West Nile Virus Diagnosis

  • Diagnosis

    The West Nile virus (WNV) is a mosquito-borne virus that was initially discovered in the district of Uganda in 1937. The first detected case of West Nile virus in the western hemisphere occurred in New York, 1999.

    With the first U.S. outbreak of WNV in 1999, there were 62 cases of serious illness. Of these, seven people died from West Nile encephalitis. The number of states affected by WNV had been increasing for several years, but now seems to have levelled off. By the end of 2004, human illness from WNV was documented in every state. Human illness cases in the U.S. exceeded 2740 in 2005, and resulted in at least 88 deaths.

    Most humans infected with WNV have no symptoms of illness. Some develop mild, flu-like symptoms that are referred to as West Nile fever.

    With West Nile fever, the symptoms are often so mild that those affected don't feel it is necessary to seek medical attention. The symptoms can easily be mistaken by individuals as having the flu.

    There is a small risk of developing West Nile encephalitis when infected with WNV. This is a serious illness that requires hospitalization.

    Encephalitis occurs when an infection causes an inflammation of the brain. Cases of West Nile encephalitis in the U.S. are rare. However, hospitalizations and deaths due to West Nile encephalitis have been increasing. Statistically, there is only one case of West Nile encephalitis for every 150 cases of West Nile fever.

    The West Nile virus (WNV) is transmitted through mosquito bites.

    There are over 30 types of mosquitoes that carry WNV. The virus is most often transmitted through the bites of northern and southern House Mosquitoes. These mosquitoes are most active at dawn and dusk.

    Only a small percentage of the mosquito population carries WNV during an outbreak. This is why the size of the mosquito population has such a direct influence on the severity of a WNV outbreak in a particular area. If temperature and rainfall are favorable for mosquito breeding, there will be a large mosquito population. This results in more human and equine (horse) cases of West Nile illness.

    Mosquitoes pick up the virus from infected birds.

    Birds act as hosts to WNV. Infected birds carry a large amount of the virus in their bloodstream. This allows the virus to be passed on to mosquitoes that feed from infected birds. The mosquitoes then transmit the virus to other birds, animals, or humans. Although most WNV infected birds survive, an increase in dead bird sightings can indicate to health officials that WNV is in their state. Ravens, crows, and blue jays are the most likely birds to become ill or die when infected with WNV.

    Testing live chickens, or samples of dead birds (such as crows and blue jays) can provide scientists with early warning of WNV being in a particular region, before humans are infected.

    Humans cannot acquire the virus from insects other than mosquitoes, or from other infected animals.

    There is no evidence to suggest that West Nile virus can be transmitted to humans by ticks, or insects other than mosquitoes. The virus also cannot be transmitted to humans directly from infected birds, horses, or contact with other humans. Researchers have determined that pregnant women who contract West Nile virus can transmit the virus to their fetuses. Researchers are still trying to determine if infected mothers can transmit the virus to their babies via breast milk.

    Humans and domestic animals are referred to as “dead-end” hosts. They may be infected with WNV, but do not transmit the virus back to mosquitoes. There is also no evidence that WNV can be transmitted to people or horses from contact with infected people or horses. This is because the virus in the blood of humans and horses does not reach a high enough level to be infective to other mammals, or to mosquitoes.

    A special circumstance where humans may be vulnerable to WNV from other humans is when they receive a blood transfusion or an organ transplant. This risk, although small, peaks during times of WNV outbreaks in the affected regions of the country. If you need a blood transfusion or organ transplant, your physician will discuss with you the risks and the benefits of the treatment. Transmission in dialysis centers has also been reported.

    You cannot contract WNV or any other infectious disease through donating blood.

    Most people bitten by a mosquito carrying West Nile virus (WNV) do not get sick.

    The majority of people who are bitten by an infected mosquito do not notice any symptoms. It is thought that those who have been exposed to WNV develop lifelong immunity.

    West Nile fever is a minor, self-limiting illness that resembles the flu Table 01.

    Only about 20 - 30% of those infected with the West Nile virus will notice symptoms of illness. If noticeable symptoms of illness occur, they will develop 3 to 14 days after being infected. Those exhibiting mild symptoms are said to have West Nile fever Table 01. Those with West Nile fever usually do not require medical treatment, and will recover fully.

