High doses of a drug called Ribavirin and another called interferon alpha-2b were found to be effective in research studies, but currently no controlled clinical trials in humans have been initiated for these or other types of medications in the therapeutic management of West Nile virus infections and encephalitis.


The prognosis for persons with West Nile virus infection is quite favorable in patients that are young and in otherwise good health. Older persons and patients with health complications can have a poorer prognosis. In rare cases, death is possible.

Special concerns

It is important to contact the local health department when finding dead birds or other animals that die suddenly of an unknown cause during suspected or confirmed local outbreaks of West Nile virus. Health officials monitor mosquito and bird populations to determine local risk for West Nile virus activity.

A person's exposure to mosquitoes and other insects that harbor arboviruses can be reduced by taking precautions when in a mosquito-prone area. Insect repellents containing DEET provide effective temporary protection from mosquito bites. Long sleeves and pants should be worn when outside during the evening hours of peak mosquito activity. When camping outside, intact mosquito netting over sleeping areas reduces the risk of mosquito bites. Communities also employ large-scale spraying of pesticides to reduce the population of mosquitoes, and encourage citizens to eliminate all standing water sources such as in bird baths, flower pots, and tires stored outside to eliminate possible breeding grounds for mosquitoes.



Despommier, Dickson. West Nile Story. New York: Apple Trees Productions, 2001.

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


Nash, D., et al. "The Outbreak of West Nile Virus Infection in the New York City Area in 1999." New England Journal of Medicine 344, no. 24 (June 14, 2001): 180714.


Bren, Linda. "West Nile Virus: Reducing the Risk." U. S. Food and Drug Administration. May 1, 2004 (June 3, 2004). <>.

"West Nile Virus: Statistics, Surveillance, and Control." United States Centers for Disease Control. May 1, 2004 (June 3, 2004).

"West Nile Virus." United States Centers for Disease Control. May 1, 2004 (June 3, 2004). <>.

"What You Should Know About West Nile Virus." American Veterinary Medical Association. May 1, 2004 (June 3, 2004). <>.


Centers for Disease Control and Prevention (CDC) Division of Vector-Borne Infectious Diseases. P.O. Box 2087, Fort Collins, CO 80522. (800) 311-3435. <>.

U. S. Food and Drug Administration. 5600 Fishers Lane, Rockville, MD 20857-0001. (888) INFO-FDA. <>.

Bryan Richard Cobb, PhD

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West Nile Virus

West Nile virus

The West Nile virus is a member of the family Flaviviridae, a virus that has become more prominent in Europe and North America in the past decade. The virus, which is closely related to the St. Louis encephalitis virus found in the United States, causes an encephalitis (swelling of the brain) in domestic animals (such as horses, dogs, cats), wild animals, and wild birds. When transferred from an infected animal to a human, the viral infection can produce encephalitis as well as inflammation of nerve cells of the spinal cord (meningitis ).

In 1937, the virus was isolated from a woman in the West Nile District of Uganda. This locale was the basis for the designation of the virus as the West Nile virus. During the 1950s, the ability of the virus to cause the serious and life-threatening human disease was recognized. In the 1960s, the virus was established as a cause of equine encephalitis.

Whether the virus has spread geographically from Uganda, or whether increased surveillance has detected the virus in hitherto unsuspected regions is not clear. However, the pattern of detection has been that of a global dissemination. Long found in humans, animals, and birds in Africa, Eastern Europe, West Asia, and the Middle East, the virus was first detected in North America in 1999.

The virus has come to prominent attention in North America following its 1999 appearance on the continent. That year, 62 cases of the disease were reported in New York City. Seven people died. The following year 21 more cases occurred, and two of the people died. In 1999 and 2000, the West Nile virus was confined to the northeastern coastal states of the United States. However, an inexorable spread to other regions of the country and the continent has begun. In the summer of 2001, dead birds that tested positive for the virus were found as far north as Toronto, Canada, as far south as the northern portion of Florida, and as far west as Milwaukee, Wisconsin. Scientists anticipate that the virus will continue to disseminate. During the summer of 2002, more than 300 cases and at least 14 deaths were reportedwith a continued spread of the virus into the western United States. By August 2002, West Nile virus was reported in 41 states.

The mosquitoes are the prime vector of the West Nile virus. When mosquitoes obtain a blood meal from an infected animal or a bird, they acquire the virus. The virus resides in the salivary glands of the mosquito, to be passed on to a human when the mosquito seeks another blood meal. The cases in New York City, especially those in 2000, are thought to have been caused by the bite of virus-infected mosquitoes that survived the cold winter months. The emergence of the mosquito in the spring can facilitate the re-emergence of the virus. For example in North America, there were large die-offs of crow populations due to West Nile virus in the Spring of 2000 and then again in the Spring of 2001.

