The range of Ixodes ricinus and the risk of contracting Lyme borreliosis will increase northwards when the vegetation period becomes longer.

In Sweden, the geographical distribution of Lyme borreliosis corresponds to that of its vector Ixodes ricinus. Both tick activity and the length of the vegetation period are determined by daily mean temperatures ≥5°C. We analysed the correspondence between the distribution of I. ricinus in Sweden, the start date, end date, and length of the vegetation period, and the distributions of tick habitat-associated plant species. The geographical distribution of I. ricinus in Sweden corresponds to a vegetation period averaging approximately 170 days, an early start (before May 1st) of spring, and to the distribution of black alder (Alnus glutinosa). Based on scenario models for these parameters, changes in the range and abundance of I. ricinus were projected for the periods 2011-2040, 2041-2070, and 2071-2100. We conclude that climate change during this century will probably increase the geographic range of I. ricinus as vegetation communities and mammals associated with high tick densities will increase their geographic ranges due to a markedly prolonged vegetation period. By the end of this century, the ranges of I. ricinus and Borrelia burgdorferi sensu lato may, in suitable habitats, encompass most of Sweden, Norway, and Finland as far as 70°N, except the mountainous regions. This will lead to an increased Lyme borreliosis risk in northern Scandinavia.