Rising temperatures and longer warm seasons are helping mosquitoes carry West Nile virus across a wider area of Italy and for more months of the year, according to Italian health authorities. More than 650 infections have been reported in Italy so far this year, roughly half of all cases recorded in Europe.
The virus has been considered endemic in Italy since 2020, when cases began to rise sharply. It is now reported each summer and early fall, and transmission can sometimes continue as late as November. Infections are also increasingly being registered in areas where West Nile was previously almost never recorded.
Italian surveillance data recorded 432 confirmed human cases from the start of surveillance to August 26. There have been 17 deaths among those confirmed cases. The number of provinces with proven West Nile circulation has risen to 66.
Antonino Bella, a researcher at the Italian National Institute of Health, said higher temperatures support mosquito reproduction and lengthen the period in which the virus can be passed to people. The evidence points to warmer conditions as an important driver of the virus's expanding seasonal and geographic range, although the available information does not establish whether temperature is the sole factor.
West Nile virus usually causes no symptoms. About 80% of infected people do not develop symptoms, while most other cases are mild and resemble influenza. A small share of patients develop severe neurological complications. These cases occur mainly among older people and people with underlying medical conditions.
The pattern is also emerging elsewhere in Europe. The Netherlands has reported more than two dozen West Nile cases this year, its first cases since 2020. Several deaths from the virus have also been reported there this year.
European health authorities have linked the changing risk to the spread of mosquitoes that thrive in warmer conditions. A decade ago, Aedes albopictus was established in eight European countries and 114 regions. This year, it is established in 13 countries and 337 regions.
Aedes aegypti has been established in Cyprus since 2022, according to the information provided in the brief. The broader expansion of invasive mosquito species has increased the areas where mosquito-borne viruses can circulate.
In June, the European Center for Disease Prevention and Control said Europe was experiencing a warming trend, with heat waves and flooding becoming more frequent and severe and summers becoming longer and warmer. Marc-Alain Widdowson, who heads the Pandemic Threats, Infectious Diseases and Antimicrobial Resistance unit at the WHO Regional Office for Europe, said warming was extending the period during which the virus and its vectors reproduce in an increasing number of countries.
Italy has the largest recorded burden in Europe so far this year. The rise in cases beyond traditional areas, together with the return of infections in the Netherlands after a four-year gap, indicates that health authorities are facing longer transmission seasons across a broader area. The long-term course of West Nile's spread, the role of factors beyond temperature, and the capacity of surveillance systems to respond to future outbreaks remain unclear.
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