Multi-source analysis reveals latitudinal and altitudinal shifts in range of Ixodes ricinus at its northern distribution limit

Abstract

Background: There is increasing evidence for a latitudinal and altitudinal shift in the distribution range of Ixodes ricinus. The reported incidence of tick-borne disease in humans is on the rise in many European countries and has raised political concern and attracted media attention. It is disputed which factors are responsible for these trends, though many ascribe shifts in distribution range to climate changes. Any possible climate effect would be most easily noticeable close to the tick's geographical distribution limits. In Norway- being the northern limit of this species in Europe- no documentation of changes in range has been published. The objectives of this study were to describe the distribution of I. ricinus in Norway and to evaluate if any range shifts have occurred relative to historical descriptions.

Methods: Multiple data sources - such as tick-sighting reports from veterinarians, hunters, and the general public - and surveillance of human and animal tick-borne diseases were compared to describe the present distribution of I. ricinus in Norway. Correlation between data sources and visual comparison of maps revealed spatial consistency. In order to identify the main spatial pattern of tick abundance, a principal component analysis (PCA) was used to obtain a weighted mean of four data sources. The weighted mean explained 67% of the variation of the data sources covering Norway's 430 municipalities and was used to depict the present distribution of I. ricinus. To evaluate if any geographical range shift has occurred in recent decades, the present distribution was compared to historical data from 1943 and 1983.

Results: Tick-borne disease and/or observations of I. ricinus was reported in municipalities up to an altitude of 583 metres above sea level (MASL) and is now present in coastal municipalities north to approximately 69°N.

Conclusion: I. ricinus is currently found further north and at higher altitudes than described in historical records. The approach used in this study, a multi-source analysis, proved useful to assess alterations in tick distribution.

Figure 1

The distribution of Ixodes ricinus in South Norway 1935-43 according to Tambs-Lyche[4]. The green colour depicts areas where I. ricinus and/or bovine besiosis occurs. The figure is adjusted and modified from Tambs-Lyche's Figure 10 to improve conformity between the map and Tambs-Lyche's written description. (Courtesy of Norsk veterinærtidsskrift)

Figure 2

The present distribution of Ixodes ricinus in Norway depicted by (a) the weighted mean obtained by the first principal component (PC1) of a PCA-analysis of four different sources (b-e). The distribution map displays tick abundance within municipalities with increasing darkness of colour. The inserted maps show: (b) the cumulative incidence of bovine babesiosis in the Norwegian Cattle Health Recording system in the period 1996-2008 (c) the cumulative incidence of human Lyme borreliosis in Norwegian Surveillance System for Communicable Diseases during the period 1991-2008 (with known place of infection) (d) the registration proportion of tick observations registered via the newspaper webpage during summer 2009 and (e) the frequency of tick observations from the veterinary survey performed in 2009 categorized as 0: never observed, 1: rarely observed (<3 tick observations during a year), 2: monthly observation, 3: weekly observation, 4: daily observation. Each of the variables is depicted on a municipal basis.

Figure 3

Distribution of Ixodes ricinus in a small area in south eastern Norway based on newspaper webpage registrations in 2009 and cervid hunters' webpage registrations in the period 2007-2009.

Figure 4

The municipalities (in blue) where the veterinary survey reported daily or weekly tick observations coupled with (a) absence of bovine babesiosis registrations in Norwegian Cattle Health Recording system 1996-2008 and (b) absence of Lyme borreliosis notifications in humans according to Norwegian Surveillance System for Communicable Diseases 1991-2008.