ROME — Italy has reported more than 650 West Nile virus infections this year as warmer temperatures and longer summers help mosquitoes survive and transmit the virus across a broader area, health authorities said. The total represents roughly half of all West Nile cases recorded in Europe so far this year. Experts at Italy’s National Institute of Health said the number is broadly consistent with recent seasons, but the virus is increasingly appearing in places that historically saw few or no infections. “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,” said Antonino Bella, a researcher at the institute. West Nile has been considered endemic in Italy since 2020, when infections began to rise sharply. Cases are now reported each summer and into early fall, and warmer conditions can extend transmission as late as November, Bella said. Most people infected with West Nile do not develop symptoms. About 20% experience a mild, flu-like illness, while a small share develop serious neurological complications. Older adults and people with underlying health conditions face the greatest risk of severe disease. The virus cycles primarily between mosquitoes and birds, with people and other mammals infected through mosquito bites. Experts say climate change is one factor among several driving increased activity. Changes in land use, declining biodiversity and shifts in bird populations may also create conditions that favor mosquitoes and bird species that efficiently carry the virus. Marc-Alain Widdowson, who leads the World Health Organization’s Europe division focused on pandemic threats and communicable diseases, said warming temperatures are likely to lengthen the period when both mosquitoes and the virus can reproduce. But he cautioned against treating climate change as the sole explanation. “A warming climate means that we have a longer mosquito season, so that the vectors can survive for longer,” Widdowson said. “Warm temperatures also mean that the virus itself will survive better.” Italy’s high case count may also reflect its unusually extensive surveillance system. The country monitors mosquitoes, birds, blood donors and human infections, allowing officials to identify more transmission than countries with less comprehensive testing. That monitoring network is drawing interest from other European nations. A Belgian research team visited the Italian institute in May to study its surveillance and research systems after Belgium began seeing locally acquired West Nile infections. The Netherlands has reported more than two dozen cases this year — its first since 2020 — along with several deaths, according to the country’s public health institute. The European Centre for Disease Prevention and Control warned in June that increasingly frequent and severe heat waves and floods, along with longer and warmer summers, are creating more favorable conditions for invasive mosquito species. Aedes albopictus, which can transmit several viruses, was established in eight European countries and 114 regions a decade ago. It is now established in 13 countries and 337 regions, the agency said. Another species, Aedes aegypti, can transmit dengue, yellow fever, chikungunya, Zika and West Nile virus. It has been established in Cyprus since 2022 and could spread to additional European countries, health officials said. The changing pattern underscores the need for sustained mosquito control, disease surveillance and public health preparedness as warmer conditions alter the seasonal risks communities face.