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General Information About Invasive Species


  West Nile Virus

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West Nile virus (WNV) is a flavivirus, a virus of the family Flaviviridae; these viruses are usually transmitted by mosquitoes and ticks and cause other serious diseases in addition to WNV, including yellow fever, dengue fever, and hepatitis C. WNV was first isolated in 1937 and has been commonly found in humans, birds, and other vertebrates in Africa, Eastern Europe, West Asia, and the Middle East, but it had not been documented in the Western Hemisphere until cases were found in the New York City area in the fall of 1999. The virus can cause encephalitis, which is an inflammation of the brain. WNV interferes with normal central-nervous-system functioning. It is closely related to the St. Louis encephalitis virus which is also found in the United States. In 2000, the disease occurred in twenty-one people in the New York City area and northern New Jersey, with only two fatalities. By 2005, the virus was found in all the conterminous United States and that year it caused a total of 116 deaths.

West Nile Virus (WNV) is transmitted to humans through the bite of a mosquito. Mosquitoes become infected when they feed on infected birds, which may have high virus levels in their blood for a few days. Infected mosquitoes can then transmit WNV when they feed on humans and animals. The virus has been found in at least 48 species of mosquitoes, over 250 species of birds, and at least 18 mammalian species. WNV is not transmitted from person to person and there is no evidence that a person can get the virus from handling infected birds or other animals, either live or dead.

Most human infections are mild and symptoms include fever, headache, and body aches, often with skin rash 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.

There is no specific treatment for WNV and, while a vaccine for horses is available, for people there is no vaccine against the disease. In more severe cases, treatment may include hospitalization, use of intravenous fluids and nutrition, airway management, ventilator support, prevention of secondary infections, and good nursing care. Anyone who thinks they have WNV should seek medical care as soon as possible. Fatality rates in humans range from 3 percent to 15 percent and are highest in the elderly.

The best way to avoid WNV is to prevent mosquito bites.

  • When you are outdoors, use insect repellent containing an EPA-registered active ingredient. Follow the directions on the package.
  • Many mosquitoes are most active at dusk and dawn. Be sure to use insect repellent and wear long sleeves and pants at these times or consider staying indoors during these hours.
  • Make sure you have good screens on your windows and doors to keep mosquitoes out.
  • Get rid of mosquito breeding sites by emptying standing water from flower pots, buckets and barrels. Change the water in pet dishes and replace the water in bird baths weekly. Drill holes in tire swings so water drains out. Keep children's wading pools empty and on their sides when they aren't being used.
  Illustration of a West Nile Virus transmission cycle
West Nile Virus transmission cycle.
Source: Centers for Disease Control and Prevention (CDC)
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  Surveillance Activity

The Centers for Disease Control and Prevention (CDC), Division of Vector-Borne Infectious Diseases collects information on the occurrence of WNV and works to develop improved methods for disease diagnosis, surveillance, prevention, and control. In the year 2000, organizations in only 19 States and localities, along the Atlantic and Gulf coasts, actively participated in this program. By 2005, 49 states, 5 cities, and the District of Columbia were participating in surveillance activities. Data are collected weekly and include information on wild birds, sentinel chicken flocks, human cases, veterinary cases, and mosquito surveillance. WNV activity is reported to the CDC through ArboNet, a web-based surveillance-data network.

Surveillance activities are intended to:

  • detect WNV activity
  • monitor trends
  • assess the impact of the disease on the human population
  • identify both geographic areas and populations that are at high risk
  • provide information for the allocation of resources
  • assess the need for and timing of interventions
  • identify larval habitats for targeted control
  • improve prevention and control measures
  • develop a better understanding of transmission cycles and potential vector species
  • provide national and regional information to public health officials, elected government officials, and the public

Wild Bird Surveillance: Both live and dead birds may be tested for evidence of the disease. Surveillance for dead crows, in particular, is a sensitive means to detect the presence of WNV in an area.

Sentinel Chicken Flock Surveillance: Uninfected chicken flocks are placed in areas where WNV transmission is expected and are then monitored for infection. Chickens readily survive WNV infection.

Human Surveillance: Physicians observe patients for clinical signs of WNV and conduct laboratory testing as appropriate. In fatal cases, additional laboratory tests may be conducted on tissue and blood specimens.

Veterinary Surveillance: Veterinarians monitor horses, dogs, cats, and other species for clinical signs of disease and conduct laboratory testing as appropriate.

Mosquito surveillance: Adult mosquitoes are trapped and tested for the presence of WNV. Information on the affected species and the density of infection is used to identify potential vector species (those mosquito species which are capable of spreading the disease) in a particular area, and to predict the subsequent risk to humans, wild birds, and other animals.

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