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Elisabet Lindgren,¹ Lars Tälleklint,² and Thomas Polfeldt^3,4

¹Natural Resources Management, Department of Systems Ecology, Stockholm University, Stockholm, Sweden
²Department of Environmental Science Policy and Management, University of California, Berkeley, California, USA
³Department of Statistics, Stockholm University, Stockholm, Sweden
⁴Statistics Sweden, Stockholm, Sweden

Abstract

We examined whether a reported northward expansion of the geographic distribution limit of the disease-transmitting tick Ixodes ricinus and an increased tick density between the early 1980s and mid-1990s in Sweden was related to climatic changes. The annual number of days with minimum temperatures above vital bioclimatic thresholds for the tick's life-cycle dynamics were related to tick density in both the early 1980s and the mid-1990s in 20 districts in central 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 seasons 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. ricinus ticks. Key words: climate change, geographic distribution, Ixodes ricinus, Lyme disease, temperature, tick, tickborne encephalitis. Environ Health Perspect 108:119-123 (2000) . [Online 29 December 1999]

http://ehpnet1.niehs.nih.gov/docs/2000/108p119-123lindgren/ abstract.html

Address correspondence to E. Lindgren, Natural Resources Management, Department of Systems Ecology, Svante Arrhenius Road 21 A, Stockholm University, SE- 106 91 Stockholm, Sweden. Telephone: 46-8-16 12 90. Fax: 46-8-15 84 17. E-mail: elisa@system.ecology.su.se

We are grateful to T. Jaenson (Uppsala University) for contributing data on the questionnaire study and C. Folke (Stockholm University) for constructive comments on the manuscript.

Financial support was provided by the Center for Research on Natural Resources and the Environment, Stockholm University, and the Swedish Council for Planning and Co-ordination of Research.

Received 26 May 1999 ; accepted 19 August 1999.