West Nile virus (WNV) causes West Nile fever. It is a member of the family Flaviviridae. West Nile virus is primarily transmitted by mosquitoes, mostly species of Culex. The primary hosts of WNV are birds, so that the virus remains within a “bird–mosquito–bird” transmission cycle. WNV has a broad host range, and is also known to be able to infect at least 30 mammalian species, including humans, some non-human primates, horses, dogs and cats. Some infected humans and horses experience disease but dogs and cats rarely show symptoms.
In humans, West Nile virus can cause a disease known as West Nile fever. Approximately 80% of infected people have few or no symptoms, around 20% of people develop mild symptoms, such as fever, headache, vomiting, or a rash, while less than 1% of people develop severe symptoms, such as encephalitis or meningitis with associated neck stiffness, confusion, or seizures. Historically, people in areas where the virus was endemic, such as the Nile Delta, usually experienced subclinical or mild disease.