At first, the patient’s symptoms were a puzzle: swelling of the brain tissue and membranes as might be seen with meningitis, but without any sign of the bacteria and viruses that typically cause it.
Medical staff at the hospital on the Italian island of Sardinia excluded potential causes one by one until they were left with something unexpected. It was West Nile virus, one of several formerly tropical diseases that have become increasingly established in Europe as climate change favours the spread of the mosquitoes that carry the illnesses with their bite.
“It was practically unknown here,” recalls Dr Maria Valentina Marras of that first case in her province of Oristano in Sardinia, little more than a decade ago. “From then until now, every summer we have a West Nile emergency.”
She now leads containment efforts as head of the department of hygiene and prevention in Oristano, a region of 147,000 people that, on the day we spoke, reported its 15th case of West Nile so far this year.
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The new patient was a 71-year-old man who presented at accident and emergency with the symptoms that have become familiar – high fever, confusion, neurological symptoms, encephalitis – which now trigger an automatic test for West Nile.
The detected cases indicate a much larger outbreak in reality, as 80 per cent of those who are infected show no symptoms. About 20 per cent suffer a flu, and 1 per cent are hospitalised. Older people, with conditions such as diabetes, heart or respiratory trouble, or obesity are more susceptible to severe illness due to their weakened immune response.
Each confirmed case triggers an immediate crackdown to contain the spread: the patient’s house is cordoned off, and fumigators move in with insecticides to treat the entire area around the home in a 200m radius. The disease does not spread from human to human, but only through infected mosquitos.
Despite these efforts, the spread of the virus is proving difficult to contain.
“This summer was a very hot summer, a terrible humid heat that started in June and is still going now in September,” Dr Marras says. “The mosquito found the ideal environment to reproduce, exponentially.”
Italy has detected 430 locally-transmitted cases of West Nile virus and 27 deaths so far in 2025, according to national data.
In the last week, France reported 71 new locally-acquired cases of Chikungunya, a virus that can cause chronic debilitating joint pain, bringing this summer’s record outbreak of a disease that originated in Tanzania to 227 cases across 30 clusters.
Various Mediterranean countries have reported cases of the viral infection Dengue, which is usually associated with tropical and subtropical regions of the world.
This summer’s record-breaking outbreaks of Chikungunya and West Nile virus represent a “new normal”, according to the European Centre for Disease Prevention and Control. It says climate change and increased international travel mean the tiger mosquito is established in 16 countries in Europe, and likely to spread further.
“As the mosquito-borne disease landscape evolves, more people in Europe will be at risk in the future,” the ECDC’s Dr Céline Gossner said in a statement.
With the new diseases, communities must learn to adjust. Information brochures sent to every house and pharmacy in the region of Oristano explain how West Nile virus spreads and how to prevent it. Never leave out bowls of water for pets overnight, as the mosquitos breed in the smallest amounts of stagnant water. Wear long sleeves and trousers in the evening, even if it’s hot. Avoid wearing dark clothing, which attracts them. Cover all windows with mosquito screens.
As it is a region that attracts plenty of tourists, English-language information campaigns target visitors, too.
To the local community facing the outbreak, the warnings and news reports are all a rather unpleasant reminder of a pandemic that seems very recent history.
“The community is afraid of this disease,” Dr Marras said. “Like all the world, we had Covid in 2020, so people are very scared of this disease. Even though there isn’t human-to-human transmission.