    Table 1.  Symptoms of West Nile fever

    Slight fever
    Headache
    Body aches in the muscles and/or the joints
    Fatigue
    Occasionally, a skin rash and swollen lymph glands may also occur

    West Nile encephalitis is a serious illness that occurs in less than 1% of those infected with the virus Table 02.

    Once the West Nile virus is transmitted to a person through a mosquito bite, the virus multiplies in the bloodstream. The blood-brain barrier exists to protect the brain from infection. However, the West Nile virus is capable of crossing the blood-brain barrier, which may cause encephalitis.

    Encephalitis is when there is inflammation (swelling) of the brain. Encephalitis can be caused by bacteria, fungi, or viruses, including the West Nile virus. Encephalitis happens when brain tissue becomes irritated due to exposure to these germs. When the tissues are irritated, they swell. See Table 02 for symptoms of West Nile encephalitis. These symptoms may last several weeks.

    Meningitis may sometimes occur with encephalitis. Meningitis is the swelling of the membranes (meninges) that surround the brain and spinal cord. A small number of people with West Nile Virus affecting the nervous system develop paralysis, usually in one part of the body. They may or may not also have meningitis or encephalitis.

    West Nile encephalitis is fatal in 10-15% of cases. Those that survive may suffer permanent disability. Lasting effects on the nervous system may include problems walking, talking, and thinking. The permanent effects may be mild or severe.

    Table 2.  Symptoms of West Nile encephalitis

    High fever
    Change in behavior, personality, or consciousness (alertness). This may include confusion, disorientation, and even memory loss or hallucinations
    Intense headache
    Severe muscle weakness that may progress to paralysis (inability to move the muscles)
    Fatigue/lethargy
    Photophobia (eyes sensitive to light)
    Tremors
    Seizures
    Coma, which may lead to death
    If accompanied with meningitis, a stiff neck may also occur

    The risk of being bitten by an infected mosquito is very small. People who live in or travel to areas where West Nile virus (WNV) is active have a higher risk of being exposed to the virus.

    Only about 2% of mosquitoes in an affected region carry the virus during the peak of a WNV outbreak. If an infected mosquito bites you, your risk of developing serious illness from the virus is very low. Only 20-30% of those infected with the virus develop West Nile fever (minor, flu-like symptoms). Less than 1% of those that get the virus become seriously ill with West Nile encephalitis.

    Advanced age or a weakened immune system can increase your risk of becoming ill after exposure to the virus.

    Persons older than 50 years of age have the greatest chance of becoming seriously ill with WNV. People with weakened immune systems, such as the chronically ill, also have a greater risk of becoming ill with WNV. Individuals in the higher-risk groups should be especially careful to avoid mosquito bites (see Prevention).

    It is not necessary to seek testing for West Nile virus (WNV) simply because you have been bitten by a mosquito.

    Your doctor can do blood tests to diagnose WNV if you present with the typical symptoms during WNV season.

    Most of the people tested for WNV are people who are participating in a scientific study, or people who have become seriously ill with West Nile encephalitis. Your doctor will determine if being tested for WNV is right for you.

    If you show signs of West Nile encephalitis, your doctor will draw a blood sample and do a spinal tap to try to diagnose this illness.

    If a person shows signs of encephalitis, it is important for clinicians to determine the cause of the illness. Some illnesses that resemble West Nile encephalitis, such as herpes simplex encephalitis or Guillain-Barré syndrome, may require special treatment.

    A blood sample can show if you've been exposed to WNV. Your clinician will decide the blood test that is right for you.

    A spinal tap, also called a lumbar puncture, is the most common way to diagnose meningitis and encephalitis. After numbing the skin, your clinician inserts a small needle in between the vertebrae (small spinal bones) in your lower back. Several samples of spinal fluid are obtained. Analyzing this fluid, which circulates around the brain and spinal cord, can help your clinician determine what may be causing your symptoms.

    Other tests may include a MRI and/or an EEG (electroencephalogram).

    An EEG uses small sensors resting on your scalp to map the electrical activity of your brain. Your brain waves will be recorded during the test, and your clinician will review the findings for any abnormalities.

