Human West Nile Virus infections in Europe: weather or climate?

Climate change is increasing Europe’s vulnerability to vector-borne diseases. One example are the increasing number of outbreaks caused by the West Nile Virus. Whereas in the early 2010s, the virus was only found in south-eastern Europe, local infections are now also detected in more northern and western countries. To inform health care institutions as well as citizens it is necessary to be able to predict where and when the next outbreak will happen. There have been studies that show that land use, climate and weather influence the risk of human West Nile Virus infections, but it is less clear what the relative contributions of land use, climate change and weather are. In particular, it is not determined yet if the northward expansion of WNV can be better explained by the gradual change in climate, or by the occurrence of specific weather conditions that increase the risk of WNV infections. In this study, a random forest model is used to determine what is the best predictor of WNV infections: weather or climate. It shows that on a spatial scale of NUTS3-regions, the climate mean seasonal cycle of the 2m temperature is the best predictor for human WNV outbreaks, and that including the weather in the model does not improve its performance. Moreover, results indicate that WNV risk is higher in areas in which the climate mean seasonal cycle of temperature is in between 20-26 °C for one or more weeks. This can help explain and predict the emergence of human WNV infections in new regions in Europe.