West Nile virus (WNV) is the most common mosquito-borne disease in the US. It can also be transmitted by blood transfusions, organ transplants, from mother to baby during pregnancy, at birth, and through breastfeeding. The risk of transmission from mother to baby is considered low, but pregnant patients should take precautions against mosquito bites. Cases have been reported of workers exposed in the laboratory.
In 2024, 49 out of 50 US states had reported cases in people, birds, or mosquitoes. At least 63 species of mosquitoes transmit WNV in the US, and WNV has been found in at least 330 species of birds. The virus is amplified in birds, which are the predominant host of WNV. People, horses, and other mammals are considered "dead-end" hosts. They may get ill, but the virus is not transmitted to mosquitoes in subsequent bites because the viral load in the blood is low.
Cases are reported from May until November, with most cases occurring in July, August, and September when people are outside while the mosquitoes are active.
