Animal surveillance is providing some of the clearest signals of the intensity of the 2026 West Nile virus season. By 26 September, France had already recorded at least 253 equine cases, the Netherlands 136 infected horses and 171 positive birds, Belgium dozens of equine and avian infections, while Cyprus had reported its first cluster involving five horses. In the United States, infections continue to be reported in horses across multiple states, while California alone has detected West Nile virus in 657 dead birds and 3,730 mosquito samples. The animal data show a transmission season extending well beyond the traditional European hotspots and underline the central role of veterinary and wildlife surveillance in identifying where the virus is actively circulating.
West Nile virus is fundamentally an animal and vector infection. Its natural transmission cycle is maintained between birds and mosquitoes, particularly Culex species. Horses and humans are incidental hosts and do not normally contribute to onward transmission. This makes the number of human infections only one component of the 2026 epidemiological picture. By late September, surveillance of horses, wild birds and mosquitoes was showing intense transmission on both sides of the Atlantic.
There is no harmonised global real-time count of infected animals. National reporting systems also differ considerably: some countries count individual positive animals, others outbreaks or epidemiological units. Direct totals therefore require caution. But the available figures as of 26 September 2026 leave little doubt about the scale of the current season.
Europe: September reveals a new face in animals
The most recent harmonised ECDC-EFSA report covered notifications submitted only up to 3 September 2026. At that point, Europe had recorded 121 outbreaks in equids and 280 outbreaks in birds. Italy accounted for 34 equine outbreaks and 218 avian outbreaks, France for 43 equine outbreaks and Belgium for 14.
Those figures are no longer representative of the situation at the end of September. National surveillance systems recorded a sharp increase during the following three weeks, particularly in France, the Netherlands and Belgium.
France: at least 253 equine cases and continuing detections after mid-September
France is experiencing one of the strongest equine West Nile seasons documented in the country. 253 equine cases were reported in 18 departments by 11 September. Of these, 143 had been detected during August and another 84 during the first eleven days of September. Between July and 11 September, France had recorded 233 cases, compared with only 25 during the equivalent period in 2025.
The largest concentrations were reported in Bouches-du-Rhône with 65 cases, Pyrénées-Orientales with 40, Aude with 29, Gard with 21, Hautes-Pyrénées with 19, Hérault with 18 and Vaucluse with 14. The distribution is no longer restricted to Mediterranean France. Cases were also detected in Haut-Rhin, Essonne, Loire-Atlantique, Val-d'Oise, Ille-et-Vilaine, Drôme, Gers and Dordogne.
And the 253 total is not an end-of-September figure. RESPE subsequently published additional West Nile alerts on 22 September in Loire-Atlantique, Haut-Rhin, Haute-Garonne, Tarn-et-Garonne, Vaucluse and Corse-du-Sud. The national consolidated figure therefore exceeded 253 by 26 September, although a newer complete national total had not yet been published. The geographical spread of equine disease is one of the most significant features of the French season.
Netherlands: 136 horses and 171 birds
No European country better illustrates the acceleration seen during September than the Netherlands. At the beginning of the month, only five horses and six birds had been reported positive. By 9 September, the numbers had increased to 58 horses and 39 birds. On 16 September, they stood at 88 horses and 100 birds.
By 23 September, the RIVM reported 136 WNV-positive horses, 171 positive birds and four positive mosquito pools. Forty-eight horses and 71 birds had been added in a single week. Of the 171 birds, 61 were dead when investigated and 110 had been sampled alive. Positive birds were particularly numerous in South Holland, but infections were also detected in North Brabant, North Holland, Utrecht, Gelderland and Flevoland. Positive horses were reported from South Holland, North Holland, Utrecht, Gelderland and North Brabant.
This simultaneous detection in birds, mosquitoes, horses and humans provides unusually clear evidence of an established and geographically extensive enzootic cycle. The human total had reached 44 infections by 23 September, but from an animal-health perspective the far larger number of equine and avian detections is arguably the more informative indicator of the intensity of transmission.
Belgium: animal infections preceded human cases
Belgium provides another particularly clear One Health sequence. The joint ECDC-EFSA report had already identified widespread animal transmission by early September, at a time when Belgium had not yet reported locally acquired human infections. National veterinary surveillance subsequently showed much greater activity. By mid-September, DGZ had reported West Nile virus in 34 horses, 80 wild birds and one flamingo in Flanders, with additional infections detected in captive canaries.
