The West Nile virus is a virus that is typically transmitted to humans through mosquito bites. In most cases, infection with the West Nile virus does not present a serious health risk. However, some people who are infected may develop a more severe form of illness that is potentially life-threatening.
Mosquitoes become infected with the West Nile virus after biting birds that carry the virus. The virus moves into the mosquitoes’ bloodstream and circulates for several days before settling into the salivary glands. The mosquitoes then go on to bite and infect people and other animals.
Rarely, the West Nile virus may be spread through blood transfusions, organ transplants, breastfeeding and transmission from a mother to her fetus. Casual contact (e.g., touching, kissing) cannot spread the virus.
The vast majority of people who become infected with the West Nile virus display no symptoms. Within a couple of weeks, they develop antibodies that help clear the virus from their bodies and protect them against future infections. However, some patients may experience serious illness as a result of infection. Brain damage, paralysis and even death may occur.
Various tests may be performed to identify the presence of the West Nile virus. A serology test that can identify the presence of certain antibodies in the blood or cerebrospinal fluid is considered the chief method of diagnosis.
In most cases, no medical treatment is necessary. Milder cases usually resolve on their own, although this may take several weeks. In more severe cases, hospitalization may be required.
The best way to prevent infection by the West Nile virus is to avoid being bitten by mosquitoes. People are urged to use mosquito repellents that have an active ingredient such as Diethyltoluamide(commonly referred to as DEET). People can also reduce mosquito breeding grounds by eliminating standing water collected in buckets, barrels, flower pots and other containers.
To date, more than 550 medications have been researched as a potential treatment for West Nile virus infection, according to the U.S. Food and Drug Administration. Research also continues into developing a vaccine that can prevent illnesses associated with infection by the virus.
About West Nile virus
The West Nile virus is a virus that does not present a health threat for most people, but can be potentially life-threatening to others. It is among a group of disease-causing viruses known as flaviviruses spread by arthropods (insects and insect-like animals that have a hard outer skeleton and jointed limbs) such as mosquitoes and ticks. Infections with the West Nile virus tend to increase from summer to fall in North America.
The West Nile virus first was identified in Africa in 1937, and first found in the Western Hemisphere (in New York City) in 1999. It has since spread throughout the United States. Of the 200 mosquito species in the United States, about 36 are known to carry the virus. The West Nile virus also is present in Asia, Europe, the Middle East, and North and South America.
Mosquito bites are the chief source of West Nile virus infection in humans. Mosquitoes become infected after biting birds that contain the virus. The virus moves into the mosquitoes’ bloodstream and circulates for several days before settling into the salivary glands. The mosquitoes then go on to bite and infect people and other animals.
Experts are not sure exactly how transmission to humans takes place. However, it is believed that the virus enters the bloodstream, multiplies and crosses the blood-brain barrier that separates the blood in the rest of the body from the central nervous system (brain and spinal cord). After crossing the barrier, the virus may infect the brain or its lining, triggering symptoms.
More than 130 species of birds are known to be hosts of the West Nile virus. In the United States, crows and jays are the most likely hosts. In addition, certain types of mosquitoes carrying the virus appear to be more likely to spread it among birds, while other types of mosquitoes are more likely to transmit it to humans and other mammals.
Rarely, the West Nile virus may be transmitted to humans through blood transfusions, organ transplants, breastfeeding and transmission from a mother to her fetus. Casual contact (e.g., touching, kissing) cannot spread the virus.
The vast majority of people who become infected display no symptoms. Within a couple of weeks, antibodies develop that help clear the virus from the body and prevent against future infections. There is some speculation that immunity against infection by the West Nile virus may decrease in later years.
However, some patients may experience serious and even life-threatening illness as a result of infection by the West Nile virus, including encephalitis (inflammation of the brain).
Risk factors and causes of West Nile virus
The West Nile virus is a disease-causing virus from the group of viruses known as flaviviruses. These viruses are spread by arthropods (insects and insect-like animals with hard outer skeletons and jointed limbs), especially mosquitoes. Other infectious diseases caused by viruses in the flavivirus group include dengue, St. Louis encephalitis and yellow fever.
