Change in tick range and tick abundance
The results from both 1994 and 2008 surveys suggest that the range of I. ricinus increased markedly in Sweden during the period from the early 1980s to 2008. This range expansion appears quite distinct at the northern parts of the tick's range. Thus, I. ricinus now occurs in many localities in the interior and northernmost regions of Norrland in places where it was not present about three decades ago, albeit at lower densities compared to South and Central Sweden and the southern part of North Sweden. It should be noted that in contrast to South Sweden homesteads are to a greater extent located in the climatologically most suitable places which are along the coast and rivers in inland river valleys, around lakes and other "low-land" areas as far as possible protected from the cold northern climate. Therefore, the records of I. ricinus in Northern Sweden mainly refer to such clusters of respondents living in places with a milder climate, with a more "southern" vegetation and fauna than that of the surrounding boreal areas dominated by spruce and pine (taiga) forest.
Both questionnaire-based studies showed that there was a significant increase in the proportion of respondents that considered that "ticks are presently" (referring to the previous year's tick season) occurring near the respondents' homes, compared to 11-16 years ago. Comparison of the percentage increase in tick occurrence was greater, i.e., significantly more rapid during the second questionnaire study period (1993-2008) than during the first period (1983-1993). The winters of 1988 to 1995 were warm or exceptionally warm in Sweden . As mentioned, this corresponds to the period when, due to the mild winters and a reduced fox population the number of roe deer increased dramatically to reach more than 1 million deer in 1994. The subsequent increase in fox and lynx numbers together with hunting began to reduce the roe deer population.
In view of the extended life cycle duration of I. ricinus which may be as long as 6 years in Northern Europe , there is a time lag of several years between a peak in the deer population and the resulting high I. ricinus population . Thus, it is inferred that the greater "% change" per year of tick occurrence during the second study period (1993-2008) compared to that of the first period (1983-1993) is due to two factors: First, the high availability of large maintenance hosts, i.e. mainly roe deer, in particular during the late 1980s and early part of the 1990s should have been favourable especially for the adult ticks to reproduce and thereby for the tick population to increase. This conclusion is supported by data recorded at the Danish Pest Infestation Laboratory (DPIL) : The annual numbers of requests for information on I. ricinus - a proxy for tick abundance - to DPIL was fairly stable between 1965 to 1985, but doubled during the late 1980s to reach a higher level in the early 1990s. The perceived tick abundance correlated with estimates of annual deer abundance and temperature records .
Second, for most European meteorological stations' temperature records the mean annual temperature shows a marked step-increase around 1989 and has thereafter been followed by consistently warm conditions [4, 38]. Such an increased warming, most of which occurred from January to early August  should both directly and indirectly have favoured survival and reproduction of both roe deer and ticks . Indirectly, climatic changes can significantly influence vegetation communities and tick host populations. Recent studies have shown a close correspondence between the durations of the vegetation period and snow cover period and the distributional area for I. ricinus in Sweden [7, 39]. The tick is unlikely to become established in an area where the snow cover period is > 150 days/year and where the mean temperature is < 5°C for > 170 days. This suggests that a direct or indirect effect of the climate, i.e., temperatures within a certain range, determine the potential geographical distribution of I. ricinus - given that precipitation and humidity are adequate.
Like many other vectors of zoonotic pathogens I. ricinus is a host- and habitat-generalist with a very wide host range and has been recorded from many different biotopes. The survival and proliferation of such ectoparasites are obviously dependent, in part, on the community of host species which they infest . In general, the main hosts of adult I. ricinus on the Swedish mainland are cervids . If, during a period of several years the climate is above "average favourable" for these mammalian tick hosts, they would presumably expand their population sizes and ranges - on the assumption that disease, predation and hunting pressure do not increase and that other tick hosts do not change their impact on the tick population. A corollary would be that I. ricinus, in the same area where the cervids occur, would most likely increase its geographical range and become more abundant. This is most likely what happened on the Swedish mainland during the last 30 years, in particular since 1988 to present time.
Population dynamic aspects
In response to climate change, populations can shift their distribution, adapt to the new climate or go extinct; shifts of their distributions can be pole ward to higher latitudes or upwards to higher altitudes . "Theory and limited empirical data suggest that shifts in population abundance along the edge of the range should be one of the first and most sensitive signs of a broader species response to environmental change" . The range extension of I. ricinus in Sweden is at the northern edge of the tick's geographical distribution. It is in such marginal areas that we obtained the first indications about a response of I. ricinus to the changing climate and host abundance . Here, at the limit of its range, environmental conditions are, in general, less optimal. The tick population will therefore be more likely to go extinct here than in its core area. Thus, it is likely that the distributional area of I. ricinus in North Sweden, as presented on the right map of Figure 3, might change: during some years the tick's range may enlarge whereas during other less favourable years it may retract.
