July 11, 2001 July 5, 2001 -- The increase in incidence of tick-borne encephalitis in Sweden reported over the past two decades is directly related to the country's increasingly mild climate over the same period, conclude authors of a study in this week's issue of THE LANCET.
Tick-borne encephalitis (TBE) is caused by a virus, carried by tick parasites, that causes brain inflammation; it is not usually fatal.
TBE incidence in Sweden has substantially increased since the mid-1980s. During the same period the climate has become milder and ticks have become more common. Elisabet Lindgren and Rolf Gustafson from Stockholm University, Sweden, investigated whether there was a link between the change in climate and the increase in incidence of TBE.
Since the late 1950s all cases of encephalitis admitted in Stockholm County (central Sweden) had a blood test for TBE. The investigators analysed the period 1960-98; 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 TBE incidence rate to account for the tick's long life-span.
Increases in disease incidence was significantly related to a combination of two consecutive mild winters, temperatures favouring spring development (8-10ºC) and extended autumn activity (5-8ºC) in the year prior to the incidence year, and temperatures allowing tick activity (5-8ºC) early in the incidence year.
Elisabet Lindgren comments: "The findings indicate that the increase in TBE incidence since the mid-1980s is related to the trend towards milder winters and the 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."
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