Italy's West Nile virus cases near 700 as warming climate stretches mosquito season

Italy has recorded 657 confirmed human cases of West Nile virus so far this year, including 51 deaths, according to the latest surveillance bulletin from the country's National Institute of Health, published Thursday. The figures confirm what Italian and international health officials have been warning for weeks: a warming climate is stretching the mosquito-borne virus's season later into the year and pushing it into provinces that had rarely seen it before.

The tally, current through September 16, is up from 594 cases in the institute's previous weekly report. It makes 2026 one of Italy's worst West Nile seasons on record, and health authorities say the country now accounts for roughly half of all cases reported across Europe this year, according to Associated Press reporting that cited Italian officials.

The numbers

Of the 657 confirmed infections, 345 involved the neuro-invasive form of the disease, which can cause encephalitis or meningitis and is far more likely to be fatal, according to the institute, known by its Italian acronym ISS. Another 80 cases were asymptomatic infections caught through routine screening of blood donors, 224 were milder fever cases, and the rest involved other symptoms. Five of the neuro-invasive cases were imported, contracted in France, Belgium, Greece, the Netherlands and the Maldives.

The virus has now been confirmed circulating in 82 provinces across 19 of Italy's 20 regions, up from 79 provinces a week earlier, the bulletin says. Lombardy, in the north, has recorded the most infections this season with 158, followed by Veneto with 116, Lazio with 96 and Piedmont with 87. The case-fatality rate among confirmed, locally acquired neuro-invasive infections stands at 14.7 percent this year, the institute said, only slightly better than the 15.8 percent recorded over the same period in 2025.

Public health officials note that the great majority of people infected with West Nile virus never know it. Roughly 80 percent of infections cause no symptoms at all, and most others produce only a mild, flu-like illness. Severe, potentially fatal neurological complications strike a small minority, concentrated among older adults and people with weakened immune systems.

A longer, wider mosquito season

West Nile virus has been endemic in Italy since 2020, transmitted chiefly by Culex mosquitoes that pick up the virus from infected birds and then bite humans and horses. What has changed, researchers say, is the length and geographic reach of the transmission season, which historically ran from June through early October but this year has extended toward November in some areas.

"Higher temperatures linked to climate change help mosquitoes reproduce and extend the transmission season, increasing the chances that infected mosquitoes will spread the virus to people," Antonino Bella, a senior researcher in ISS's infectious diseases department, told the Associated Press.

"There's no doubt that warming weather will expand the period of replication for the virus and the vector in more countries," said Marc-Alain Widdowson, who leads the pandemic threats, communicable diseases and antimicrobial resistance division at the World Health Organization's European office. "You've got to be careful not to say that climate change is all of it, but certainly a warming climate means that we have a longer mosquito season."

Widdowson pointed to a broader set of drivers alongside warming — including biodiversity loss and land-use change — but said the direction of travel across the region is unmistakable. Invasive mosquito species are also expanding their range: the tiger mosquito, Aedes albopictus, capable of transmitting other viruses such as dengue and chikungunya, has gone from being established in 8 European countries and 114 regions a decade ago to 13 countries and 337 regions today, while Aedes aegypti, the primary vector for yellow fever and Zika, has been established in Cyprus since 2022.

Cases spreading beyond Italy

Italy is not alone. The Netherlands, which had not recorded a locally acquired human case since 2020, has logged 28 confirmed infections this year and four deaths, according to the Dutch national institute for public health, RIVM. One of the fatal cases involved a patient who was infected not by a mosquito bite but through a blood transfusion from a donor later found to be carrying the virus, underscoring the pressure the virus's spread is putting on blood-safety screening across the continent. Infections in the Netherlands have been detected in the provinces of Utrecht, Noord-Brabant, Noord-Holland and Zuid-Holland, along with cases in horses and birds.

Belgium, which recorded its own locally acquired cases for the first time this year, sent a research team to Rome in May to study Italy's surveillance methods, health officials said. The European Centre for Disease Prevention and Control tracks confirmed cases across the continent on a weekly public dashboard as the season continues, part of a broader effort to harmonize how member states detect and report the virus.

Alongside West Nile, Italy's latest surveillance report also recorded 14 cases this year of Usutu virus, a related and generally milder mosquito-borne pathogen, spread across six regions including Lombardy, Emilia-Romagna and Lazio.

What happens next

ISS publishes its West Nile surveillance bulletins weekly through the autumn, and officials caution that figures from the most recent four weeks are provisional and likely to rise as delayed diagnoses and confirmatory lab tests come in. Regional health authorities in the hardest-hit areas, including Lombardy, Veneto and Lazio, have continued mosquito-control spraying and public advisories urging residents to eliminate standing water and use repellent into the autumn, a season when such measures were once considered unnecessary.

Blood transfusion services across affected regions are maintaining enhanced screening of donors in circulation zones, a practice Italy adopted after the virus became endemic in 2020 and that the Netherlands has now had to confront directly following its transfusion-linked death. Public health researchers say the coming weeks, as European nations close out this year's transmission season and compare notes on caseloads, are likely to sharpen debate over how much of the increase reflects better detection versus a genuine expansion of the disease driven by warming temperatures — a distinction the World Health Organization and national institutes are still working to disentangle season by season.