- West Nile virus (WNV) is a mosquito-borne flavivirus that was first identified in Uganda in 1937. Since then, it has spread across Africa, the Middle East, Europe, Asia, and the Americas. WNV gained global attention when it appeared in the United States in 1999, leading to widespread outbreaks and becoming the most common cause of domestically acquired arboviral disease in North America.
- The virus is primarily maintained in nature through a bird–mosquito–bird transmission cycle, with humans and other mammals considered incidental, or “dead-end,” hosts, as they usually do not develop high enough levels of the virus in their blood to infect mosquitoes.
- The principal vectors for West Nile virus are mosquitoes of the Culex genus, particularly Culex pipiens in urban areas and Culex tarsalis in rural or agricultural zones. These mosquitoes typically feed at dusk and dawn and are strongly associated with stagnant water sources, including storm drains, ponds, and containers. Birds, especially certain passerine species like crows and jays, serve as the primary reservoir hosts. The virus amplifies in the bird population and is passed to mosquitoes when they feed on infected birds.
- Clinically, most WNV infections in humans are asymptomatic—only about 20% of infected individuals develop West Nile fever, a mild flu-like illness with symptoms such as fever, headache, body aches, joint pains, vomiting, diarrhea, or rash. However, approximately 1 in 150 infected people develop severe neuroinvasive disease, particularly older adults and immunocompromised individuals. This can include encephalitis (inflammation of the brain), meningitis (inflammation of the membranes surrounding the brain and spinal cord), or acute flaccid paralysis that resembles polio. In such cases, symptoms can be long-lasting or fatal, and recovery may take weeks or months.
- There is currently no specific antiviral treatment or vaccine available for West Nile virus in humans. Management is supportive and focuses on alleviating symptoms and preventing complications, especially in severe cases requiring hospitalization. Diagnosis is typically confirmed through laboratory testing of blood or cerebrospinal fluid, often using IgM antibody detection or nucleic acid amplification tests.
- Prevention of West Nile virus centers on vector control and personal protective measures. This includes eliminating mosquito breeding sites, using insect repellents, installing window screens, and wearing long sleeves and pants during peak mosquito activity hours. Surveillance of mosquito populations and infected birds is also a key strategy in anticipating and managing outbreaks. As the virus is now endemic in many regions, especially across the continental U.S., ongoing public health vigilance is critical to reducing its impact on human and animal populations.
