21. West Nile Virus Eruptions in Summer 2010 – What Is the Possible Linkage with Climate Change?

West Nile Virus (WNV) is a vector-borne pathogen of global importance. Many factors impact the transmission, epidemiology and geographic distribution of WNV. However, climate and especially warm conditions were found to be crucially important causes that instigated the outbreaks. New areas of the WNV transmission with the occurrence of human cases have been identified during summer 2010. According to the European Center for Disease Control (ECDC), infections by WNV have occurred in Greece, Romania, Hungary, Israel, Russia, and Italy. The precise reasons for the existence of the current outbreak of WNV infection in humans in Eurasia remain unclear. However, climatic factors are believed to have increased the abundance of mosquitoes and shortened the transmission cycle in the vectors, leading to increased human cases.

Mean monthly temperature and precipitation data show the extreme behavior of the air temperature as well as the rainfall patterns during summer 2010 in selected areas where WNV circulation occurred – Macedonia (Greece), Western Turkey, Southeastern Romania and Southwestern Russia. The results show that the warming tendency during the hot season over recent years continued in summer 2010. Moreover, the air temperature was extremely higher than normal in the selected study sites. This might have an impact on the risk for WNV outbreaks.

As for the precipitation, the picture is more complex. The increase in WNV cases could be related to the unusual increase in the rainfall amounts during that summer (Macedonia and SE Romania). Alternatively, WNV may increases after an extreme dry period (SW Russia), since standing water pools become richer in organic materials.

During the summer of 2010, Eurasia had to deal with exceptional heat-waves while a record of high numbers of extreme warm nights had been documented in parts of south-eastern Europe.

In summary, although the WNV transmission is multi-factorial, it seems that the increase in the summer temperature should be considered when evaluating the risk of WNV transmission.