Increasing geographical distribution and density of Ixodes ricinus (Acari: Ixodidae) in central and northern Sweden.

The geographical distribution and density of Ixodes ricinus (L.) in the 2 northern regions, Svealand and Norrland, in Sweden were studied by using a questionnaire in Swedish magazines for house owners and dog owners, and in provincial newspapers. Analysis of the approximately 1,200 answers revealed that ticks are present in all parts of Svealand (except northern Värmland and northern and western Dalarna), the southeastern part of Norrland (i.e., Gästrikland and Hälsingland), and along the Baltic Sea coast of central and northern Norrland. The proportion of answers reporting ticks and the estimated tick density (i.e., the number of ticks infesting dogs and cats) decreased from south to north. The answers to the questionnaire and data from field sampling of ticks suggest that tick density decreased distinctly along a narrow boundary zone, coinciding with the biological limit of Norrland (Limes Norrlandicus), crossing Sweden through southern Värmland, southeastern Dalarna, and central Gästrikland. The results of the questionnaire suggest that I. ricinus ticks are more widespread today than in the early 1980s, especially in Värmland, western and central Dalarna, Hälsingland, and the coastal areas of Medelpad, Angermanland, and Västerbotten, and that the proportion of the human population at risk for tick-borne pathogens in Svealand and Norrland is increasing.