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West Nile Virus

(Last updated June 2012)

What are West Nile virus, West Nile fever, and West Nile encephalitis?

West Nile virus is 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. The virus can infect humans, birds, mosquitoes, horses, and some other mammals.

West Nile fever is a case of mild disease in people, characterized by flu-like symptoms. West Nile fever typically lasts only a few days and does not appear to cause any long-term health effects.

More severe disease caused by a person’s being infected with this virus can be West Nile encephalitis, West Nile meningitis, or West Nile meningoencephalitis. Encephalitis refers to an inflammation of the brain; meningitis is an inflammation of the membrane around the brain and the spinal cord; and meningoencephalitis refers to inflammation of the brain and the membrane surrounding it.

Where did West Nile virus come from?

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 where the U.S. virus originated, but genetically, it is most closely related to strains found in the Middle East.

Is West Nile virus now established in the Western Hemisphere?

The continued expansion of West Nile virus in the United States indicates that it is permanently established in the Western Hemisphere.

Is the disease seasonal in its occurrence?

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.

I understand West Nile virus has been found in mosquitoes in this area. What does this mean?

One of the species of mosquitoes found to carry West Nile virus is the Culex species, which survives through the winter, or overwinters, in the adult stage. The virus, along with the mosquitoes, have been documented in Western Pennsylvania in the past. This does not mean that if you are bitten by a mosquito, you will get the disease.

Who is at risk for getting West Nile encephalitis?

All residents of areas where virus activity has been identified are at risk of getting West Nile encephalitis; persons over 50 years of age have the highest risk of severe disease. It is unknown if immunocompromised persons are at increased risk for WNV disease.

What are the symptoms of West Nile virus infection?

Most people who are infected with the West Nile virus will not have any type of illness. It is estimated that 20 percent of the people who become infected will develop West Nile fever—mild symptoms including fever, headache, and body aches, occasionally with a skin rash on the trunk of the body and swollen lymph glands.

The symptoms of severe infection (West Nile encephalitis or meningitis) include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis. It is estimated that one in 150 persons infected with the West Nile virus will develop a more severe form of disease.

What is the incubation period in humans (i.e., time from infection to onset of disease symptoms) for West Nile encephalitis?

Usually three to fourteen days.

How long do symptoms last?

Symptoms of West Nile fever will generally last a few days, although even some healthy people report having been sick for several weeks. Symptoms of severe disease (encephalitis or meningitis) may last several weeks, although neurological effects may be permanent.

What can I do to reduce my risk of becoming infected with West Nile virus?

Here are preventive measures that you and your family can take:

Protect yourself from mosquito bites

  • Apply insect repellent 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 you use 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, as 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.
  • Help reduce the number of mosquitoes in areas outdoors where you work or play by draining sources of standing water. In this way, you reduce the number of places mosquitoes can lay their eggs and breed.
  • At least once or twice a week, empty water from flower pots, pet food and water dishes, birdbaths, swimming pool covers, buckets, barrels, and cans.
  • Check for clogged rain gutters and clean them out.
  • Remove discarded tires and other items that could collect water.
  • Be sure to check for containers or trash in places that may be hard to see, such as under bushes or under your home. Note: Vitamin B and ultrasonic devices are not effective in preventing mosquito bites.

What can be done to prevent outbreaks of West Nile virus?

Prevention and control of West Nile virus and other arboviral diseases are most effectively accomplished through integrated vector management programs. These programs should include surveillance for West Nile virus activity in mosquito vectors, birds, horses, other animals, and humans, and implementation of appropriate mosquito-control measures to reduce mosquito populations when necessary. Additionally, when virus activity is detected in an area, residents should be alerted and advised to increase measures to reduce contact with mosquitoes.

Is there a vaccine against West Nile encephalitis?

No, but several companies are working toward developing a vaccine.

What is the basic transmission cycle of West Nile virus?

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.

If I live in an area where birds or mosquitoes with West Nile virus have been reported and a mosquito bites me, am I likely to get sick?

No. Even in areas where the virus is circulating, very few mosquitoes are infected with the virus. Even if the mosquito is infected, less than 1 percent of people who get bitten and become infected will get severely ill. The chances you will become severely ill from any one mosquito bite are extremely small.

Can you get West Nile encephalitis from another person?

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.

—This information is from the Bayer Advanced website.

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