West Nile Virus in Southern Europe

12 August 2025

West Nile virus, a mosquito-borne disease, has become a notable concern in Southern Europe. Transmitted primarily by Culex mosquitoes, the disease has been increasingly reported across the region. By early August, Italy, Greece, France, Romania and Bulgaria had confirmed human infections. Italy has been the hardest hit, reporting over 170 confirmed human cases and 20 deaths until now, mostly occurring in the central Lazio region, and the rest distributed in the Northern and Southern regions. Other countries like Greece have seen cases occurring in Athens and its surrounding regions. Romania and Bulgaria also reported a handful of cases.

The spread of West Nile Virus in Southern Europe is driven by a combination of environmental, climatic, and ecological factors. Culex mosquitoes, the primary vectors for West Nile Virus, are native to the region and thrive in warm, humid conditions. Climate change has exacerbated the situation by extending mosquito breeding seasons and expanding their habitats. Rising temperatures and altered precipitation patterns, such as heavy rains, create ideal breeding conditions in stagnant water pools.

West Nile Virus primary target are birds, with humans and horses serving as incidental hosts. The virus’s ability to infect a wide range of bird species, particularly migratory ones, enables its geographic spread across borders. Culex mosquitoes can survive winter in a dormant state (diapause). Research has shown that West Nile Virus can survive within infected mosquitoes, making the current outbreak worth of consideration also for the future season.

Approximately 80% of West Nile Virus infections in humans are asymptomatic, 20% cause flu-like symptoms (West Nile fever), and only 1% lead to severe neurological conditions like meningitis or meningoencephalitis. These severe cases, though rare, are a significant concern due to their high mortality rate, as evidenced by Italy’s 20 deaths to date. Owing to the high proportion of asymptomatic cases, the true extension of the West Nile Virus outbreak is unclear. The nature of West Nile Virus transmission patterns, coupled with delays in reporting and data aggregation, complicates real-time monitoring.

At Zetesim, we are studying general models for mosquito-human interactions. The complexity of these models, incorporating factors such as mosquito population dynamics, biting rates, pathogen transmission probabilities, and human behavior, demands extensive data calibration and validation to account for environmental influences and vector control measures, making them particularly challenging. With advancements in data collection and computational techniques, we hope to develop robust forecasting tools that may accurately predict, in the future, disease risk from key mosquito-transmitted viruses.