West Nile virus death raises concern as locally acquired infections emerge in Netherlands

A person has died after contracting West Nile virus within the Netherlands, as health authorities increase surveillance following evidence that infected mosquitoes are circulating in several parts of the country.

The patient, who lived in the province of Utrecht, contracted the virus in the Netherlands and subsequently died in hospital, according to the country’s National Institute for Public Health and the Environment (RIVM). Authorities have not released further information about the individual out of respect for their family.

The death is significant because the infection was acquired domestically rather than during travel overseas. Until this summer, the last known locally acquired human cases of West Nile virus in the Netherlands had been recorded in 2020.

Three infections believed to have been contracted within the Netherlands had been reported by RIVM by the end of August. A further locally acquired infection has now been confirmed by public health authorities in the Amsterdam-Amstelland region, indicating that human infections are not confined to the initial cases identified around Utrecht.

The first confirmed locally acquired human infection of 2026 was discovered in August through an unusual route. A blood donor from Utrecht tested positive for West Nile virus during a Sanquin study examining donors living in areas where the virus had previously been detected in humans, horses, birds or mosquitoes.

The donor had not recently travelled abroad and had no symptoms, leading authorities to conclude that the infection was probably acquired from a mosquito in the Netherlands. At the time, RIVM said the simultaneous detection of West Nile virus in a human, a bird and a horse provided evidence that mosquitoes capable of carrying the virus were active within the country.

West Nile virus occurs naturally among birds and is primarily spread through mosquitoes. A mosquito can acquire the virus after feeding on an infected bird and subsequently transmit it when biting another bird, a horse or a human.

Humans and horses are considered dead-end hosts because they generally do not develop sufficiently high levels of virus in their blood to continue the normal transmission cycle. The virus therefore does not ordinarily spread directly from one infected person to another.

For most people who become infected, West Nile virus causes little or no illness. RIVM estimates that around 80% of infected people experience no symptoms at all, while approximately 19% develop relatively mild flu-like symptoms including fever, headache, muscle pain and sometimes gastrointestinal symptoms or a rash.

Around 1% of infections can result in serious neurological disease, including encephalitis or meningitis. Among people who develop these severe forms of West Nile disease, RIVM estimates that between 4% and 14% may die, with the risk increasing substantially among older people. For those aged 70 and above who develop severe disease, the fatality rate can reach between 15% and 29%.

People over the age of 50 and those whose immune systems are weakened by illness or medication are considered at greater risk of developing severe disease.

There is currently no human vaccine against West Nile virus and no specific antiviral treatment. People with mild illness generally recover with supportive treatment, while severe neurological disease can require hospitalisation.

Despite the recent infections and death, Dutch health authorities have emphasised that the likelihood of an individual contracting West Nile virus and becoming ill in the Netherlands remains very small.

What has changed is the evidence that the virus is circulating locally.

During 2026, West Nile virus has been detected in the provinces of Utrecht, North Brabant and South Holland, and authorities say it may also be present elsewhere. RIVM has asked doctors across the Netherlands to remain alert when treating patients with symptoms consistent with West Nile fever.

The appearance of locally acquired cases in northern Europe is being watched closely because West Nile virus has expanded its geographical range over recent decades. It has long circulated across parts of Africa, the Middle East, West Asia and North America and is established in southern and central parts of Europe.

The virus has increasingly been detected further north, including in Germany and the Netherlands. Temperature and environmental conditions can influence mosquito populations and the transmission of mosquito-borne diseases, making changes in climate an important part of understanding how their geographical distribution may develop.

The Netherlands first detected West Nile virus in a wild bird in the Utrecht region in September 2020. Shortly afterwards, the country’s first locally acquired human infection was identified in someone believed to have been bitten by an infected mosquito in the same region. Several locally acquired human infections were ultimately identified that year.

The return of human infections six years later does not mean the Netherlands is facing a widespread outbreak. The number of confirmed cases remains very small, and the overwhelming majority of mosquito bites do not pose a West Nile risk.

Health authorities are nevertheless monitoring infections across humans, birds, horses and mosquitoes to understand where the virus is circulating and whether transmission is increasing.

People can reduce their risk by avoiding mosquito bites, particularly around dusk when the common house mosquitoes responsible for transmitting West Nile virus are most active. RIVM recommends wearing clothing that covers the skin, using insect repellent on exposed areas, fitting screens to windows and doors and using mosquito nets where appropriate.

For European health authorities, the significance of the Dutch cases extends beyond a single fatality. A locally acquired infection demonstrates that the entire transmission chain – infected birds, mosquitoes capable of carrying the virus and humans being exposed to them – is occurring within the country.

The risk to any individual in the Netherlands remains very low. But the reappearance of locally acquired human infections after six years, combined with detections in animals and mosquitoes, provides another reason for European public health agencies to watch closely how mosquito-borne diseases are changing their geographical range.

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