Elisabet Lindgren

Changes in the geographical distribution and abundance of the tick Ixodes ricinus during the past 30 years in Sweden

BackgroundIxodes ricinus is the main vector in Europe of human-pathogenic Lyme borreliosis (LB) spirochaetes, the tick-borne encephalitis virus (TBEV) and other pathogens of humans and domesticated mammals. The results of a previous 1994 questionnaire, directed at people living in Central and North Sweden (Svealand and Norrland) and aiming to gather information about tick exposure for humans and domestic animals, suggested that Ixodes ricinus ticks had become more widespread in Central Sweden and the southern part of North Sweden from the early 1980s to the early 1990s. To investigate whether the expansion of the ticks northern geographical range and the increasing abundance of ticks in Sweden were still occurring, in 2009 we performed a follow-up survey 16 years after the initial study.MethodsA questionnaire similar to the one used in the 1994 study was published in Swedish magazines aimed at dog owners, home owners, and hunters. The questionnaire was published together with a popular science article about the ticks biology and role as a pathogen vector in Sweden. The magazines were selected to get information from people familiar with ticks and who spend time in areas where ticks might be present.ResultsAnalyses of data from both surveys revealed that during the near 30-year period from the early 1980s to 2008, I. ricinus has expanded its distribution range northwards. In the early 1990s ticks were found in new areas along the northern coastline of the Baltic Sea, while in the 2009 study, ticks were reported for the first time from many locations in North Sweden. This included locations as far north as 66°N and places in the interior part of North Sweden. During this 16-year period the ticks range in Sweden was estimated to have increased by 9.9%. Most of the range expansion occurred in North Sweden (north of 60°N) where the ticks coverage area doubled from 12.5% in the early 1990s to 26.8% in 2008. Moreover, according to the respondents, the abundance of ticks had increased markedly in LB- and TBE-endemic areas in South (Götaland) and Central Sweden.ConclusionsThe results suggest that I. ricinus has expanded its range in North Sweden and has become distinctly more abundant in Central and South Sweden during the last three decades. However, in the northern mountain region I. ricinus is still absent. The increased abundance of the tick can be explained by two main factors: First, the high availability of large numbers of important tick maintenance hosts, i.e., cervids, particularly roe deer (Capreolus capreolus) during the last three decades. Second, a warmer climate with milder winters and a prolonged growing season that permits greater survival and proliferation over a larger geographical area of both the tick itself and deer. High reproductive potential of roe deer, high tick infestation rate and the tendency of roe deer to disperse great distances may explain the range expansion of I. ricinus and particularly the appearance of new TBEV foci far away from old TBEV-endemic localities. The geographical presence of LB in Sweden corresponds to the distribution of I. ricinus. Thus, LB is now an emerging disease risk in many parts of North Sweden. Unless countermeasures are undertaken to keep the deer populations, particularly C. capreolus and Dama dama, at the relatively low levels that prevailed before the late 1970s - especially in and around urban areas where human population density is high - by e.g. reduced hunting of red fox (Vulpes vulpes) and lynx (Lynx lynx), the incidences of human LB and TBE are expected to continue to be high or even to increase in Sweden in coming decades.

Climate change: present and future risks to health, and necessary responses

Abstract.  McMichael AJ, Lindgren E (The Australian National University, Canberra, Australia; and Karolinska Institute, Stockholm, Sweden). Climate change: present and future risks to health, and necessary responses (Review). J Intern Med 2011; 270: 401–413.

Monitoring EU Emerging Infectious Disease Risk Due to Climate Change

Climate change, globalization, and other drivers have made Europe a “hot spot” for emerging infectious diseases, which calls for changes in monitoring systems. In recent years, we have seen transmission of traditionally “tropical” diseases in continental Europe: chikungunya fever (CF) in Italy in 2007, large outbreaks of West Nile fever in Greece and Romania in 2010, and the first local transmission of dengue fever in France and Croatia in 2010 (1–3). These events support the notion that Europe is a potential “hot spot” for emerging and re-emerging infectious diseases (EIDs) (4). Major EID drivers that could threaten control efforts in Europe include globalization and environmental change (including climate change, travel, migration, and global trade); social and demographic drivers (including population aging, social inequality, and life-styles); and public health system drivers (including antimicrobial resistance, health care capacity, animal health, and food safety) (5, 6). Climate change is expected to aggravate existing local vulnerabilities by interacting with a complex web of these drivers (6). For example, increases in global trade and travel, in combination with climate change, are foreseen to facilitate the arrival, establishment, and dispersal of new pathogens, disease vectors, and reservoir species.

