Europe recorded its worst West Nile virus season on record by early October, with 1,715 locally acquired human cases confirmed across 13 EU and EEA countries, driven by expanding mosquito ranges and warmer summers.
In Plain English: The Clinical Takeaway
- Vector-borne flavivirus: West Nile virus is a single-stranded RNA virus belonging to the same family as dengue and Zika, transmitted primarily through the bites of Culex mosquitoes.
- Asymptomatic prevalence: Roughly 80 percent of human infections cause no symptoms.
- Crepuscular activity: Culex mosquitoes feed most actively during the hours just before sunset through early morning, making targeted evening and night protection essential.
Croatia Reports Twenty-Five Infections and One Death
Croatia’s public health institute, the Hrvatski zavod za javno zdravstvo (HZJZ), released its final seasonal update reporting 25 confirmed human infections and one death nationwide. That figure marks more than four times the six cases recorded across all of 2025. Of the 25 confirmed infections, 19 were symptomatic, with 17 classified as neuroinvasive disease, a severe manifestation causing meningitis, encephalitis, or acute flaccid paralysis. Two patients developed mild West Nile fever, while the remaining six infections were asymptomatic detections found through routine blood screening. Zagreb dominated the geographic distribution, accounting for 15 of the country’s confirmed cases.
The high proportion of neuroinvasive cases—representing roughly 89 percent of symptomatic infections in Croatia—stems from passive surveillance ascertainment bias rather than an unusually virulent viral strain. Mild cases of West Nile fever are largely self-limiting, causing flu-like illness that resolves at home without medical testing. Surveillance systems focusing primarily on hospitalized individuals capture severe neurological complications while missing the broader pool of mild infections. Seroprevalence studies indicate the true ratio of mild fever to neuroinvasive illness is closer to 20-to-1.
Continental Expansion Reaches Belgium and the Netherlands
The domestic surge in Croatia unfolded against a wider continental outbreak documented in a joint October report by the European Centre for Disease Prevention and Control (ECDC) and the European Food Safety Authority (EFSA). By October 7, the bloc registered 1,715 locally acquired human cases across 13 EU and EEA countries, alongside 141 total continental deaths. Greece and France each established national records for surveillance totals, while animal monitoring revealed 510 equid and 591 bird outbreaks. The transmission season also produced documented firsts in Belgium and the Netherlands, which logged 15 and 59 locally acquired cases respectively after previously recording only travel-associated infections.
The northward shift aligns with environmental factors governing virus replication. Viral replication inside the mosquito accelerates at temperatures above 20 degrees Celsius and peaks near 30 degrees Celsius. As European summers lengthen and warm, Culex pipiens mosquito populations are establishing further north, prompting public health authorities to project continued geographic expansion in future transmission seasons.
| Region / Country | Reported 2026 Metric | Key Clinical or Epidemiological Feature |
|---|---|---|
| EU / EEA (Bloc Total) | 1,715 local human cases (by Oct 7) | Most geographically widespread West Nile season ever documented, with 187 NUTS 3 regions affected. |
| Croatia (HZJZ Data) | 25 confirmed cases, 1 death | Eighty-nine percent of symptomatic cases presented as neuroinvasive disease. |
| Belgium | 15 confirmed cases | First-ever locally acquired infections recorded in the country. |
| Netherlands | 59 confirmed cases | Marked the country’s transition from travel-associated to local transmission. |
Wild Bird Reservoirs Trigger Early Public Health Response
Wild bird populations provided the first biological signal of viral activity weeks before human cases peaked. In late August, the Croatian Veterinary Institute identified West Nile virus in dead crows collected across Zagreb neighborhoods including Zapruđe, Kvaternikov trg, and Dubrava. Crows serve as amplifying hosts, acting as an early-warning indicator for wildlife health authorities.
To safeguard the blood supply, Croatia maintained routine nucleic acid testing (NAT) on donations throughout the transmission season, complying with EU Directive 2002/98/EC. Because no licensed human vaccine exists, prevention relies entirely on personal and community measures. These include applying EPA-registered insect repellents containing DEET, picaridin, or oil of lemon eucalyptus during crepuscular feeding hours, wearing long sleeves and trousers.
Contraindications & When to Consult a Doctor
Individuals utilizing insect repellents should adhere strictly to product labeling. Blood donation centers defer donors with active diagnoses or recent travel to endemic zones in accordance with established safety protocols.
As autumn temperatures cool, HZJZ expects the transmission season to wind down, though sporadic late-season cases remain possible while surviving Culex adults maintain activity. European health agencies will maintain monitoring protocols as the geographic distribution of the virus continues to challenge traditional surveillance boundaries.
Worth a look
