West Nile Virus: History, Distribution, and Characterization(updated 9/11/02)
In an attempt to help provide factual information about the transmission and prevention of infection by the West Nile virus, the Disease Prevention Education Division of Health and Safety, Youth and Community Services has compiled the following information from several federal agencies. This information has been compiled for general information purposes only and anyone using the information should not represent the American Red Cross as an authority in this area.
Background: The Virus' History, Distribution and Characterization
Introduction: West Nile Virus (WNV) has emerged in recent years in temperate regions of Europe and North America, presenting a threat to public, equine (horse) and animal health. The most serious manifestation of the WNV infection is fatal encephalitis (inflammation of the brain) in humans and horses, as well as mortality in certain domestic and wild birds.
Background: WNV is transmitted to humans through mosquito bites. Mosquitoes become infected when they feed on infected birds that have high levels of WNV in their blood. Infected mosquitoes can then transmit WNV when they feed on humans or other animals.
WNV is not transmitted from person to person and there is no evidence that a person can become infected by handling live or dead infected birds. But, to add a further level of safety, if birds or other potentially infected animals must be handled, a protective barrier (e.g., gloves, plastic bags) should be used.
Most humans infected with WNV have no symptoms. A small proportion develops mild symptoms that include fever, headache, body aches, skin rash and swollen lymph glands. Less than 1 percent of infected people develop more severe illness that includes meningitis (inflammation of the spinal cord) or encephalitis (inflammation of the brain). The symptoms of these illnesses can include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness and paralysis. Of the few people that develop encephalitis, a small proportion die but, overall, this is estimated to occur in less than 1 out of 1000 infections. The risk of severe disease is higher for persons 50 years of age and older.
There is no specific treatment for WNV infection or vaccine to prevent it. Treatment of severe illnesses includes hospitalization, use of intravenous fluids and nutrition, respiratory support, prevention of secondary infections and good nursing care. Medical care should be sought as soon as possible for persons who have symptoms suggesting severe illness.
Geographic Distribution:* In the U.S. from 1999 through August 14, 2002, WNV has been documented in Alabama, Arkansas, Connecticut, Delaware, the District of Columbia, Florida, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Nebraska, New Hampshire, New Jersey, New York, North Carolina, North Dakota, Ohio, Oklahoma, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Vermont, Virginia, West Virginia and Wisconsin.
From 1999 through 2001, there were 149 cases of West Nile virus human illness in the United States reported to the Centers for Disease Control and Prevention and confirmed, including 18 deaths. In 2002, WNV activity has spread to most eastern and mid-western states, with 113 cases and 5 deaths as of August 2002.
*It is anticipated that the geographic distribution information will change. For the most up-to-date disease surveillance, please check case information at http://www.cdc.gov/ncidod/dvbid/westnile/index.htm
Questions and Answers
Q. Where did West Nile Virus come from?
A. The West Nile Virus has been commonly found in humans and birds and other vertebrates in Africa, Eastern Europe, West Asia and the Middle East, but until 1999 had not previously been documented in the Western Hemisphere. It is not known from where the U.S. virus originated, but it is most closely related genetically to strains found in the Middle East.
Q. How is West Nile virus transmitted?
A. Mosquitoes become infected when they feed on infected birds, which may circulate the virus in their blood for a few days. Infected mosquitoes can then transmit West Nile virus to humans and animals while biting to take blood. The virus is located in the mosquito's salivary glands. During blood feeding, the virus may be injected into the animal or human, where it may multiply, possibly causing illness.
Note: Different viruses are transmitted in different ways. Unlike the West Nile Virus, HIV (human immunodeficiency virus), the virus that causes AIDS, is not spread through mosquito bites.
Q. How does West Nile Virus actually cause severe illness and death in humans?
A. Following transmission by an infected mosquito, West Nile Virus multiplies in the person's blood system and crosses what is called the 'blood-brain barrier' to reach the brain. The virus interferes with normal central nervous system functioning and causes inflammation of brain tissue or encephalitis.
Q. What is West Nile encephalitis?
A. "Encephalitis" means an inflammation of the brain and can be caused by viruses and bacteria, including viruses transmitted by mosquitoes. West Nile encephalitis is an infection of the brain caused by West Nile Virus, a flavivirus commonly found in Africa, West Asia, and the Middle East. It is closely related to St. Louis encephalitis virus found in the United States.
Q. Can you get West Nile encephalitis from another person?
A. No. West Nile encephalitis is NOT transmitted from person-to-person. For example, you cannot get West Nile Virus from touching or kissing a person who has the disease, or from a health care worker who has treated someone with the disease.
Q. What are the symptoms of West Nile encephalitis?
A. Most infections are mild, and symptoms include fever, headache, and body aches, occasionally with a skin rash on the trunk of the body and swollen lymph glands. More severe infection may be marked by headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, paralysis and, rarely, death.
Symptoms of mild disease will generally last a few days. Symptoms of severe disease may last several weeks, although neurological effects may be permanent.
Q. What is the incubation period in humans (i.e., time from infection to onset of disease symptoms) for West Nile encephalitis?
A. Usually 3 to 15 days.
Q. What should I do if I think I have symptoms of West Nile Virus?
A. Contact your health care provider if you have concerns about your health. If you or your family members develop symptoms such as high fever, confusion, muscle weakness and severe headaches, you should see your doctor immediately.