    Different causes of encephalitis (e.g., herpes encephalitis) may affect the brain in different ways. Because some kinds of neurological illnesses have specific treatments, it is important that caregivers find the cause of your illness if possible. A MRI (magnetic resonance imaging) may show your clinician the kinds of changes taking place in your brain. This may help your doctor determine the cause of your illness, and be better able to treat it.

    Sometimes, a brain biopsy may be necessary.

    If you are seriously ill and other tests are inconclusive, your clinician may need to take a sample of tissue from the brain itself. This tissue is examined in the laboratory to see what is causing your symptoms.

    There is currently no proven vaccine or drug that prevents or cures West Nile virus (WNV) in humans, though researchers are trying to develop a vaccine.

    A number of companies are testing vaccines and medicines that may prevent WNV infection or decrease the severity of symptoms. However, none have yet been proven effective in humans. Horse and animal trials with a vaccine have been conducted over the last few years. Preliminary results are showing that the vaccine may be effective in preventing some animals from contracting WNV.

    A medication that is currently being used primarily in cancer treatment, Interferon, is being studied to see if it can decrease the symptoms of West Nile illness in humans. Preventing mosquito bites remains the most effective means available to decrease your chance of being infected with WNV.

    Eliminate mosquito breeding areas from your property Table 03.

    Mosquitoes reproduce in wet areas, and areas that have decaying organic matter such as grass clippings, leaves, or animal droppings. If puddles are left standing, a colony of mosquitoes can breed there in less than a week. Puddles as small as those left after watering your garden or washing your car are sufficient to allow mosquitoes to reproduce. See Table 03 for steps you can take to eliminate mosquito breeding areas from your property.

    Table 3.  The do's and don'ts of eliminating mosquito breeding areas on your property

    Remove tires, buckets, and other water-holding objects from your property. Drill drainage holes in water-collecting items that cannot be removed.
    Make sure that your gutters are in working order and free from obstructions. Water left standing in one section of a house gutter can produce millions of mosquitoes in just one breeding season.
    Dispose of litter such as cans, bottles, and containers. Any items that can collect even a small amount of water can provide a breeding ground for mosquitoes.
    Do not allow water to collect in garbage cans or lids.
    Rinse out birdbaths and potted plant reservoir trays at least twice a week.
    Store pet food and water bowls inside when not in use. Replace your pet's water daily.
    If you have outdoor fishponds, consider stocking them with fish that eat mosquitoes.
    Clean and properly chlorinate swimming pools and hot tubs. Be aware that mosquitoes can breed in the small amount of standing water that may collect on pool and hot tub covers.
    Remove weeds and tall grasses near your home. Mow your lawn regularly.

    Know how to avoid mosquito bites Table 04.

    Mosquitoes are most likely to bite in the evening, at night, and early in the morning. They are also attracted by carbon dioxide, which is released in your breath when you exhale. That is why you may notice more mosquito bites when attending an outdoor gathering with many people. Carbon dioxide can attract mosquitoes from over 100 feet away. At closer range, sweating during outdoor activities, and floral fragrances in perfumes or personal care products may attract mosquitoes. See Table 04 for some easy ways you can avoid mosquito bites.

    Table 4.  Easy ways to avoid mosquito bites.

    Keep windows and doors to your home tightly screened (16-18 gauge mesh).
    Avoid going outside during dawn, dusk, and at night.
    Wear protective clothing such as long pants, long-sleeved shirts, and socks.
    Mosquitoes are attracted to darker clothing, so wear light-colored clothing.
    During the day, avoid shaded areas of your property, where mosquitoes are more likely to bite.
    Precautions against mosquito bites should continue until there have been two hard frosts in the fall.

    The most effective personal insect repellants against mosquitoes are those that contain DEET.

    DEET is a commonly used insect repellent. It is effective against numerous insects that bite or feed on humans. It has been found to be very effective in preventing mosquito bites, and can be safely used on exposed skin (follow manufacturer's directions). It can also be applied to clothing.

    DEET has been used for over 40 years, and has been thoroughly studied. It has been found to be practically non-toxic to mammals (including humans) when used appropriately. However, toxic reactions can occur. Toxic are most likely to occur when the product is used incorrectly (such as overuse, or using products with too high of a concentration of DEET).

    Scientists aren't sure how DEET works, but it may interfere with the mosquito's ability to sense chemicals and odors given off by humans that usually attract the mosquito. DEET does not actually kill insects, but repels them.

