West Nile Virus belongs to the Flaviviridae family of viruses, along with diseases such as dengue and yellow fever. It emerged in Uganda in the 1930s but has since spread to North America, Europe, Africa, and parts of Asia. The virus circulates mainly between birds and mosquitoes, which then go on to infect humans and other mammals.
Most people infected do not develop symptoms, but a small proportion of those who do may experience serious complications. Outbreaks occur during warmer months when mosquitoes are most active.
Only about 20 per cent of people infected with West Nile Virus will become ill. When symptoms do occur, they typically appear two to 14 days after being bitten.
Mild symptoms may include:
Severe symptoms include:
Severe West Nile Virus infections can lead to encephalitis (inflammation of the brain) and meningitis (inflammation of the membranes surrounding the brain and spinal cord). Long-term complications include muscle weakness, hearing and memory loss and depression.
West Nile Virus is primarily spread via mosquitoes of the Culex species, who feed on infected birds and then pass the virus to humans. Human-to-human transmission can occur through blood transfusions, organ transplants, and from mother to baby during pregnancy, delivery, or breastfeeding. There is no evidence that it can be spread through day-to-day contact.
There is no specific treatment for West Nile Virus and care is focused on relieving symptoms. Anyone with severe symptoms should seek treatment urgently. People with mild symptoms should rest and stay hydrated.
To prevent the virus those living in at-risk areas should avoid mosquito bites and control the insects. Measures include using insect repellent containing DEET, wearing long-sleeved clothing, and installing window and door screens. Culex mosquitoes bite at dusk and night time so sleeping under a bed net is also advised.
Mosquitoes lay eggs on stagnant or still water so eliminating standing water around homes, gardens and neighbourhoods is important.
Since being first identified in the West Nile region of Uganda in 1937 the disease has spread around the world. The first case in the United States occurred in 1999 and now several thousand people are infected in the US every year. In 2024 there were nearly 1,800 cases and 164 deaths.
The first large outbreak in Europe took place in Romania in 1996 and there have been cases in Spain, France, Italy, Portugal and Greece. The Culex mosquito was detected in southern England in 2010 for the first time since 1944 – and it is now established in wetlands in Essex and Kent. However, there have been no cases of the disease in the UK and the risk is considered very low.
gov.uk/guidance/west-nile-virus
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