West Nile Virus

West Nile virus (WNV) is a vector-borne disease transmitted by the bite of an infected mosquito. Mosquitoes carry the virus after feeding on infected birds and transmit the disease to humans and other animals.

Studies show that 8 in 10 infected people are asymptomatic (experience no symptoms),  and 1 in 5 infected people develop a mild flu-like illness called ‘West Nile fever’ (New York State Department of Health, 2017). Symptoms can include fever, headache, tiredness, muscle aches, nausea, vomiting, and a rash.

1 in 150 people infected with WNV will develop more severe disease (WHO, 2017), such as encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal cord) (CDC, 2023). Recovery from severe illness may take several months. Among those who experience severe central nervous system illness, 1 in 10 people succumb to a fatal outcome.

West Nile virus takes its name not from the Nile River, but from Uganda’s West Nile district, where it was first identified in 1937.

Anyone who resides in or travels to regions with WNV activity is at risk of infection. In tropical climates, cases occur throughout the year. In the USA and other temperate Northern Hemisphere climates, WNV occurs through June and November, peaking around July and September. Outbreaks often follow major bird migratory routes.

In the USA WNV is endemic in New York State, and is considered a seasonal infection, with cases peaking in some parts of the state each summer.

In 2021, 2,700 cases were reported in the USA and 30 cases were reported in Canada. During 2019–2021, fewer than 1,000 cases were reported in Europe.

• Those over the age of 50 years are at higher risk for severe illness, however, serious infections can occur in people of any age (WHO, 2017).
• Immunocompromised people, for example, people living with HIV or transplant recipients, are at the highest risk for becoming severely ill with WNV infections (ECDC, 2023).
• Those with underlying illnesses, such as diabetes, cancer, and kidney disease, are also associated with more severe disease.

The species of mosquito responsible for the vast majority of WNV transmission to humans is the Culex species. These mosquitoes are mainly active during the evening, but their peak activity occurs at dusk and dawn. The best way to prevent infection is to protect yourself against mosquito bites.