The progression is important because Belgium only documented WNV circulation in wild birds relatively recently. In 2026, infections in birds were followed by severe neurological disease in horses and then by locally acquired human cases. By 23 September, nine human infections had been notified to ECDC. The animal surveillance therefore provided evidence of local transmission before the human burden became apparent.
Italy: extensive avian circulation remains central to the European epidemic
Italy remains Europe's largest established WNV transmission zone. At the beginning of September it accounted for 218 of Europe's 280 reported avian outbreaks, together with 34 equine outbreaks. By 23 September, integrated Italian surveillance showed evidence of viral circulation in 83 provinces across 19 regions, based on human, animal and vector detections.
Regional animal surveillance illustrates how extensive that circulation has become. In Lombardy alone, for example, the IZSLER surveillance system had recorded numerous WNV-positive wild birds by 26 September, including 51 positives in Milan province, 31 in Pavia, 15 in Cremona and 14 in Varese. Italy therefore remains important not only because of its approximately 700 human infections by 23 September, but because WNV is circulating simultaneously across avian, equine, mosquito and human surveillance systems.
Cyprus: five horses in the first Paralimni cluster
Cyprus added another important animal event late in the month. On 23 September, veterinary authorities confirmed WNV in horses at a premises in the Paralimni area. Subsequent investigations identified five infected horses among the 16 animals kept at the establishment. As of 25 September, no additional infected horses had been detected outside that group and all five animals were recovering following treatment.
The cases were the first reported equine detection in the Paralimni area and again demonstrate the value of horses as sentinels for local WNV circulation.
Austria, Germany, Spain and other European countries
Austria had reported equine and avian infections during the season, particularly in eastern parts of the country. Germany also recorded WNV-positive horses and wild or captive birds, although activity remained more moderate than in neighbouring Belgium and the Netherlands.
Spain continued to report equine outbreaks in September. Spain's official RASVE animal-disease system recorded new WNV outbreaks after the 3 September European cut-off, including detections in Catalonia and Andalusia.
United States: horses remain part of a much larger animal and vector signal
The same pattern is visible in North America. The United States does not provide a single real-time national animal dashboard equivalent to the Dutch integrated system, and equine cases are reported through state authorities and the Equine Disease Communication Center.
New equine cases continued to appear through late September. On 24 September, for example, Maryland reported a West Nile case in a 15-year-old Quarter Horse mare from Cecil County. The mare had developed neurological disease and was euthanised. The EDCC has documented cases during the 2026 season across numerous states, demonstrating geographically widespread equine exposure. California provides particularly detailed integrated animal and vector data. By 25 September, California had recorded 657 WNV-positive dead birds, 3,730 positive mosquito samples, 143 positive sentinel chickens and 14 infected horses.
Positive dead birds had been reported from 24 counties and infected mosquito pools from 29 counties. The 14 equine cases were distributed across 13 counties.
Several equine cases involved neurological disease. California's 11th equine case, an unvaccinated two-year-old Friesian stallion in Amador County, was euthanised after developing severe neurological signs. Other affected horses recovered, including vaccinated animals, illustrating that infection can occur despite vaccination although immunisation remains the principal means of reducing the risk of clinical equine disease.
The enormous number of positive mosquito samples and dead birds makes clear that equine cases represent only a small fraction of the broader ecological circulation.
Canada: birds and mosquitoes also indicate substantial transmission
Canada's provincial surveillance provides another animal perspective. In Manitoba, by 25 September, WNV had been identified in 65 birds, one horse and 432 positive pools of Culex tarsalis mosquitoes during the 2026 season. The mosquito total alone was almost three times the 148 positive pools reported in Manitoba during 2025.
Equine infections have also been reported elsewhere in Canada during the season, including Ontario. As in Europe and the United States, the number of clinically affected horses represents only the visible edge of a much larger bird–mosquito transmission system.
Why animal data matter more than ever in 2026
West Nile surveillance is sometimes presented primarily through human case numbers. That approach misses much of the epidemiology. Birds are the amplifying hosts. Mosquitoes connect infected bird populations. Horses reveal spillover into susceptible mammals.
The striking numbers reported in 2026 (more than 250 equine cases already documented in France, 136 horses and 171 birds in the Netherlands, dozens of infected horses and birds in Belgium, 657 positive dead birds in California and hundreds of positive mosquito pools in both California and Manitoba) demonstrate that viral circulation is much broader than human case counts alone suggest.
The geographical pattern is also changing. Established WNV zones in Italy, southern France and the Balkans remain active, but substantial animal transmission is now being documented farther north and west, most clearly in the Netherlands and Belgium.
As of 26 September 2026, that expansion of the animal transmission cycle is one of the defining features of the season.
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