West Nile virus: symptoms, risk in the UK and how to avoid it

TravelHealthPro says most people infected do not have any symptoms. Approximately 20 percent develop symptoms, including fever, headache, tiredness, body aches, nausea and a rash. When symptoms occur they usually take the form of a mild, self-limiting flu-like illness.

The incubation period (the time it takes for symptoms to appear after being bitten by an infected mosquito) is usually two to six days, and ranges from two to 14 days, and can be up to 21 days in immunosuppressed people.

About one in every 150 cases develops more serious neurological symptoms. TravelHealthPro describes meningitis (inflammation of the lining of the brain and spine), encephalitis (brain inflammation) and acute flaccid paralysis (when one or more limbs become weak or floppy).

• Most infections: no symptoms, or symptoms so mild they go undiagnosed.

• Milder illness: fever, headache, tiredness, body aches, muscle pain, nausea and a red, bumpy rash.

• Neurological disease: high fever, headache, neck stiffness, disorientation, muscle weakness, tremors, convulsions, flaccid paralysis and coma.

• Extreme tiredness is common after both the infection and the rare neurological complications, and depression and personality changes have been observed after the acute illness resolved.

West Nile virus (WNV) belongs to the family of viruses called Flaviviridae, the same family as dengue and yellow fever. UKHSA explains that birds are its natural reservoir and that the virus can be transmitted to humans through mosquitoes. It was first detected in the West Nile district of Uganda in 1937.

TravelHealthPro describes it as spread to humans by mosquitoes and found on every continent except Antarctica. Humans and horses are accidental hosts. The factsheet explains that humans are dead-end hosts, so the virus does not spread directly from person to person through casual contact.

Transmission is mainly by mosquito bite. TravelHealthPro notes isolated reports of non-mosquito spread, such as through blood transfusion and organ transplants, and says UK blood donors who have travelled to affected areas are monitored in collaboration with UKHSA.

TravelHealthPro says supportive care is an essential part of managing people with West Nile virus, but there is no specific treatment. There is also currently no vaccine to prevent the infection in humans.

Clinicians are advised to take a full travel history when assessing returned travellers, and to consider West Nile virus in anyone in the UK with unexplained neurological symptoms associated with recent or concurrent fever when no other diagnosis has been identified. Specialist advice from local infection services must be sought, and appropriate serum or cerebrospinal fluid samples can be sent to the UKHSA Rare and Imported Pathogens Laboratory.

Infection in humans is notifiable to the relevant UK health agencies (UKHSA and Public Health Scotland). For severe disease, care is provided in hospital.

Transmission occurs primarily through the bite of an infected mosquito. TravelHealthPro says humans and animals are dead-end hosts: while they may become infected, they do not spread the infection, so West Nile virus does not spread directly from person to person. No human-to-human spread through casual contact has been documented, and no transmission to health care workers has been reported when standard infection control precautions are followed.

There have been isolated reports of non-mosquito spread, such as through blood transfusion and organ transplants. NHS Blood and Transplant, with UKHSA, monitors blood donors who have travelled to affected areas. The World Health Organization is quoted as advising of one reported case of a pregnant woman passing the virus to her unborn baby, and of a very small proportion of infections through breastfeeding.

UKHSA says there are no known cases of locally acquired WNV in the UK, although there have been a small number of travel-associated cases. A mosquito species able to carry the virus, Culex modestus, was found in the UK in 2010 for the first time since 1944 and is now established in wetlands in Essex and Kent either side of the Thames estuary.

In March 2025, fragments of WNV genetic material were detected in mosquitoes collected in Nottinghamshire in July 2023 during a research study. UKHSA describes this as the first evidence of WNV in any mosquito in the UK, and says it does not necessarily mean mosquitoes are involved in ongoing transmission. Its assessment remains that the probability of a human outbreak is currently, at most, very low.

TravelHealthPro notes that the main risk for UK residents continues to be travel to areas overseas where the virus is established, including parts of mainland Europe, where it has become more widespread with an expanding geographical range and longer transmission seasons.

TravelHealthPro says immunocompromised travellers, people over 50 years old, and people with underlying medical conditions such as cancer, diabetes, hypertension and kidney disease are at greatest risk of severe illness, which can include infection of the central nervous system. It lists older age, underlying illnesses such as cancer, high blood pressure and kidney disease, and genetic factors as risk factors.

Risk of catching the virus during travel depends on how many infected mosquitoes are found at the destination and how much time the traveller spends there, so a longer stay in a risk area may result in more exposure. As of June 2025, TravelHealthPro reports seven confirmed human cases in UK residents since 2000, all linked to travel abroad.

There is no vaccine, so prevention depends on avoiding mosquito bites. TravelHealthPro notes that Culex mosquitoes mainly bite in the evening and at night, but that other mosquitoes capable of spreading WNV bite at other times, so 24-hour bite protection is recommended when visiting risk areas.

Travellers are advised to be aware of WNV risks in destinations, to follow effective bite prevention advice, and to take particular care if they are older, have a weakened immune system or have long-term conditions, since these groups are at greatest risk of severe illness.

Get urgent medical advice if you develop a fever together with a stiff neck, severe headache, confusion, weakness or convulsions after travel to a risk area, because these are the features TravelHealthPro lists for neurological disease. Tell the assessing clinician about your travel, since a full travel history is advised for returned travellers.

If you are unsure how urgent your symptoms are, NHS 111 can advise. Seek emergency help for anyone who is very unwell, confused or has seizures.

For most people the infection is trivial or unnoticed. In the small proportion who develop neurological disease, TravelHealthPro says the fatality rate can be up to 17 per cent, and persistent problems affecting the brain, spinal cord and nerves have been reported in people who develop severe disease. It adds that immunity is considered to follow infection, although how long it lasts is unknown.

Care for severe disease is provided in hospital under NHS infection or neurology teams, with private care an option for those who choose it. Because the infection is rare in UK residents, most people will only need general reassurance and bite-avoidance advice before travel.