Organisms are usually more abundant at the centre of their range, where optimal biotic and abiotic conditions usually prevail, than towards the periphery . The data on median numbers of ticks recorded by the respondents on themselves, and on their dogs and cats suggest that these medians are greater in southern and central Sweden than in northernmost Sweden. This supports the view that tick density is, in general, much lower near the periphery of its range.
Near the core of the distributional range of I. ricinus in Central Europe a phenomenon similar to that observed by us in North Sweden was observed: In the Czech Republic I. ricinus extended its range to higher altitudes. Thus, ticks infected with the TBE virus (TBEV) and several species of the B. burgdorferi s.l. complex were collected during the last decade at higher altitudes than before [20, 21, 43]. Tick range expansion can be facilitated by human activity, for example when tick-infested dogs, cattle or other domesticated mammals are brought into previously tick-free areas. A few such cases where it is suspected that ticks, found in previously tick-free localities in northern Sweden, had dropped from dogs that had shortly before visited tick-infested areas in central or south Sweden were reported by informants during our survey (TGT Jaenson DGE Jaenson, unpublished data). However, we believe that in Sweden the main vehicle for rapid and effective transportation of ticks of all stages into new areas are roe deer and to a lesser extent other large mammals. Birds are important transporters of immature I. ricinus, which may be infected with Borrelia bacteria and other pathogens of humans [44–46]. In contrast to some other bird-parasitizing tick species, I. ricinus ticks on birds are very rarely adults [44–46]. Therefore, they are unlikely to establish new tick populations in tick-free areas. However, such immature ticks may introduce "new" pathogens into previously non-endemic areas.
Roe deer - the main host for females of Ixodes ricinus
The abundance of ticks is largely determined by the availability of suitable hosts [22, 23, 29, 30, 47–49]. In many parts of Europe including Sweden, the roe deer has for the last decades up to the present time been the most important blood meal host for females of I. ricinus [13, 14, 22, 23, 48] and a mate-seeking site for the tick males . Consequently, the roe deer population is a key factor for the reproductive success of the tick population. In North America, the white-tailed deer (Odocoileus virginianus) plays a similar role as an important host for I. scapularis [42, 47, 50]. In Central Sweden in the late summer, one single roe deer can harbour > 2000 I. ricinus ticks; Mean infestation rates of 30 females, 17 males, 93 nymphs and 265 larvae of I. ricinus were recorded on 37 roe deer by Tälleklint & Jaenson . A growing number of roe deer is therefore likely to have been of major importance for the ticks increasing abundance and range expansion from the 1980s.
To obtain support for the hypothesis that there is a causal link between changes in deer distribution and abundance and similar changes in tick distribution and abundance, one would need to establish if such changes indeed have coincided in space and time . In Great Britain people consider that the increases in tick numbers over recent years coincide spatially with increases in deer numbers; people's perceptions were supported by data showing simultaneous increases in tick infestation rates on grouse and roe deer .
For reasons explained earlier, the roe deer population in Sweden expanded to very high levels during the 1980s and 1990s, reaching a peak in 1993-94. The subsequent spread of ticks northwards in Sweden were most likely a result of the greater availability of hosts, particularly roe deer, and a changing climate that was directly and indirectly favourable for both ticks and roe deer [8, 14, 26]. Extended vegetation periods and mild winters have continued in most years after the early 1990s.
On the Swedish mainland the number of roe deer has recently declined, partly due to increasing numbers of predators but more importantly due to two cold winters (2009/2010 and 2010/2011) with heavy snow cover . However, the roe deer has a great reproductive potential and great dispersal capacity , and during winter-time many hunters are providing fodder to increase deer survival and reproduction in order to maintain large numbers of deer for hunting. Therefore, if counter-measures are not undertaken, to keep the roe deer and other deer populations at low, acceptable levels in line with public health objectives, they are likely to rapidly regain high population levels.
The geographic range of roe deer in Sweden covers a larger area than the one where I. ricinus has been recorded. Thus, in 1990 roe deer were only absent from the north-western part of Lapland in North Sweden . This implies that, provided the climate will be suitable for I. ricinus this tick may be able to establish permanent populations in areas of North Sweden where the tick is still absent but where the main host for the adult stage, the roe deer occurs.
A characteristic behavioural trait of young roe deer is their tendency to rapidly disperse to new areas, often far away from their place of birth . Nearly all young deer in North Sweden appear to leave their place of birth . While young deer in South Sweden usually do not disperse more than 20 km from their place of birth the corresponding mean distance in North Sweden is > 40 km with some of the migrations as far as 200 km , presumably due to greater distances between favourable habitats in North Sweden than in South Sweden. This behaviour strongly supports the hypothesis that roe deer has greatly contributed to the recent rapid and massive spread of I. ricinus throughout northern Sweden.