    There is a wide variety of products that contain DEET. Aerosols, liquids, lotions, towlettes, and wristbands are the most common examples. These products contain varying percentages of DEET. It is important to read the label and determine if the product contains an appropriate amount of DEET to meet your needs Table 05.

    If a reaction to any insect repellent does occur, immediately wash the product from your skin. Call a physician, and take the repellent container with you when seeking medical care. If you get repellent in your eyes, rinse your eyes immediately with water for at least 5-10 minutes and seek medical care.

    Adults should use an insect repellant containing 30% DEET, and follow precautions for the safe use of the product Table 05.

    Table 5.  Insect repellant use for adults: what you need to know

    Always read the entire label and follow manufacturer's instructions. Be sure the repellent is approved by the Environmental Protection Agency (EPA).
    Choose a product with the correct concentration of DEET. For most typical outdoor situations, adults should use products containing no more than 30 to 35% DEET.
    Do not saturate the skin. Apply just enough for a light covering.
    Frequent re-application is not necessary. A 30% concentration of DEET will provide approximately four to six hours of protection. Re-apply only as often as necessary to maintain protection.
    During application, avoid using the product near the eyes, nose, mouth, ears, and genitals. Instead of spraying directly on the face, apply to hands and carefully rub onto the face. Wash hands with soap and water after application.
    Avoid applying insect repellant to any rash, open wound, sunburn or break in the skin. Avoid inhaling the product during application. Do not apply the repellant in an enclosed space, such as a tent. Do not use repellant near food.
    Minimizing exposure to chemicals during pregnancy and nursing is always advisable. Ask your doctor about the benefits and risks of using insect repellant when pregnant or nursing.
    Apply only to skin not covered by clothing. Do not use on skin underneath clothing. Instead, insect repellents can usually be applied to the surface clothing. Check manufacturer's recommendations on the label.
    DEET can damage plastics, leather, materials that are varnished or painted, and some synthetic fabrics. It is safe to use on natural fibers and nylon. Check manufacturer's recommendations before applying to clothing or objects.
    Wash the product from your skin with soap and water after returning indoors.

    Children should use insect repellant containing no more than 10% DEET. Special safety precautions should be followed when using insect repellant on children Table 06.

    Table 6.  Tips for using insect repellant on children 2-12 years of age:

    Always read the entire label and follow manufacturer's instructions.
    Do not let children apply the insect repellent. An adult should always apply the product to children's skin or clothing.
    Do not use on children younger than two years old. Instead, cover the child's stroller or playpen with mosquito netting.
    Apply in a well-ventilated area, so that you and the child can avoid inhaling the product.
    Do not use repellant on children's bedding or pajamas.
    Products used on children should have a concentration of 10% DEET or less. Avoid applying adult-strength insect repellants to children's skin.
    Avoid saturating the skin, or re-applying the product too frequently. One application should last about four to six hours.
    Do not apply the product to children's hands. Their hands may come in contact with their eyes and mouth, or with food.
    Do not apply the product directly to the child's face. Spray onto your hands, then rub your hands on the child's face, avoiding the eyes, nose and mouth.
    Never use repellents on irritated skin, sunburns, or open wounds.
    Children should wash the product from skin immediately after returning indoors.

    Use pesticides containing pyrethroids on fabrics for long-lasting insecticide protection.

    Pyrethroids are a group of synthetic contact insecticides. Where DEET works by repelling mosquitoes, pyrethroids work by killing mosquitoes that come in contact with it. When applied to fabric, pyrethroids maintain potency for at least two weeks, even after the fabric is laundered. Pesticides containing pyrethroids should not be applied directly to the skin. The concurrent use of pyrethroids on clothing and DEET on exposed skin provides powerful protection against mosquito bites. Always follow manufacturer's directions on the label of the product.

    Become familiar with local and state health department policies regarding West Nile virus (WNV) prevention and monitoring.

    Surveillance for the spread of WNV in the U.S. involves random blood sampling of humans, birds, and animals. Surveillance also includes collecting samples of mosquitoes and dead birds. The Health Department in your area can provide information on steps that you should take if you find dead birds. Do not pick up dead birds with your bare hands.

