| Summary: | We examined whether a reported northward expansion of the geographic distribution limit of the disease-transmitting tick Iowdes ricinws and an increased tick density between the early 1980s and mid-1990s in Sweden was related to dimatic changes. The annual number of days with minimum temperatures above vital bioclimatic thresholds for the tick's i-cycle dynamics were related to tick density in both the early 1980s and the mid-1990s in 20 districts in centra and northern Sweden. The winters were markedly milder in all of the study areas in the 1990s as compared to the 1980s. Our results indicate that the reported northern shift in the distribution limit of ticks is related to fewer days during the winter sesons with low minimum temperatures, i.e., below-12`C. At high latitudes, low winter temperatures had the clearest impact on tick distribution. Further south, a combination of mild winters (fewer days with minimum temperatures below-7°C) and extended spring and autumn seasons (more days with minimum temperatures from 5 to 8°C) was related to increases in tick density. We conclude that the relatively mild climate of the 1990s in Sweden is probably one of the primary reasons for the observed increase of density and geographic range of I. ricinuw ticks. Key won cimate change, geographic distrbution, Ixodes ricinus, Lyme disease, temperature, tick, tickborne encephalitis. Environ Health Persect 108:119-123 (2000). [Online 29 December 1999]
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