West Nile Virus Spreads in Europe as Climate Change Expands

As climate change reshapes disease patterns across Europe, the West Nile virus has emerged as a growing public health concern. The mosquito-borne pathogen has now been detected in 12 European countries this year, up from nine at the same point in 2025, with Italy and Greece recording the highest case numbers at 311 and 157 respectively, according to VRT NWS.

What Is the West Nile Virus?

West Nile virus is a flavivirus related to dengue, yellow fever, and Japanese encephalitis, first identified in the West Nile region of Uganda in 1937. It circulates naturally between mosquitoes—primarily of the Culex genus—and birds, which serve as reservoirs without typically developing symptoms. Humans and horses are “dead-end hosts”: they can become infected through mosquito bites but cannot transmit the virus further.

The virus has circulated in Europe since the 1950s, introduced by migratory birds from Africa. However, it was only in 1996 that a large outbreak was detected in Romania, where 393 people became infected. Since then, annual summer outbreaks have been reported across Southern and Central Europe, with the most significant activity concentrated in the Mediterranean region and the Balkans.

A Growing Threat in Belgium

Belgium has seen increasing signs of West Nile virus activity in recent years. The country recorded its first official human case in 2019, and in 2025, three crows tested positive for the virus. The virus was also detected in birds in May and July 2026, with infected wild birds found in Gembloux, Schaarbeek, and Kapellen.

On 7 August 2026, the first confirmed horse infection in Belgium was officially confirmed by the Federal Agency for the Safety of the Food Chain (FAVV). The horse showed severe neurological symptoms and was euthanized. The infection was confirmed by Sciensano, Belgium’s health institute. “The West Nile virus has also been detected in neighboring countries and many other regions in Europe in recent years. It is therefore no surprise that this virus is now also being noticed in our country,” the FAVV stated.

Research has confirmed that Belgian mosquitoes are capable of transmitting the virus. A 2023 study by KU Leuven researchers, published in PLOS Neglected Tropical Diseases, demonstrated for the first time that Culex pipiens mosquitoes captured in the Leuven area can be infected with the virus and transmit it through their saliva.

“Our research group is actively researching this and has for the first time demonstrated that mosquitoes captured in the Leuven area can be infected with the virus and transmit it in their saliva,” said Leen Delang, virologist at KU Leuven.

The Climate Change Connection

Scientists have now formally established the link between climate change and the virus’s expansion in Europe. A 2024 study by researchers at VUB and ULB, published in Nature Communications, used ecological niche modeling to compare factual simulations with counterfactual scenarios without climate change. The results showed that the area ecologically suitable for West Nile virus circulation increased notably during 1901–2019, while remaining largely unchanged in the no-climate-change counterfactual.

“Until now, the formal establishment of the link between the spread of the virus and climate change was lacking, but our results point to a major role of climate change in the advance of the virus in southeastern Europe,” said Diana Erazo, lead author of the study and researcher at the Spatial Epidemiology Lab (SpELL).

“Our results also show a recent and drastic increase in the number of people at risk of exposure,” Erazo added. “This is partly due to an increase in population density, but climate change remains a crucial factor.”

Wim Thiery, climate scientist at VUB and co-author of the study, was blunt about the implications: “This is the face of climate change. 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.”

Thiery also warned that the virus could continue advancing northward. “Currently we mainly see the Mediterranean region and the Balkans emerging as red zones, but it’s quite possible that in the future this will advance further north towards our regions.”

The spread of West Nile virus is linked to high temperatures in spring and summer, dry periods in summer, and warm winters—all conditions that climate change is making more common across Europe. Warmer temperatures accelerate the virus’s incubation period in mosquitoes, while extended warm seasons allow for longer transmission windows.

Current Outbreak in North Macedonia

A major epidemic is currently ongoing in North Macedonia, where at least 5 deaths and 43 confirmed cases have been reported as of 22 August 2026. The health authorities declared an epidemic emergency in early August in several municipalities around Skopje. “Almost daily new patients are being admitted and currently 20 to 21 people are in the hospital,” said Fadil Cana, director of the Clinic for Infectious Diseases in North Macedonia, as reported by VRT NWS.

The European Centre for Disease Prevention and Control (ECDC) reported 99 affected regions across the 12 countries with cases. “The expansion of West Nile virus transmission as well as the spread of invasive mosquito species demonstrate why Europe needs to strengthen its mosquito-control capacity,” said Celine Gossner, ECDC executive.

How Dangerous Is It for Humans?

About 80% of human infections are asymptomatic. Approximately 20% develop flu-like symptoms, and in about 1% of cases, severe neurological complications such as encephalitis or meningitis can occur, primarily in older people or those with weakened immune systems.

In the United States between 1999 and 2021, more than 55,000 cases were reported, with over half leading to neurological disorders and about 2,500 deaths. There is no vaccine for humans and no specific treatment—only symptom management and supportive care are available.

Challenges in Controlling Outbreaks

The Institute of Tropical Medicine (ITG) in Antwerp has warned of widespread insecticide resistance in the common house mosquito (Culex pipiens) in Europe, making outbreak control more difficult. “At this moment, we have no effective weapon in Belgium against a West Nile outbreak. There is an urgent need for new control agents that are efficient and acceptable,” said Ruth Müller, head of Entomology at the ITG, in a statement.

Belgium also lacks systematic mosquito monitoring. “We don’t have systematic monitoring here yet,” said Isra Deblauwe, entomologist at the ITG.

Meanwhile, the Asian tiger mosquito (Aedes albopictus)—which can transmit dengue, chikungunya, and Zika viruses—has been observed in 12 Belgian municipalities in 2026, according to Sciensano and the Institute of Tropical Medicine, as VRT NWS reported.

What to Watch For

As climate change continues to create favorable conditions for mosquito vectors, experts expect the West Nile virus to expand its range further north in Europe. The situation in North Macedonia demonstrates the potential severity of outbreaks, while Belgium’s first confirmed horse infection signals that the virus is now circulating within the country.

With no human vaccine available and insecticide resistance complicating control efforts, public health authorities face significant challenges. The ECDC has urged European countries to strengthen mosquito-control capacity through integrated approaches combining monitoring, community engagement, and targeted control solutions.

For now, prevention remains the primary defense: avoiding mosquito bites, particularly during peak transmission season from July to September, and reducing standing water where mosquitoes breed. As Wim Thiery noted, addressing the root cause—climate change—will be essential in the long term to curb the spread of tropical diseases across the continent.