West Nile Virus (WNV)
The Disease
West Nile virus (WNV) was first isolated in 1937 from a human in the West Nile province of Uganda, Africa. Since then, West Nile viruses have been reported from Africa, Europe, the Middle East, West and Central Asia, and, most recently, the United States and Canada. West Nile virus was first found in the United States in New York City in September 1999. West Nile infection was detected in mosquitoes, birds, horses, and a single human case in South Carolina for the first time during 2002.
The incubation period for WNV in humans (the time from infection to onset of disease symptoms) is usually 3 to 14 days. Most people who are infected with WNV will not have any type of illness. An estimated 20% 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. Symptoms of mild disease will generally last a few days. Severe infection causes West Nile encephalitis (brain inflammation) or meningitis (inflammation of the membranes that surround the brain and spinal cord), and symptoms include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis. Symptoms of severe disease may last several weeks, although effects on the nervous system might be permanent. An estimated 1 in 150 persons infected with the West Nile virus will develop a more severe form of disease.
Clinical signs of WNV infection also have been observed in horses, even though most infected horses do not show symptoms. In general, horses experience a 35% death rate. Like humans, virus levels in horses are generally too low to infect arthropod vectors like mosquitoes. For this reason, humans and horses are considered dead-end hosts incapable of continuing the virus transmission cycle. Birds, however, do experience virus levels capable of infecting mosquitoes, and they help to maintain the virus in nature.
Transmission Cycle
West Nile virus exists primarily as an infection of birds, and it is transmitted from bird to bird by several species of mosquitoes. The principle route of human infection with West Nile virus is through the bite of an infected mosquito. Mosquitoes become infected when they feed on infected birds, which may have the virus in their blood for a few days. The virus eventually finds its way into the mosquito's salivary glands. When a mosquito bites, the virus may be injected into humans and animals, where it can multiply and possibly cause illness. Mosquitoes are capable of transmitting the virus to another animal 10 to 14 days after feeding on an infected bird.
Many types of mosquitoes may carry the virus. One of the main types of mosquitoes that carry the virus is the Southern House Mosquito (Culex quinquefasciatus). That type of mosquito breeds anywhere foul water (water with high organic content) stands for more than a week, such as in rain barrels, tubs, catch basins, cesspools, ditches, ground pools, dairy drains, sewage lagoons, and other similar habitats. It bites at night and may enter homes. West Nile virus has been isolated from ticks, mites, and ked flies, but the importance of non-mosquito arthropods as potential human vectors is not known.
Song birds, shorebirds, owls, and hawks develop sufficient levels of virus to infect most feeding mosquitoes. Certain birds, including Common Grackles, various corvids (American Crows, Blue Jays, Magpies, and Ravens), House Finches, and House Sparrows are highly infectious to mosquitoes. The corvids in North America are highly susceptible to infection and are thus useful as wild indicators for WNV surveillance. Infection is rarely fatal for several common bird species, including House Sparrows, Cardinals, Catbirds, Mourning Doves, and Rock Doves. Bird species such as pigeons, woodpeckers, and ducks do not develop sufficient levels of virus to infect most feeding mosquitoes.