West Nile Virus

Typically, West Nile virus spreads to humans and animals via infected mosquitoes. Mosquitoes become infected when they feed on infected birds, which circulate the virus in their blood for a few days. The virus may also be transmitted through contact with other infected animals, their blood, or other tissues. However, approximately 80% of people who are infected will not show any symptoms.

There also have been reports of possible transmission of the virus from mother to child during pregnancy or breast-feeding or exposure to the virus in a lab, but these are rare and not conclusively confirmed.

Most West Nile virus infections occur during warm weather, when mosquitos are active. The incubation period — the period between when you’re bitten by an infected mosquito and the appearance of signs and symptoms of the illness — ranges from two to 14 days.

West Nile virus has occurred in Africa, Asia, Europe, Middle East and the United States. In a few cases, it might have spread through other routes, screened for the virus, substantially reducing the risk of infection from blood transfusions. Vaccines are available for use in horses but not yet available for people.

Global Distribution of West Nile Virus by Country

Red: human cases or human seropositivity;

Blue: nonhuman/mosquito cases or seropositivity;

Grey: no data or no positives reported. Black lines represent worldwide distribution of the main West Nile virus mosquito vectors, excluding areas of extreme climate denoted by dashed lines. Circled numbers indicate the reported presence of West Nile virus lineages other than lineage 1 in that specific area. For Japan, South Korea, Finland, and Sweden, seropositivity has been detected only in nonresident birds, which was not considered indicative of local transmission.