    Since it has been found that some adult mosquitoes carrying the virus do survive over the winter, your local health department may need to implement mosquito control methods. This can include the aerial spraying or targeted ground application of pesticides. Your health department will have information about how you can minimize your exposure to any pesticides that may be used in your area.

  • Prevention and Screening

    There is currently no proven vaccine or drug that prevents or cures West Nile virus (WNV) in humans, though researchers are trying to develop a vaccine.

    A number of companies are testing vaccines and medicines that may prevent WNV infection or decrease the severity of symptoms. However, none have yet been proven effective in humans. Horse and animal trials with a vaccine have been conducted over the last few years. Preliminary results are showing that the vaccine may be effective in preventing some animals from contracting WNV.

    A medication that is currently being used primarily in cancer treatment, Interferon, is being studied to see if it can decrease the symptoms of West Nile illness in humans. Preventing mosquito bites remains the most effective means available to decrease your chance of being infected with WNV.

    Eliminate mosquito breeding areas from your property Table 03.

    Mosquitoes reproduce in wet areas, and areas that have decaying organic matter such as grass clippings, leaves, or animal droppings. If puddles are left standing, a colony of mosquitoes can breed there in less than a week. Puddles as small as those left after watering your garden or washing your car are sufficient to allow mosquitoes to reproduce. See Table 03 for steps you can take to eliminate mosquito breeding areas from your property.

    Table 3.  The do's and don'ts of eliminating mosquito breeding areas on your property

    Remove tires, buckets, and other water-holding objects from your property. Drill drainage holes in water-collecting items that cannot be removed.
    Make sure that your gutters are in working order and free from obstructions. Water left standing in one section of a house gutter can produce millions of mosquitoes in just one breeding season.
    Dispose of litter such as cans, bottles, and containers. Any items that can collect even a small amount of water can provide a breeding ground for mosquitoes.
    Do not allow water to collect in garbage cans or lids.
    Rinse out birdbaths and potted plant reservoir trays at least twice a week.
    Store pet food and water bowls inside when not in use. Replace your pet's water daily.
    If you have outdoor fishponds, consider stocking them with fish that eat mosquitoes.
    Clean and properly chlorinate swimming pools and hot tubs. Be aware that mosquitoes can breed in the small amount of standing water that may collect on pool and hot tub covers.
    Remove weeds and tall grasses near your home. Mow your lawn regularly.

    Know how to avoid mosquito bites Table 04.

    Mosquitoes are most likely to bite in the evening, at night, and early in the morning. They are also attracted by carbon dioxide, which is released in your breath when you exhale. That is why you may notice more mosquito bites when attending an outdoor gathering with many people. Carbon dioxide can attract mosquitoes from over 100 feet away. At closer range, sweating during outdoor activities, and floral fragrances in perfumes or personal care products may attract mosquitoes. See Table 04 for some easy ways you can avoid mosquito bites.

    Table 4.  Easy ways to avoid mosquito bites.

    Keep windows and doors to your home tightly screened (16-18 gauge mesh).
    Avoid going outside during dawn, dusk, and at night.
    Wear protective clothing such as long pants, long-sleeved shirts, and socks.
    Mosquitoes are attracted to darker clothing, so wear light-colored clothing.
    During the day, avoid shaded areas of your property, where mosquitoes are more likely to bite.
    Precautions against mosquito bites should continue until there have been two hard frosts in the fall.

    The most effective personal insect repellants against mosquitoes are those that contain DEET.

    DEET is a commonly used insect repellent. It is effective against numerous insects that bite or feed on humans. It has been found to be very effective in preventing mosquito bites, and can be safely used on exposed skin (follow manufacturer's directions). It can also be applied to clothing.

    DEET has been used for over 40 years, and has been thoroughly studied. It has been found to be practically non-toxic to mammals (including humans) when used appropriately. However, toxic reactions can occur. Toxic are most likely to occur when the product is used incorrectly (such as overuse, or using products with too high of a concentration of DEET).

    Scientists aren't sure how DEET works, but it may interfere with the mosquito's ability to sense chemicals and odors given off by humans that usually attract the mosquito. DEET does not actually kill insects, but repels them.

    There is a wide variety of products that contain DEET. Aerosols, liquids, lotions, towlettes, and wristbands are the most common examples. These products contain varying percentages of DEET. It is important to read the label and determine if the product contains an appropriate amount of DEET to meet your needs Table 05.

