West Nile Virus
West Nile virus is a virus of the family Flaviviridae, found in both tropical and temperate regions. It mainly infects birds, but is known to infect humans, horses, dogs, cats, bats, chipmunks, skunks, squirrels, and domestic rabbits. The main route of human infection is through the bite of an infected mosquito.
Image reconstructions and cryoelectron microscopy reveal 50-nm virions covered with a relatively smooth protein surface. This structure is remarkably similar to the dengue fever virus. Both belong to the genus flavivirus within the family Flaviviridae.
The virus is mostly maintained in birds. Female mosquitoes, mainly of the species Culex pipiens, Culex restuan, and Culex quinquefasciatus, bite infected birds, carry the virus in their salivary glands, and infect other birds when they bite again. Culex pipiens is thought to be the main mosquito species which transmits the virus from birds to mammals. In mammals the virus does not multiply as readily, and it is believed that mosquitoes biting infected mammals do not further transmit the virus. A 2004 paper in Science found that Culex pipiens mosquitoes existed in two populations in Europe, one which bites birds and one which bites humans. In North America 40% of Culex pipiens were found to be hybrids of the two types which bite both birds and humans, providing a vector for West Nile virus. This is thought to provide an explanation of why the West Nile disease has spread more quickly in North America than Europe.
There is no vaccine for humans. A vaccine for horses based on killed viruses exists; some zoos have given this vaccine to their birds, although its effectiveness there is unknown. Dogs and cats show few if any signs of infection. There have been no cases of direct canine-human or feline-human transmission, but these common pets may incubate the virus and pass it along through mosquitoes.
West Nile control is achieved through mosquito control, by elimination of mosquito breeding sites, larviciding active breeding areas and encouraging personal use of mosquito repellants containing DEET. The public is also encouraged to spend less time outdoors, wear long covering clothing and ensure that mosquitos cannot enter buildings.
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