Q. Is the disease seasonal in its occurrence?
A. In the temperate zone of the world (i.e., between latitudes 23.5° and 66.5° north and south), West Nile encephalitis cases occur primarily in the late summer or early fall. In the southern climates where temperatures are milder, West Nile Virus can be transmitted year round.
Q. How many cases of West Nile encephalitis in humans have occurred in the U.S.?
A. In 1999, 62 cases of severe disease, including 7 deaths, occurred in the New York area. In 2000, 21 cases were reported, including 2 deaths in the New York City area. In 2001, there were 66 human cases of severe disease and 9 deaths. No reliable estimates are available for the number of cases of West Nile encephalitis that occur worldwide. Please see CDC's current case count for information on 2002 cases in the U.S.
Q. What can I do to reduce my risk of becoming infected with West Nile Virus?
A. Here are preventive measures to take:
- Apply insect repellent containing DEET sparingly to exposed skin. The more DEET a repellent contains the longer time it can protect you from mosquito bites. A higher percentage of DEET in a repellent does not mean that your protection is better-just that it will last longer. DEET concentrations higher than 50 percent do not increase the length of protection. Choose a repellent that provides protection for the amount of time that you will be outdoors.
- Repellents may irritate the eyes and mouth, so avoid applying repellent to the hands of children.
- Whenever using an insecticide or insect repellent, be sure to read and follow the manufacturer's DIRECTIONS FOR USE, as printed on the product.
- Spray clothing with repellents containing permethrin or DEET since mosquitoes may bite through thin clothing. Do not apply repellents containing permethrin directly to exposed skin. If you spray your clothing, there is no need to spray repellent containing DEET on the skin under your clothing.
- When possible, wear long-sleeved shirts and long pants whenever you are outdoors.
- Place mosquito netting over infant carriers when you are outdoors with infants.
- Consider staying indoors at dawn, dusk, and in the early evening, which are peak mosquito biting times.
- Install or repair window and door screens so that mosquitoes cannot get indoors.
- To avoid helping mosquitoes breed in your environment, drain standing water. Routinely empty water from flower pots, pet bowls, clogged rain gutters, swimming pool covers, discarded tires, buckets, barrels, cans and other items that collect water in which mosquitoes can lay eggs.
Note: Vitamin B and "ultrasonic" devices are NOT effective in preventing mosquito bites.
Q. Which mosquito repellent works the best?
A. The most effective repellents contain DEET (N,N-diethyl-m-toluamide), which is an ingredient used to repel pests like mosquitoes and ticks. DEET has been tested against a variety of biting insects and has been shown to be very effective. The more DEET a repellent contains the longer time it can protect you from mosquito bites. A higher percentage of DEET in a repellent does not mean that your protection is better-just that it will last longer. DEET concentrations higher than 50% do not increase the length of protection.
Q. Is DEET safe for pregnant or nursing women?
A. There are no reported adverse events following use of repellents containing DEET in pregnant or breastfeeding women.
Q. How should products containing DEET be used on children?
A. No definitive studies exist in the scientific literature about what concentration of DEET is safe for children. The American Academy of Pediatrics has recommended that a cautious approach is to use products with a low concentration of DEET, 10 percent or less, on children aged 2 - 12. Most guidelines cite that it is acceptable to use repellents containing DEET on children over 2 years of age. It is recommended that for children under 2 years of age only one application per day of repellent containing DEET should be used.
Parents should choose the type and concentration of repellent to be used by taking into account the amount of time that a child will be outdoors, exposure to mosquitoes, and the risk of mosquito-transmitted disease in the area. Persons who are concerned about using DEET or other products on children may wish to consult their health care provider for advice. The National Pesticide Information Center (NPIC) can also provide information through a toll-free number, 1 (800) 858-7378 or http://npic.orst.edu.
Always follow the recommendations appearing on the product label when using repellent.
- When using repellent on a child, apply it to your own hands and then rub them on your child. Avoid children's eyes and mouth and use it sparingly around their ears.
- Do not apply repellent to children's hands. (Children tend to put their hands in their mouths.)
- Do not allow young children to apply insect repellent to themselves; have an adult do it for them. Keep repellents out of reach of children.
- Do not apply repellent to skin under clothing. If repellent is applied to clothing, wash treated clothing before wearing again.
- Do not apply aerosol or pump products directly to your face. Spray your hands and then rub them carefully over the face, avoiding eyes and mouth.
- Do not apply repellent to cuts, wounds or irritated skin.
- After returning indoors, wash treated skin with soap and water.
Q. Is there a vaccine against West Nile encephalitis?
A. No, but several companies are working towards developing a vaccine.
Q. Is West Nile virus transmitted through blood transfusions?
A. No cases of West Nile Virus attributed to transfusion of blood or blood products have been reported either in the US or in other areas of the world with previous epidemics. While there is a theoretical possibility that WNV can be transmitted by blood transfusion the risk is regarded as extremely low. In areas where there are human cases, the low theoretical risk is further reduced by careful attention to existing donor screening procedures.
This information has been compiled from materials provided by several federal agencies. For additional information or resource documentation, please reference the following Web sites:
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