Diseases > Mosquitoes > West Nile

West Nile

West Nile infections, caused by West Nile virus (WNV), were initially diagnosed in New York City in 1999. WNV spread westward during subsequent years with the first confirmed cases (a total of 292) in Indiana in 2002. Since then, West Nile infections have been the most common mosquito-borne disease reported to CDC in the U.S. (nearly 3,000 cases in 2005) and in Indiana (69 cases in 2005). Most people who become infected with WNV will experience no or very mild symptoms. Approximately 20% of infected individuals will experience flu-like symptoms. However, WNV can cause severe disease and death in humans. The fatality rate in patients with diagnosed infections and clinical disease is 3-15%. Purdue University extension publication E-240-W "West Nile virus in Indiana" (PDF 920KB) provides detailed information on WNV, disease in humans, and practical information regarding mosquito control. WNV virus also can cause severe disease and death in horses.

Causative agent

• A virus in the Family Flaviviridae, genus Flavivirus.

Geographical distribution of cases

• Africa, Europe, and North America.

Symptoms of infection

• Typically flu-like, including headache, fever, muscle ache, and nausea.
• Severe symptoms include encephalitis and/or meningitis.
• Disease usually is most severe in the elderly, but serious illness can occur in younger people.
• NOTE: an infection can result in long-term sequelae.

Reservoir hosts of WNV

• Numerous species of birds, including the house sparrow and perhaps the robin.
• NOTE: birds infected with WNV can die, the most noticeable being crows, blue jays, hawks, and owls.

Vectors of WNV in Indiana

• Culex restuans is an amplifying vector among birds, initiating WNV transmission as early as mid-April.
• C. pipiens and C. salinarius are bridge vectors, transmitting WNV from birds to humans by late summer.
• NOTE: female Culex mosquitoes feed from early evening into night, when your risk of exposure to infected mosquitoes is greatest.
• NOTE: Culex larvae typically develop in water with a high organic content, including highly polluted habitats, and are also noted for developing in small containers such as discarded tires, cans, rain gutters, etc.

Modes of transmission

• Via the bite of infected Culex females.
• Via contact with body fluids of infected birds.
• NOTE: human-to-human transmission via blood transfusion and organ transplants has been documented.

Diagnosis of infection

• Symptoms listed above together with their onset in late summer-early fall when Culex numbers are high.
• Laboratory tests that detect antibodies to WNV in a patient's blood.

Treatment of infection

• Supportive measures only; there are no anti-viral drugs currently available.

Prevention of infection

• There is no vaccine for humans, but an effective vaccine exists for horses.
• Avoid outdoor summer activities from early evening into night when Culex females seek blood meals.
• Wear clothing consisting of a long sleeved shirt, long pants, and a hat.
• Use a repellent recommended by CDC and treat clothing with permethrin.

Control of vectors

• Eliminate larval developmental sites such as containers in yards and on business properties that hold water.
• Larvicides as needed based on monitoring aquatic sites in which Culex larvae develop.
• Adulticides if warranted by monitoring WNV in birds and Culex mosquitoes, and disease cases in humans.