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.

Mapping climate change vulnerabilities to infectious diseases in Europe.

Background: The incidence, outbreak frequency, and distribution of many infectious diseases are generally expected to change as a consequence of climate change, yet there is limited regional information available to guide decision making. Objective: We surveyed government officials designated as Competent Bodies for Scientific Advice concerning infectious diseases to examine the degree to which they are concerned about potential effects of climate change on infectious diseases, as well as their perceptions of institutional capacities in their respective countries. Methods: In 2007 and 2009/2010, national infectious disease experts from 30 European Economic Area countries were surveyed about recent and projected infectious disease patterns in relation to climate change in their countries and the national capacity to cope with them. Results: A large majority of respondents agreed that climate change would affect vector-borne (86% of country representatives), food-borne (70%), water-borne (68%), and rodent-borne (68%) diseases in their countries. In addition, most indicated that institutional improvements are needed for ongoing surveillance programs (83%), collaboration with the veterinary sector (69%), management of animal disease outbreaks (66%), national monitoring and control of climate-sensitive infectious diseases (64%), health services during an infectious disease outbreak (61%), and diagnostic support during an epidemic (54%). Conclusions: Expert responses were generally consistent with the peer-reviewed literature regarding the relationship between climate change and vector- and water-borne diseases, but were less so for food-borne diseases. Shortcomings in institutional capacity to manage climate change vulnerability, identified in this assessment, should be addressed in impact, vulnerability, and adaptation assessments.

Risk indicators for the tick Ixodes ricinus and Borrelia burgdorferi sensu lato in Sweden

The distributional area of the tick Ixodes ricinus (L.), the primary European vector to humans of Lyme borreliosis spirochaetes (Borrelia burgdorferi sensu lato) and tick‐borne encephalitis virus, appears to be increasing in Sweden. It is therefore important to determine which environmental factors are most useful to assess risk of human exposure to this tick and its associated pathogens. The geographical distribution of I. ricinus in Sweden was analysed with respect to vegetation zones and climate. The northern limit of I. ricinus and B. burgdorferi s.l. in Sweden corresponds roughly to the northern limit of the southern boreal vegetation zone, and is characterized climatically by snow cover for a mean duration of 150 days and a vegetation period averaging 170 days. The zoogeographical distribution of I. ricinus in Sweden can be classified as southerly–central, with the centre of the distribution south of the Limes Norrlandicus. Ixodes ricinus nymphs from 13 localities in different parts of Sweden were examined for the presence of B. burgdorferi s.l. and found to be infected with Borrelia afzelii and Borrelia garinii. Tick sampling localities were characterized on the basis of the density of Borrelia‐infected I. ricinus nymphs, presence of specific mammals, dominant vegetation and climate. Densities of I. ricinus nymphs and Borrelia‐infected nymphs were significantly correlated, and nymphal density can thus serve as a general indicator of risk for exposure to Lyme borreliosis spirochaetes. Analysis of data from this and other studies suggests that high densities of Borrelia‐infected nymphs typically occur in coastal, broadleaf vegetation and in mixed deciduous/spruce vegetation in southern Sweden. Ixodes ricinus populations consistently infected with B. burgdorferi s.l. can occur in: (a) biotopes with shrews, rodents, hares and birds; (b) biotopes with shrews, rodents, hares, deer and birds, and (c) island locations where the varying hare (Lepus timidus) is the only mammalian tick host.

