Emergence of hantavirus in South Germany: rodents, climate and human infections.

Human hantavirus (serotype Puumala) infections are prevalent throughout Europe. The bank vole is the main reservoir of the Puumala virus (PUUV). Between 2001 and 2006, the annual incidences in Germany ranged from 0.1 to 0.5 per 100,000 inhabitants. About half of the cases were reported from the state of Baden-Württemberg (BW) in southwest Germany. In 2007, 1,089 PUUV infections were reported from BW. This reflects an incidence of 10.1:100,000, which is more than 11 times higher than the mean incidence of the previous 6 years. Spatial analysis highlights incidences up to 90:100,000 in the most affected district. The winter season 2006/2007 showed an all time high in reported mean temperature. The previous summer and autumn led to a beech mast year, resulting in favourable feed conditions for bank voles in the winter season 2006/2007. The causes of the observed increase in PUUV infections in 2007 cannot be restricted to known cycles in the bank vole population. Favourable feed conditions, a mild winter and an early onset of spring may have influenced bank vole population size as well as human exposure to infectious rodent excretions. Further epidemiologic studies are necessary to better understand the interaction between environmental factors, occurrence of Puumala virus in bank voles and the risk for human disease.