West Nile: Causes and How It Spreads

Key points

  • West Nile virus disease (West Nile) is caused by a virus primarily spread to people through the bite of infected mosquitoes.
  • West Nile virus is maintained in the environment between mosquitoes and birds.
  • People do not spread the infection other than rarely through blood transfusions, organ transplantation, and mother to baby.
Mosquito full of blood on a person's arm

Primary cause

West Nile virus disease is caused by a virus that is found in the United States and many other places in the world.

The virus belongs to a group of viruses called flaviviruses. St. Louis encephalitis virus and Powassan virus are two other flaviviruses that also circulate in the United States.

How it spreads

  • West Nile virus circulates in the environment between mosquitoes (primarily Culex species) and birds.
  • People become infected with the virus when mosquitoes feed on infected birds and then bite people.
  • People are considered dead-end hosts because unlike birds, they do not develop high enough levels of virus in their bloodstream and cannot pass the virus on to other biting mosquitoes.
  • The virus is rarely transmitted from person-to-person by blood transfusion, organ transplantation, and mother to baby, during pregnancy, delivery, or breastfeeding.
  • Because the virus can be transmitted through blood and organ transplantation, persons who were recently diagnosed with West Nile virus infection should not donate blood or bone marrow for 120 days following infection.
  • Prevent getting sick with West Nile by preventing mosquito bites.