West Nile Virus Reaches Flanders: Five Probable Cases
Belgian health authorities have confirmed the first locally acquired human infections of West Nile virus (WNV) in Flanders, reporting five probable cases — four of whom showed neurological symptoms and one of whom was hospitalised. The Flemish Department of Care (Departement Zorg) said on 11 September that the cases are classified as probable pending confirmatory testing, and that the overall risk to the public remains very low.
“Further tests will have to provide certainty,” Anneleen De Sadeleer, spokesperson for the Departement Zorg, told VRT NWS. She stressed that “the news of infections in people does not mean Flemings should worry,” adding that “quarantine is not necessary.”
The infections were recorded in the provinces of Antwerp and Flemish Brabant — the same regions where WNV had already been detected in horses and birds in the weeks before. To protect privacy, no further details about the patients’ identity or residence have been released.
A Virus Spread by Mosquitoes
West Nile virus is a flavivirus spread mainly through mosquito bites. Mosquitoes become carriers after biting an infected bird and can then transmit the virus to humans and other mammals. In this transmission cycle, humans and horses are “dead-end hosts” — they do not produce enough virus in their blood to infect feeding mosquitoes, so the virus cannot spread from person to person.
In Belgium, the primary vector is Culex pipiens, the common house mosquito. A KU Leuven study demonstrated for the first time that Culex pipiens mosquitoes caught in the Leuven area can be infected with the virus and can transmit it through their saliva. “Our research group is actively studying this,” virologist Leen Delang said, per the VRT NWS explainer.
For most people, the infection is mild or entirely silent. According to the Dutch public health institute RIVM, roughly 80 percent of infected people show no symptoms at all. About 19 percent develop flu-like symptoms such as fever, headache, muscle pain, rash and swollen lymph nodes. In approximately 1 percent of cases, the virus triggers severe neurological complications, including encephalitis or meningitis.
The severe form is more common among people over 50 and those with weakened immune systems, and it can be fatal. There is no specific treatment and no human vaccine; care focuses on relieving symptoms.
More Cases Likely, Virologists Say
The five probable cases are almost certainly an undercount. “There are certainly more West Nile virus infections than the 5 we have already found,” virologist Steven Van Gucht of the public health institute Sciensano told HLN. “It is not really a human virus. Occasionally it ends up in humans almost by accident.”
Van Gucht described WNV as fundamentally a virus of birds that only rarely spills over into people. “This remains something rare and it will remain rare in the future,” he said, noting that across Europe there are roughly a thousand serious cases each year, most of them in Italy. He also expects the virus to persist: “I suspect we will still see the virus here next spring and next summer.”
Flemish authorities said they expect further infections until the mosquito season ends in October. “The risk decreases as the days get shorter and temperatures fall,” the Departement Zorg said.
A Regional and European Picture
The Belgian cases follow a sharp rise in infections in neighbouring countries. In the Netherlands, 18 people had been infected and three had died as of 10 September — a doubling from nine cases the week before, according to VRT NWS. One of the Dutch patients contracted the virus during a holiday in Southeastern Europe.
Across Europe, the European Centre for Disease Prevention and Control (ECDC) had registered 1,086 human infections in 15 countries this season as of early September, dominated by Italy (516), Greece (284) and Spain (88).
In Belgium, animal surveillance had already signalled the virus’s arrival. The country recorded its first confirmed WNV infection in a horse on 7 August; since then, 25 horses and several birds have been infected, and two horses have been euthanised. That animal data pointed authorities toward the regions where human cases later appeared.
The Climate Connection
Scientists increasingly link the virus’s northward march to climate change. A 2024 study published in Nature Communications by researchers at the VUB and ULB found that the area of Europe ecologically suitable for WNV circulation increased markedly over the 20th century, while it remained largely unchanged in a counterfactual scenario without climate change. “The drastic increase in the human population at risk of exposure is partly due to historical changes in population density, but climate change has also been a critical driver,” the study concluded.
“Unlike corona, this is a virus where climate factors are very important for possible infections,” climate scientist Wim Thiery of the VUB said. “Besides escalating climate extremes, the emergence of tropical diseases in Europe is unfortunately one of the many logical consequences of our addiction to oil, coal and gas.”
Protecting Yourself
With no vaccine or targeted treatment available, prevention centres on avoiding mosquito bites. Health authorities recommend wearing long clothing when mosquitoes are active, using insect repellent, installing screens on windows and doors, using mosquito nets where needed, and removing standing water from flower pots and other containers — prime breeding sites.
What to Watch
Belgium still lacks systematic monitoring of WNV in mosquitoes, a gap researchers have flagged. Whether the country moves to close it — and whether the five probable cases are confirmed by pending tests — will shape how authorities respond as warming continues to push the virus further north. For now, officials’ message to the public is one of measured calm: the virus is here, but the risk of serious illness for most people remains small.