West Nile Virus

Transmission & History

West Nile virus is carried by mosquitoes and if transmitted to humans, can cause serious encephalitis. The virus was first reported in 1937 from a woman in the West Nile province of Uganda in Central Africa. West Nile virus was first documented in the United States in New York City during an epidemic in August 1999 and reached Northwest Florida in summer of 2001.

Virus Cycle & Affects

The disease caused by the virus is known as encephalitis and can be severe in the elderly, but it is usually mild in healthy adults and children. Mosquitoes obtain the virus from feeding on infected birds. The virus is then passed along to humans when an infected female mosquito takes a second blood meal from a human instead of a bird. As they are feeding on human blood, they release saliva that contains the virus. The virus enters the human bloodstream along with the mosquitoes' saliva. The mosquito species responsible for the 1999 New York City outbreak have not yet been identified with certainty. The peak time for mosquito blood-feeding is during the time between sunset and sunrise.

Recommended Prevention Steps

Personal protection against biting arthropods, particularly when they are infected with dangerous pathogens, remains one of the most important ways to avoid disease. Avoid mosquitoes. Make sure screens are in good repair to prevent mosquitoes entering areas where there is a threat of encountering infected mosquitoes, wear protective clothing. Fortunately, the USA has some of the best mosquito and arthropod control programs in the world. Vector control and personal protection against vectors and the diseases they carry are the best way to avoid infection with vector-borne pathogens.

The 5 D's of Prevention

  • Dusk and Dawn: Avoid being outdoors when mosquitoes are most active.
  • Dress: Cover your skin with clothing.
  • Deet: Use mosquito repellent on bare skin and clothing.
  • Drain: Remove standing water in which mosquitoes can lay eggs.

Credit: J. Rutherford, Monroe CHD


For more information please visit the Florida Department of Health website and the Florida Cooperative Extension's Electronic Information website.