Tick-borne encephalitis in Sweden and climate change

Background The incidence of tick-borne encephalitis (TBE) in Sweden has substantially increased since the mid-1980s. During the same period the climate has become milder and ticks have become more abundant. We investigated whether there is a link between the change in climate and the increase in incidence of TEE. Methods Since the late 1950s all cases of encephalitis admitted in Stockholm County have been serologically tested for TEE. We analysed the period 1960-98 with multiple regressions. The number of days per season with temperatures of known importance for tick prevalence and pathogen transmission were studied. 2 years of temperature data were related to each TEE incidence rate to account for the tick's long life-span. Findings Increases in disease incidence was significantly related (R-2=0.58; p<0.0001) to a combination of two consecutive mild winters, temperatures favouring spring development (8-10C) and extended autumn activity (5-8 degreesC) in the year prior to the incidence year, and temperatures allowing tick activity (5-8 degreesC) early in the incidence year. Interpretations The findings indicate that the increase in TBE incidence since the mid-1980s is related to the period's change towards milder winters and early arrival of spring. Other factors may have influenced TBE incidence such as more people in endemic locations, and increases in host animal populations; factors which are partly climate related. Access to TBE vaccination since 1986 and increased awareness of ticks might have caused an underestimation of the links found. Our findings also suggest that the incidence of other tick-borne zoonoses might have been affected by the milder climate.