¿Qué significa el virus del Nilo Occidental en alrededor de diez caballos flamencos para la gente de Bélgica?

Flemish health authorities said on Thursday, 3 September, that around ten horses in Vlaanderen had been infected with West Nile virus, while Belgium had still recorded no locally acquired human case. A spokesperson for the Flemish Department of Care told Belga that laboratories had identified almost one new equine infection a day during the preceding week, with affected areas including Willebroek, Duffel and Heist-op-den-Berg.

The increase is a marked change from the situation at the beginning of August. Belgium’s Federal Agency for the Safety of the Food Chain, known as FAVV, confirmed the country’s first recognised equine case on 7 August after a horse with severe neurological symptoms was euthanised. Sciensano, Belgium’s national public-health institute, confirmed the virus. The Flemish Department of Care subsequently listed cases in Vilvoorde, Willebroek and Meise, but its public information page had not yet been updated to reflect the roughly ten cases reported on Thursday. The precise total, the condition of every affected horse and a complete municipality-by-municipality breakdown therefore remain to be published officially.

The virus normally circulates between wild birds and mosquitoes, particularly common Culex mosquitoes. An infected mosquito can transmit it to a horse or a person, but neither horses nor humans normally carry enough virus in their blood to continue the mosquito-borne cycle. The FAVV consequently describes them as epidemiological dead-end hosts: an infected horse is not a direct danger to neighbouring horses, its owner or stable staff.

That distinction is central to the public-health message. The Flemish Department of Care says there is currently no reason for alarm, although residents should remain vigilant and reduce mosquito bites. Most human infections cause no symptoms; the European Food Safety Authority estimates that about 20% produce a fever-like illness and fewer than 1% affect the nervous system. Older people and people with weakened immune systems face the greatest risk of severe disease. Anyone who develops fever, headache, muscle pain or neurological symptoms after substantial mosquito exposure should seek medical advice, but the present findings do not imply person-to-person transmission.

For horse owners, the risk is more immediate. Equine infections are also frequently asymptomatic, but animals that become clinically ill can develop fever, poor coordination, muscle tremors, weakness or paralysis. The FAVV says 30% to 40% of horses that develop a clinical disease picture may die. There is no virus-specific treatment, although supportive veterinary care is possible and preventive vaccines for horses are available. Suspected cases are notifiable under Belgian and European animal-health rules and should be reported immediately through a veterinarian to the FAVV.

The discoveries also show how Belgium’s divided institutional system works in a zoonotic outbreak. The Flemish Department of Care, politically overseen by Caroline Gennez, the Flemish Minister for Welfare, Poverty Reduction, Culture and Equal Opportunities, is responsible for preventive human-health policy in Vlaanderen and alerts municipalities, general practitioners and hospitals around detected clusters. Animal-disease notification and epidemiological investigation belong primarily to the federal FAVV, while Sciensano supplies laboratory expertise and coordinates surveillance across human, animal and environmental health. Blood and blood-product safety falls under the federal medicines agency, the FAMHP.

That federal-regional division has already produced practical action. After the first equine case near Vilvoorde, the FAMHP asked Belgian blood establishments to question donors who lived in or had stayed overnight in Vilvoorde, Steenokkerzeel, Machelen, Zemst or Grimbergen and to test relevant donations. The agency stressed that transmission through blood products is rare and that the measures were precautionary. They nevertheless matter because an infection without symptoms can otherwise escape notice.

Two legitimate policy frames now sit alongside each other. The Flemish public-health frame emphasises proportionality: there are no confirmed locally acquired human cases, most infections are mild or silent, and protection against mosquito bites is preferable to public alarm. The federal veterinary and blood-safety frame places more weight on early detection, mandatory reporting and targeted safeguards because infected horses and birds demonstrate local circulation before doctors necessarily see a human patient. These positions are not contradictory; they reflect different responsibilities and different thresholds for intervention.

The broader change is ecological rather than merely administrative. Sciensano’s One Health surveillance project first detected West Nile virus in Belgium in wild corvids sampled in 2025. More positive birds appeared in Antwerpen and Vlaams-Brabant in August 2026, while Wallonia recorded its first detection in a wild magpie in Namur province in July. The Walloon biodiversity administration describes the virus as still rare but subject to reinforced monitoring. Across Europe, the European Centre for Disease Prevention and Control updates West Nile surveillance weekly during the mosquito season as the virus’s range moves northwards.

Earlier Belgian surveillance reports had warned that the necessary ingredients were already present: suitable wild birds, common Culex mosquitoes and environmental conditions capable of sustaining transmission. The move from detected birds to clinically affected horses is therefore significant less because horses spread the virus than because they serve as sentinels. Each equine case indicates that an infected local mosquito was active somewhere in the surrounding area.

The immediate advice remains modest. Residents can remove standing water around homes, use mosquito repellent and wear covering clothes, particularly around dusk. Horse owners should discuss vaccination with a veterinarian and urgently report neurological symptoms. Authorities will continue testing birds, horses and suspected human cases through the active mosquito season, which generally runs into October. The central unanswered questions are whether the roughly ten equine infections belong to several distinct local clusters, how widely the virus is circulating in mosquitoes and birds, and whether Belgium can complete the 2026 season without its first confirmed locally acquired human case.