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. Risk of transmission from mother to baby is considered low, but pregnant women should take precautions against mosquito bites. Cases have been reported of workers exposed in the laboratory.
All but three of the 50 US states reported cases in 2016 in people, birds, or mosquitoes. More than 2,000 cases were reported in humans and 119 (5.8%) died in 2015. There are at least 63 species of mosquitoes that 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 are considered "dead-end" hosts, as are horses. People and horses 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.