What other factors are causing the increasing range and abundance of the tick and increased human incidence of tick-borne pathogens?
Changes in climate and vegetation are two key factors that will profoundly impact both I. ricinus and its host animals. Many other factors may be regarded as additional drivers that have affected and will affect the abundance and range of I. ricinus in Sweden and neighbouring countries. These include increased availability of large blood hosts, especially cervids, for the tick females and for both sexes to use as mating sites; migration and dispersal capacity of wild animals, especially deer; and migration and transportation of medium-sized to large domesticated mammals. Other factors are diseases affecting deer, hares and pheasants or their predators; changed hunting pressures on tick hosts or on their predators; provision of winter feed to deer, hares and pheasants; changed agricultural and farming practices; changed grazing methods; changed land use patterns; and reduced or lost diversity of tick host animals and their predators in the "tick's food web" [40, 52] including increased abundance or disappearance of vertebrates affecting the abundance or behaviour of blood hosts of ticks; importation and spread of non-native tick host animals; changed environmental and conservation legislation and strategies; creation or establishment of protected or recreational areas such as nature reserves, national parks, forest reserves and urban parks; feeding of deer and other wild animals close to or in urban areas; immigration of deer into urban areas and increased availability for ticks of deer, hares, dogs and pheasants in such areas; greater human exposure to ticks due to changed leisure activities with more outdoor activities to promote well-being and health; increased berry and mushroom picking; more leisure time; changed/increased awareness of and greater ability to detect and remove ticks; increased awareness by doctors and laymen of tick-borne disease symptoms and increased testing for infection leading to increased reporting of ticks bites and tick-borne diseases; greater awareness of "tick high-risk areas" and "TBE high-risk areas"; and changed incidence of TBE due to altered TBE vaccination rates.
Lyme borreliosis and TBE incidences in Sweden
Roe deer can be infested with Borrelia-  and presumably also with Anaplasma-, Rickettsia-, Babesia- and TBE-virus-infected I. ricinus. Therefore, roe deer are likely to play an important role in the dispersal to new locations of ticks infected with human pathogens. Many new TBE-foci have been detected in Sweden during recent years [54–56]. The large roe deer population and the great dispersal potential of deer  infested with pathogen-infected ticks, in combination with a warmer climate with more rapid rise in late spring and early summer temperature permitting simultaneous co-feeding of infectible tick larvae together with infected nymphs, may help to explain why such new TBEV-foci have appeared far away from the "old" TBEV-endemic areas. Also, the rapid spread of the tick and tick-borne infections into central and northern Sweden is presumably mainly due to "transportation" of infected ticks on migrating roe deer. It is presumably to a much lesser extent due to dogs, which have visited more southern parts of Sweden, infested with Borrelia- infected ticks and northward-migrating birds infested with Borrelia- and TBEV-infected ticks [44–46].
In Sweden, the geographical distribution of human-pathogenic LB spirochaetes, B. burgdorferi s.l., coincides with that of their main vector, I. ricinus [27–30]. In other words, changes in tick distribution and tick density are likely to have increased the risk of human LB and TBE and are likely to lead to further changes in risk areas of these and other tick-borne infections.
The highest incidence of LB reported for Europe is in Eastern Central Europe, with incidence figures of 120-130 human cases per 100,000 inhabitants/year recorded in Slovenia and Austria, respectively . We found that the LB endemic area in Sweden may have a similar, high annual incidence rate: about 125 LB cases/100,000 inhabitants . There are significant relationships between roe deer density and abundance of I. ricinus [23, 29, 30, 58], nymphal abundance and density of Borrelia- infected nymphs [29, 30, 39] and between density of Borrelia- infected nymphs and LB incidence in humans . Thus, the abundance of roe deer may be used as a crude indicator of risk for human exposure to LB spirochaetes. However, since roe deer are incompetent hosts for B. burgdorferi s.l.  they may at very high densities divert larval ticks from feeding on reservoir-competent hosts to feeding on deer. This will result in a negative relationship between the density of I. ricinus nymphs and the density of nymphs infected with B. burgdorferi s.l. . Similar relationships as for LB also exist between roe deer density and tick density and incidence of human TBE cases . Thus, if the abundance and range of roe deer is maintained in Sweden, tick density and the tick's range are likely to increase further. Consequently, the incidences of human diseases vectored by I. ricinus are likely to become even higher. The fallow deer, Dama dama, occurs in Sweden. It is generally much less abundant than the roe deer and has a lower tendency to disperse compared to that of the roe deer. However, the fallow deer population is on the increase in some localities and is likely to become another important tick host and public health problem, especially in periurban areas in Sweden.