Birds are the primary host of the West Nile virus, although other animals may host the virus. Animals such as horses, cats, dogs, bats, chipmunks, skunks, squirrels and domestic rabbits may become inadvertent hosts of the virus. While horses do frequently experience symptoms of West Nile Virus, most other animals do not appear ill as a result of infection. However, spread of the virus appears to require a carrier such as a mosquito. Thus, infection does not appear to be transmitted from animal to animal, from human to human, or between humans and animals.
Mosquitoes carry the highest amounts of the virus in early fall, and the peak of viral transmission usually occurs in late August and early September. As the weather turns colder, these mosquitoes die off and the risk of becoming infected by the West Nile virus decreases.
People who are age 50 or older are at an increased risk of severe illness as a result of a West Nile virus infection. People who spend a great deal of time outdoors may also have a higher risk of becoming infected because they are more likely to be bitten by mosquitoes. It is suspected that people with compromised immune systems have an increased risk of contracting illness as a result of a West Nile virus infection.
People who receive blood transfusions are not believed to be at significantly greater risk of being infected by the West Nile virus than the rest of the population. All donated blood in the United States is checked for the presence of the West Nile virus using experimental tests, and potential donors are screened and asked not to give blood if they meet certain criteria that suggest the possibility of infection. In addition, screening procedures are now in place to identify and prevent transplant of infected organs.
Still, there are rare but documented cases of West Nile virus being spread through both donated blood and organs. Researchers and organizations such as the U.S. Food and Drug Administration are working on new technologies (e.g., chemical treatments to kill the virus in the blood) that will lower the risk of these types of transmission. Most experts stress that the life-saving benefits of blood and organ transfusion far outweigh the very small risk of becoming infected with the West Nile virus.
Signs and symptoms of West Nile virus
About 80 percent of all people infected by the West Nile virus will not experience any symptoms of illness, according to the U.S. Centers for Disease Control and Prevention. However, the remaining 20 percent of infected people will display symptoms, usually between three and 14 days after being infected.
For most people, these will be mild and last anywhere from a few days to several weeks. Such minor symptoms of infection may include:
Abdominal or back pain
Body aches
Diarrhea
Fever
Headache
Loss of appetite
Nausea
Rash on the back, chest or stomach
Sore throat
Swollen lymph glands
Vomiting
Patients who experience mild symptoms typically do not require medical care. However, pregnant and breastfeeding women should consult with a physician if they experience any of these symptoms.
For some infected people, serious illness may result from infection. Severe illnesses associated with the West Nile virus are known as neuroinvasive diseases because a person’s nervous system is affected. Examples of these diseases include West Nile encephalitis (inflammation of the brain), West Nile meningitis (inflammation of the membranes surrounding the brain and spinal cord) and West Nile meningoencephalitis (disease that combines the two).
Severe symptoms may last for several weeks and may cause permanent neurological damage. These symptoms may include:
Coma
Convulsions
Disorientation
Headache
High fever
Muscle weakness
Neck stiffness
Lack of coordination
Numbness
Paralysis
Stupor
Tremors
Vision loss
People who experience these symptoms should seek immediate medical care. These severe illnesses associated with a West Nile virus infection can cause brain damage, lasting paralysis and even death. Between 3 percent and 15 percent of people who experience a neuroinvasive disease due to the West Nile virus do not survive.
Diagnosis methods for West Nile virus
The first step to identify whether a person has been infected by the West Nile virus involves visiting a physician. A physician will perform a physical examination and compile a medical history.
In addition, antibodies produced by the body in response to the virus may be present in blood or cerebrospinal fluid. A serology test that can identify these antibodies is considered the chief method of diagnosis. In 2003, the U.S. Food and Drug Administration approved a test called the IgM Capture ELISA that also can detect the presence of these antibodies in a person’s blood. This newer test is now generally recommended over a serology test, but serology tests may still be used in some circumstances.
Other tests may be performed to identify the presence of a West Nile virus infection, including:
Complete blood count. This blood test may reveal an elevated white blood cell count, which can indicate the presence of infection (although not necessarily its cause).
Spinal tap. Procedure in which a needle is used to collect a sample of cerebrospinal fluid from the spinal canal for evaluation. This test may reveal an elevated white blood cell count and elevated protein levels. This test can be especially valuable in diagnosing West Nile encephalitis or meningitis.
Magnetic resonance imaging (MRI) test. An MRI of the patient’s head may show evidence of tissue inflammation in about one-third of patients.