Upon entry to a host's bloodstream, multiplication of the virus in the blood occurs. Then, by a mechanism that is not yet deciphered, the virus crosses the barrier between the blood and the brain. Subsequent multiplication of the virus in brain tissue causes nervous system malfunction and inflammation of the infected brain tissue.

Although a large population of mosquitoes may be present, the chances of acquiring West Nile virus via a mosquito bite is small. Data from the examination of mosquito populations indicates that less than one percent of mosquitoes carry the virus, even in areas where the virus is known to be present.

The mosquito to human route of infection is the only route known thus far. The virus is known to infect certain species of ticks. However, as of early 2002, tick-borne out-break of the disease has not been documented in humans. Person to person contact cannot occur. Even exchange of body fluids between an infected human and an uninfected person will not transmit the virus.

Currently no human vaccine to the West Nile virus exists. Prevention of infection consists of repelling mosquitoes by conventional means, such as the use of repellent sprays or creams, protective clothing, and avoiding locations or times of the day or season when mosquitoes might typically be encountered.

See also Viruses and responses to viral infection; Zoonoses

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West Nile virus

West Nile virus, microorganism and the infection resulting from it, which typically produces no symptoms or a flulike condition. The virus is a flavivirus and is related to a number of viruses that cause encephalitis. It usually is transmitted through the bite of several mosquito species, and can infect humans and more than 200 animal species, including alligators, horses, and many common birds. A number of North American bird species, including the blue jay, crow, and house sparrow, act as reservoirs of the virus.

About one fifth of humans infected with the virus develop West Nile fever, which in most people is characterized by fever, headache, muscle ache, joint pain, nausea and vomiting, and in some cases rash and swollen lymph nodes (swollen glands). Less than 1% of all persons infected may develop a severe case, progressing to encephalitis or meningitis, about a week after the initial symptoms of West Nile fever appear. Loss of vision, tremors and convulsions, paralysis, coma, and, in some cases, death may result. Older persons and persons with a weakened immune system are most susceptible to severe cases of the disease, and they may suffer from longer-term effects including weakness and fatigue, headaches, memory loss and confusion, and depression. There is no treatment or vaccine for the virus except to alleviate the symptoms; controlling the mosquitoes that carry the disease is the most effective way to limit its spread.

West Nile virus was first identified in 1937 in the West Nile district of Uganda. It was subsequently found in much of the rest of Africa, the Middle East, and warmer regions of Asia and Europe. Its first recorded appearance in the United States was in Queens, N.Y., in 1999, and it since has spread to most of the United States and neighboring areas in North America.

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West Nile Fever

West Nile Fever

What Is West Nile Fever?

Do Many People Contract WNF?

Is WNF Contagious?

How Do People Know They Have WNF?

How Is WNF Diagnosed and Treated?

Can WNF Be Prevented?


West Nile fever is a viral infection that can result in inflammation of the brain, called encephalitis (en-seh-fuh-LYE-tis). The virus that causes it is spread to humans by infected mosquitoes.

What Is West Nile Fever?

West Nile fever (WNF) is caused by West Nile virus (WNV), which is part of the flavivirus family*. First discovered in Africa, WNV can infect animals and humans, although animals (mainly birds, but also horses, cats, and bats) are the primary hosts* for the virus.

(FLAY-vih-vy-rus) family is a group of viruses that includes those that cause dengue fever and yellow fever.
are organisms that provide another organism (such as a parasite or virus) with a place to live and grow.

Most of the time, people with WNF become only mildly ill. In some cases, however, WNF can develop into a life-threatening disease. If the virus passes into the brain, the infection can cause serious inflammation and complications affecting the nervous system. Of those infected, people older than 50 have the greatest risk of developing severe disease.

Do Many People Contract WNF?

WNF is found most frequently in Africa, the Middle East, Western Europe, and Asia. It was not found in the Western Hemisphere until 1999, when the first case appeared in the United States. Since then, presence of the virus has been documented in 39 states and the District of Columbia. WNF tends to occur more often in the summer and early fall, but the vast majority of cases likely go unreported because they cause only mild illness, if any.

Is WNF Contagious?

Generally, a person cannot contract WNF from another infected person or from an infected animal (transmission of the virus through a blood transfusion has been confirmed in some cases). Likewise, infected people cannot spread the virus to animals. Scientists think that the virus is transmitted almost exclusively by the bite of an infected mosquito. The chances of becoming ill with WNF actually are very small. Of all the mosquitoes in any area where infected mosquitoes have been found, fewer than 1 percent carry the virus.