Globalization and the Sustainability of Human Health An ecological perspective

205 ozone, biodiversity, terrestrial and marine food-producing ecosystems, and the great cycles of water, nitrogen, and sulfur (Meyer 1996, Vitousek et al. 1997). These systems sustain the conditions on which life depends, and their weakening may therefore have profound long-term implications for human population health (McMichael 1993, Last 1997). Much of the recognition of how these unprecedented large-scale environmental changes may jeopardize human health has emerged, albeit tentatively, during this current decade. For example, the First Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), published in 1990 (Houghton et al. 1990), paid scant attention to the risks to human health that are a consequence of climate change, although it dealt in detail with the potential impacts of farms, forests, fisheries, water catchments, and other systems. In contrast, IPCC’s Second Assessment Report (IPCC 1996) gave a much more detailed consideration to the potential health impacts of climate change. The report noted that “The sustained health of human populations requires the continued integrity of Earth’s natural systems.” This latter statement invokes an unfamiliar idea. The dominance of urbanism and individualism within modern Western culture has diminished people’s awareness of the dependence of continued good health on the natural world. We tend to focus instead on immediate, local, tangible influences on personal health, thus viewing health primarily as an individual asset to be transacted within the health care system and enhanced by prudent individual behavior (supplemented by regulatory protection). The ethos of modern epidemiological research, with its predominantly reductionist approach to studying disease causation by cataloging proximate risk-factor behaviors and exposures, has reinforced this individual-centered view of health and disease (Loomis and Wing 1990, Pearce 1996). There are, however, important influences on health that operate at the population level—some of which do not translate directly into individual-level factors. An awareness that the health of a population reflects ecological circumstances has long been applied by ecologists to nonhuman, especially wild, species (Anderson 1982, Odum 1992). To understand these larger-scale ecological influences on human health, Globalization and the Sustainability of Human Health

Adaptation to the infectious disease impacts of climate change

Climate change has the potential to increase the challenge of preventing and controlling outbreaks of infectious diseases. An adaptation assessment is an important aspect of designing and implementing policies and measures to avoid, prepare for, and effectively respond to infectious diseases outbreaks. The main steps in conducting an adaptation assessment include: 1) evaluating the effectiveness of policies and measures that address the burden of climate-sensitive infectious diseases; 2) identifying options to manage the health risks of current and projected climate change; 3) evaluating and prioritizing the options; 4) identifying human and financial resources needs, and possible barriers, constraints, and limits to implementation; and 5) developing monitoring and evaluation programs to ensure continued effectiveness of policies and measures in a changing climate. Optimally, relevant stakeholders are optimally included throughout the adaptation assessment. Although the process of conducting an assessment is similar across nations and regions, the context and content will vary depending on local circumstances, socioeconomic conditions, legal and regulatory frameworks, and other factors. The European Centers for Disease Prevention and Control developed guidelines for conducting assessments, with sufficient consistency to facilitate learning lessons across assessments.

Determinants and Drivers of Infectious Disease Threat Events in Europe

Globalization and environment, the most frequent underlying drivers, should be targeted for interventions to prevent such events.

European monitoring systems and data for assessing environmental and climate impacts on human infectious diseases.

Surveillance is critical to understanding the epidemiology and control of infectious diseases. The growing concern over climate and other drivers that may increase infectious disease threats to future generations has stimulated a review of the surveillance systems and environmental data sources that might be used to assess future health impacts from climate change in Europe. We present an overview of organizations, agencies and institutions that are responsible for infectious disease surveillance in Europe. We describe the surveillance systems, tracking tools, communication channels, information exchange and outputs in light of environmental and climatic drivers of infectious diseases. We discuss environmental and climatic data sets that lend themselves to epidemiological analysis. Many of the environmental data sets have a relatively uniform quality across EU Member States because they are based on satellite measurements or EU funded FP6 or FP7 projects with full EU coverage. Case-reporting systems for surveillance of infectious diseases should include clear and consistent case definitions and reporting formats that are geo-located at an appropriate resolution. This will allow linkage to environmental, social and climatic sources that will enable risk assessments, future threat evaluations, outbreak management and interventions to reduce disease burden.