Treatment options for West Nile virus
There is no specific treatment for a West Nile virus infection. Antibiotics are not a treatment option because the infection is caused by a virus, and antibiotics have no effect on viruses.
Milder cases usually resolve without treatment, although this may take several weeks. People may take over-the-counter fever-reducer and pain-relief medications to alleviate symptoms. However, no one should take any type of medication, including over-the-counter varieties, without first consulting a physician.
In more severe cases, hospitalization may be required. In such situations, treatment focuses on supportive measures such as providing intravenous fluids, helping the patient with breathing, preventing other infections and providing general nursing care.
Prevention methods for West Nile virus
The best way to prevent infection by the West Nile virus is to avoid being bitten by mosquitoes. People are urged to use mosquito repellents that have an active ingredient currently registered with the U.S. Environmental Protection Agency, such as Diethyltoluamide (commonly referred to as DEET). Other approved active ingredients include picaridin and oil of lemon eucalyptus, according to the U.S. Centers for Disease Control and Prevention.
Products with between 10 percent and 30 percent DEET are often recommended. Repellent with a 10 percent concentration of DEET is effective for about two hours. Wearing repellent is especially important at dusk and dawn, when mosquitoes are most active.
Repellent should not be used on skin covered by clothing. In addition, products should not be sprayed directly onto the face. Instead, the product should be sprayed onto the hands and then applied to the face (avoiding the eyes and mouth). Repellent should not be sprayed onto skin that is irritated or cut. People are urged to stop using repellent if they develop a rash or an allergic reaction.
Products containing 10 percent or less of DEET are safest for children aged 2 to 12. Repellent should not be applied on the hands of children or used on infants under 2 months of age. Instead, the strollers and playpens of infants can be covered with mosquito netting.
Wearing long sleeves and pants reduces the amount of exposed skin on which mosquitoes may feed. However, mosquitoes can bite through thin clothing. Keeping well-maintained screens on windows and doors can help prevent mosquitoes from getting indoors.
Clothing, shoes, bed nets, camping gear and other products that contain the ingredient permethrin also can help ward off mosquitoes. Permethrin is a highly effective insecticide and repellent that continues to protect against mosquitoes, ticks and other arthropods (insects and insect-like animals with hard outer skeletons and jointed limbs) – even after repeated washings.
People can also reduce mosquito breeding grounds by removing any standing water collected in buckets, barrels, flower pots and other containers. Water in bird baths and pet dishes should be regularly changed (at least weekly), and holes should be drilled into tire swings to allow any water collected inside to drain out. Keeping children’s wading pools empty of water and resting on their sides can also help reduce potential breeding areas. Clogged gutters should be cleaned to prevent water from pooling.
Horses can become ill as the result of exposure to West Nile virus, but most other pets do not. Pet owners are encouraged to consult their veterinarian about how best to protect their pets from the virus. Insect repellents designed for humans should not be used on animals.
Ongoing research regarding West Nile virus
To date, more than 550 medications have been tested for their potential in treating a West Nile virus infection, according to the U.S. Food and Drug Administration. About 3 percent of these drugs have shown promise as a potential treatment for infection by the virus.
Research continues as to whether the antiviral drug ribavirin (usually used to treat hepatitis C) may be effective in treating a West Nile virus infection. In addition, interferon therapy (an immune-cell therapy) is being researched as another possible treatment option.
Research continues into developing a vaccine that can prevent illness associated with infection by the West Nile virus. Researchers are also investigating new diagnostic methods for detecting the virus.
Questions for your doctor on West Nile virus
Preparing questions in advance can help patients to have more meaningful discussions with healthcare professionals regarding their conditions. Patients may wish to ask their doctor the following questions about West Nile virus:
How can I tell if I have a West Nile virus infection?
Are my symptoms serious enough to require medical treatment?
What tests will you need to perform to diagnose a West Nile virus infection?
How will I need to prepare for these tests?
What are my treatment options?
What are the side effects of these treatments?
What changes in my symptoms should I report to you?
Is there anything I can do to prevent infection by the West Nile virus?
Where can I find products such as repellent with DEET or clothing treated with permethrin? Are there specific repellents you recommend?
What steps can I take to reduce the mosquito population around my house?
