West Nile virus is a mosquito-borne flavivirus that infects birds, humans and other mammals, causing febrile illness and occasionally severe neurological disease.
Transmission cycle and pathogenesis
West Nile virus circulates primarily between Culex mosquitoes and wild birds. Following ingestion of infected blood, the virus replicates in the mosquito midgut and disseminates to the salivary glands, allowing transmission during subsequent feeds. Birds develop high-level viremia and act as amplifying hosts. Humans, horses and most other mammals are incidental dead-end hosts because they do not sustain viremia at levels sufficient to infect biting mosquitoes. After inoculation in a human host, the virus replicates in dendritic cells and spreads to regional lymph nodes and the bloodstream. Approximately 80 % of infections are asymptomatic, while about 20 % manifest as West Nile fever with headache, myalgia, arthralgia and sometimes a maculopapular rash. In less than 1 % of cases, particularly among older adults or immunocompromised people, the virus crosses the blood–brain barrier, causing meningitis, encephalitis or acute flaccid paralysis. The host innate immune response, especially type I interferons, is critical for controlling viral replication and limiting disease severity.
Epidemiology and public health
The virus was first isolated in Uganda in 1937 and has since become established across Africa, Europe, Asia and the Americas. It entered North America in 1999 and rapidly spread across the continent, causing outbreaks among humans and significant mortality in corvids and other birds. Transmission peaks in late summer and early autumn when mosquito populations are abundant. Monitoring programs track infection in mosquito pools, sentinel chickens and horses to provide early warning of human risk. Laboratory diagnosis relies on detection of specific IgM antibodies, identification of viral RNA by reverse-transcription polymerase chain reaction or culture from clinical samples. There is no specific antiviral therapy or approved human vaccine, so prevention focuses on reducing mosquito breeding sites, using insect repellents, wearing protective clothing and implementing community-level vector control. Public awareness campaigns and timely reporting of cases help guide control measures. Research continues on vaccines and therapeutics to mitigate the impact of West Nile virus as it remains a public health concern.
West Nile virus illustrates how vector-borne viruses can emerge and establish new ecological niches, underscoring the importance of surveillance and integrated mosquito management.
Related Terms: Arbovirus, Mosquito, Flavivirus, Encephalitis, Vector-borne disease