The transmission cycle begins when a mosquito bites an infected bird and takes in blood that contains WNV. If the mosquito then bites a human, it can transmit the virus to that person. There is no evidence that humans can contract the disease by handling live or dead birds or any other animal that has been infected with the virus. Still, it is never a good idea to handle dead animals with bare hands; experts recommend that people always use disposable gloves and place the dead animal in a plastic bag when disposing of it.

How Do People Know They Have WNF?

The first symptoms of WNV infection are usually fever, headache, and body aches, sometimes accompanied by a rash and swollen lymph nodes*. Serious cases of the disease may cause more severe symptoms, including high fever, stiff neck, muscle weakness, convulsions*, confusion, paralysis*, and coma*. Very severe cases can result in death, but this is rare. Symptoms usually begin 3 to 15 days after infection.

(LIMF) nodes are small, bean-shaped masses of tissue that contain immune system cells that fight harmful microorganisms. Lymph nodes may swell during infections.
(kon-VUL-shuns), also called seizures, are involuntary muscle contractions caused by electrical discharges within the brain and are usually accompanied by changes in consciousness.
(pah-RAH-luh-sis) is the loss or impairment of the ability to move some part of the body.
(KO-ma) is an unconscious state in which a person cannot be awakened and cannot move, see, speak, or hear.

How Is WNF Diagnosed and Treated?

If WNF is suspected, the first thing a doctor will do is take a history, which means asking a person about prior health, when symptoms began,

West Nile Invades New York

In the summer of 1999, dead birds began appearing all over the New York metropolitan area. Public health officials were called in to find out why. They soon learned that the deaths were linked to the virus that causes West Nile fever, an infection that is spread by mosquitoes. Before 1999, West Nile fever had never been seen in the Western Hemisphere.

and recent travels and activities. This may help determine if the person might have been exposed to an infected mosquito. A blood test can confirm the presence of the virus.

For mild cases of WNF, there is no specific treatment. A doctor usually will recommend rest and over-the-counter medications, such as acetaminophen (uh-see-teh-MIH-noh-fen), to ease fever and aches. Severe cases of WNF may require hospitalization and more specialized care, such as intravenous (in-tra-VEE-nus) fluids (fluids given directly into a vein) to prevent or treat dehydration* in someone who is too sick to drink or who is vomiting. A person who is having trouble breathing may be put on a ventilator*.

(dee-hi-DRAY-shun) is a condition in which the body is depleted of water, usually caused by excessive and unre-placed loss of body fluids, such as through sweating, vomiting, or diarrhea.
(VEN-tuh-lay-ter) is a machine used to support or control a persons breathing.

How long WNV illness lasts depends on the severity of the infection. If a person has a mild infection, symptoms often will go away in about a week. Recovery from serious infection may take several weeks to months. Most people who are infected with WNV do not become very sick. Only about 1 percent of all infected people become severely ill. Of these severe cases, up to 15 percent are fatal. Elderly people have the highest risk of developing serious complications from the disease.

Can WNF Be Prevented?

There is no vaccine for WNF, so the best way to prevent the spread of the virus is to prevent mosquito bites. To do so, experts recommend that people avoid being outside at times when mosquitoes are most active (dawn, dusk, and early evening), and that they wear long sleeves and long pants and use insect repellent when outside. When using repellent, it is very important to follow the instructions on the package, especially for children.

In the United States, WNV often has been traced to areas where dead birds have been found. By tracking the disease and looking for patterns of infection, public health officials are better able to prevent future outbreaks. Experts advise people to contact the local or state health department if a dead bird is found in an area where WNV has been reported; a representative will collect the bird for testing.

See also





U.S. Centers for Disease Control and Prevention (CDC), 1600 Clifton Road, Atlanta, GA 30333. The CDC is the U.S. government authority for information about infectious and other diseases. It provides fact sheets about West Nile fever at its website.

Telephone 800-311-3435

Website KidsHealth is a website created by the medical experts of the Nemours Foundation and is devoted to issues of childrens health. It contains articles on a variety of health topics, including West Nile virus.

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West Nile fever

West Nile fever (west nyl) n. a viral disease spread by the Culex pipiens mosquito. It causes encephalitis, with influenza-like symptoms, enlarged lymph nodes, and a bright red rash on the chest and abdomen. In patients with a weakened immune system (such as the elderly) it can progress to convulsions, coma, and paralysis.

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West Nile virus

West Nile vi·rus • n. a flavivirus of African origin that can be spread to humans and other mammals via mosquitoes, causing encephalitis and flulike symptoms, with some fatalities.

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