    If a reaction to any insect repellent does occur, immediately wash the product from your skin. Call a physician, and take the repellent container with you when seeking medical care. If you get repellent in your eyes, rinse your eyes immediately with water for at least 5-10 minutes and seek medical care.

    Adults should use an insect repellant containing 30% DEET, and follow precautions for the safe use of the product Table 05.

    Table 5.  Insect repellant use for adults: what you need to know

    Always read the entire label and follow manufacturer's instructions. Be sure the repellent is approved by the Environmental Protection Agency (EPA).
    Choose a product with the correct concentration of DEET. For most typical outdoor situations, adults should use products containing no more than 30 to 35% DEET.
    Do not saturate the skin. Apply just enough for a light covering.
    Frequent re-application is not necessary. A 30% concentration of DEET will provide approximately four to six hours of protection. Re-apply only as often as necessary to maintain protection.
    During application, avoid using the product near the eyes, nose, mouth, ears, and genitals. Instead of spraying directly on the face, apply to hands and carefully rub onto the face. Wash hands with soap and water after application.
    Avoid applying insect repellant to any rash, open wound, sunburn or break in the skin. Avoid inhaling the product during application. Do not apply the repellant in an enclosed space, such as a tent. Do not use repellant near food.
    Minimizing exposure to chemicals during pregnancy and nursing is always advisable. Ask your doctor about the benefits and risks of using insect repellant when pregnant or nursing.
    Apply only to skin not covered by clothing. Do not use on skin underneath clothing. Instead, insect repellents can usually be applied to the surface clothing. Check manufacturer's recommendations on the label.
    DEET can damage plastics, leather, materials that are varnished or painted, and some synthetic fabrics. It is safe to use on natural fibers and nylon. Check manufacturer's recommendations before applying to clothing or objects.
    Wash the product from your skin with soap and water after returning indoors.

    Children should use insect repellant containing no more than 10% DEET. Special safety precautions should be followed when using insect repellant on children Table 06.

    Table 6.  Tips for using insect repellant on children 2-12 years of age:

    Always read the entire label and follow manufacturer's instructions.
    Do not let children apply the insect repellent. An adult should always apply the product to children's skin or clothing.
    Do not use on children younger than two years old. Instead, cover the child's stroller or playpen with mosquito netting.
    Apply in a well-ventilated area, so that you and the child can avoid inhaling the product.
    Do not use repellant on children's bedding or pajamas.
    Products used on children should have a concentration of 10% DEET or less. Avoid applying adult-strength insect repellants to children's skin.
    Avoid saturating the skin, or re-applying the product too frequently. One application should last about four to six hours.
    Do not apply the product to children's hands. Their hands may come in contact with their eyes and mouth, or with food.
    Do not apply the product directly to the child's face. Spray onto your hands, then rub your hands on the child's face, avoiding the eyes, nose and mouth.
    Never use repellents on irritated skin, sunburns, or open wounds.
    Children should wash the product from skin immediately after returning indoors.

    Use pesticides containing pyrethroids on fabrics for long-lasting insecticide protection.

    Pyrethroids are a group of synthetic contact insecticides. Where DEET works by repelling mosquitoes, pyrethroids work by killing mosquitoes that come in contact with it. When applied to fabric, pyrethroids maintain potency for at least two weeks, even after the fabric is laundered. Pesticides containing pyrethroids should not be applied directly to the skin. The concurrent use of pyrethroids on clothing and DEET on exposed skin provides powerful protection against mosquito bites. Always follow manufacturer's directions on the label of the product.

    Become familiar with local and state health department policies regarding West Nile virus (WNV) prevention and monitoring.

    Surveillance for the spread of WNV in the U.S. involves random blood sampling of humans, birds, and animals. Surveillance also includes collecting samples of mosquitoes and dead birds. The Health Department in your area can provide information on steps that you should take if you find dead birds. Do not pick up dead birds with your bare hands.

    Since it has been found that some adult mosquitoes carrying the virus do survive over the winter, your local health department may need to implement mosquito control methods. This can include the aerial spraying or targeted ground application of pesticides. Your health department will have information about how you can minimize your exposure to any pesticides that may be used in your area.

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