West Nile Virus (WNV)
West Nile virus belongs to the flaviviruses. The name for this group was influenced by the yellow fever virus, another member of the flavivirus group, as Latin “flavus” means “yellow”. Dengue fever, Japanese encephalitis, Tick-borne encephalitis (TBE), and the Zika virus, for example, also belong to this genus of pathogens.
The virus was discovered in 1937 in the West Nile district of Uganda. In the following decades it appeared in Israel, Egypt and finally also in France. Since then it has spread to all five continents and is now one of the most widespread flaviviruses worldwide. Today, it is the most commonly transmitted virus to humans by mosquitoes in the U.S.
The West Nile Virus transmission cycle
The virus circulates between birds and mosquitoes. When an infected bird is bitten by a mosquito, the mosquito can transmit the virus to another bird, spreading the disease further. While WNV has been detected from 65 species of mosquitoes in 10 genera, Culex mosquitoes, Cx. pipiens (L.), Cx. quinquefasciatus, and Cx. tarsalis (Coquillett), are the primary vector species and most important in maintaining the cycle. WNV has also been isolated multiple times in field-collected Ae. aegypti and Ae. albopictus, but they appear to be of only minor importance with respect to human infections.
Sometimes a virus-carrying mosquito (vector) bites another animal instead of a bird for its next meal, e.g. a mammal such as horses or humans. These are not the actual target of the virus, but they can also get infected with WNV (they are therefore called incidental hosts). Humans and horses are also considered “dead end” hosts because they don’t develop a high enough viremia to infect biting mosquitoes (see Figure 1). In rare cases, transmission from human to human is possible by blood transfusion, organ transplantation, breastfeeding or by transmission from an infected pregnant woman to her unborn child.