Landscape predictors of tick-borne encephalitis in Latvia: land cover, land use, and land ownership

Tick abundance and infection with three zoonotic bacteria are heterogeneous in a Belgian peri-urban forest

Ixodes ricinus is a vector of several pathogens of public health interest. While forests are the primary habitat for I. ricinus, its abundance and infection prevalence are expected to vary within forest stands. This study assesses the spatio-temporal variations in tick abundance and infection prevalence with three pathogens in and around a peri-urban forest where human exposure is high. Ticks were sampled multiple times in 2016 and 2018 in multiple locations with a diversity of undergrowth, using the consecutive drags method. Three zoonotic pathogens were screened for, Borrelia burgdorferi s.l., Coxiella burnetii, and Francisella tularensis. The influence of season, type of site and micro-environmental factors on tick abundance were assessed with negative binomial generalized linear mixed-effects models. We collected 1642 nymphs and 181 adult ticks. Ticks were most abundant in the spring, in warmer temperatures, and where undergrowth was higher. Sites with vegetation unaffected by human presence had higher abundance of ticks. Forest undergrowth type and height were significant predictors of the level of tick abundance in a forest. The consecutive drags method is expected to provide more precise estimates of tick abundance, presumably through more varied contacts with foliage. Borrelia burgdorferi s.l. prevalence was estimated from pooled ticks at 5.33%, C. burnetii was detected in six pools and F. tularensis was not detected. Borrelia afzelii was the dominant B. burgdorferi genospecies. Tick abundance and B. burgdorferi s.l. infection prevalence were lower than other estimates in Belgian forests.

Land system governance shapes tick-related public and animal health risks

Land cover and land use have established effects on hazard and exposure to vector-borne diseases. While our understanding of the proximate and distant causes and consequences of land use decisions has evolved, the focus on the proximate effects of landscape on disease ecology remains dominant. We argue that land use governance, viewed through a land system lens, affects tick-borne disease risk. Governance affects land use trajectories and potentially shapes landscapes favourable to ticks or increases contact with ticks by structuring human-land interactions. We illustrate the role of land use legacies, trade-offs in land-use decisions, and social inequities in access to land resources, information and decision-making, with three cases: Kyasanur Forest disease in India, Lyme disease in the Outer Hebrides (Scotland), and tick acaricide resistance in cattle in Ecuador. Land use governance is key to managing the risk of tick-borne diseases, by affecting the hazard and exposure. We propose that land use governance should consider unintended consequences on infectious disease risk.

Producing context specific land cover and land use maps of human-modified tropical forest landscapes for infectious disease applications

Satellite-based land cover mapping plays an important role in understanding changes in ecosystems and biodiversity. There are global land cover products available, however for region specific studies of drivers of infectious disease patterns, these can lack the spatial and thematic detail or accuracy required to capture key ecological processes. To overcome this, we produced our own Landsat derived 30 m maps for three districts in India's Western Ghats (Wayanad, Shivamogga and Sindhudurg). The maps locate natural vegetation types, plantation types, agricultural areas, water bodies and settlements in the landscape, all relevant to functional resource use of species involved in infectious disease dynamics. The maps represent the mode of 50 classification iterations and include a spatial measure of class stability derived from these iterations. Overall accuracies for Wayanad, Shivamogga and Sindhudurg are 94.7 % (SE 1.2 %), 88.9 % (SE 1.2 %) and 88.8 % (SE 2 %) respectively. Class classification stability was high across all three districts and the individual classes that matter for defining key interfaces between human habitation, forests, crop, and plantation cultivation, were generally well separated. A comparison with the 300 m global ESA CCI land cover map highlights lower ESA CCI class accuracies and the importance of increased spatial resolution when dealing with complex landscape mosaics. A comparison with the 30 m Global Forest Change product reveals an accurate mapping of forest loss and different dynamics between districts (i.e., Forests lost to Built-up versus Forests lost to Plantations), demonstrating an interesting complementarity between our maps and the % tree cover Global Forest Change product. When studying infectious disease responses to land use change in tropical forest ecosystems, we recommend using bespoke land cover/use classifications reflecting functional resource use by relevant vectors, reservoirs, and people. Alternatively, global products should be carefully validated with ground reference points representing locally relevant habitats.

Ecological and environmental factors affecting the risk of tick-borne encephalitis in Europe, 2017 to 2021

BackgroundTick-borne encephalitis (TBE) is a disease which can lead to severe neurological symptoms, caused by the TBE virus (TBEV). The natural transmission cycle occurs in foci and involves ticks as vectors and several key hosts that act as reservoirs and amplifiers of the infection spread. Recently, the incidence of TBE in Europe has been rising in both endemic and new regions.AimIn this study we want to provide comprehensive understanding of the main ecological and environmental factors that affect TBE spread across Europe.MethodsWe searched available literature on covariates linked with the circulation of TBEV in Europe. We then assessed the best predictors for TBE incidence in 11 European countries by means of statistical regression, using data on human infections provided by the European Surveillance System (TESSy), averaged between 2017 and 2021.ResultsWe retrieved data from 62 full-text articles and identified 31 different covariates associated with TBE occurrence. Finally, we selected eight variables from the best model, including factors linked to vegetation cover, climate, and the presence of tick hosts.DiscussionThe existing literature is heterogeneous, both in study design and covariate types. Here, we summarised and statistically validated the covariates affecting the variability of TBEV across Europe. The analysis of the factors enhancing disease emergence is a fundamental step towards the identification of potential hotspots of viral circulation. Hence, our results can support modelling efforts to estimate the risk of TBEV infections and help decision-makers implement surveillance and prevention campaigns.

First Expert Elicitation of Knowledge on Possible Drivers of Observed Increasing Human Cases of Tick-Borne Encephalitis in Europe

Tick-borne encephalitis (TBE) is a viral disease endemic in Eurasia. The virus is mainly transmitted to humans via ticks and occasionally via the consumption of unpasteurized milk products. The European Centre for Disease Prevention and Control reported an increase in TBE incidence over the past years in Europe as well as the emergence of the disease in new areas. To better understand this phenomenon, we investigated the drivers of TBE emergence and increase in incidence in humans through an expert knowledge elicitation. We listed 59 possible drivers grouped in eight domains and elicited forty European experts to: (i) allocate a score per driver, (ii) weight this score within each domain, and (iii) weight the different domains and attribute an uncertainty level per domain. An overall weighted score per driver was calculated, and drivers with comparable scores were grouped into three terminal nodes using a regression tree analysis. The drivers with the highest scores were: (i) changes in human behavior/activities; (ii) changes in eating habits or consumer demand; (iii) changes in the landscape; (iv) influence of humidity on the survival and transmission of the pathogen; (v) difficulty to control reservoir(s) and/or vector(s); (vi) influence of temperature on virus survival and transmission; (vii) number of wildlife compartments/groups acting as reservoirs or amplifying hosts; (viii) increase of autochthonous wild mammals; and (ix) number of tick species vectors and their distribution. Our results support researchers in prioritizing studies targeting the most relevant drivers of emergence and increasing TBE incidence.

Decoding the Geography of Natural TBEV Microfoci in Germany: A Geostatistical Approach Based on Land-Use Patterns and Climatological Conditions

Background: Tickborne-encephalitis (TBE) is a potentially life-threating neurological disease that is mainly transmitted by ticks. The goal of the present study is to analyze the potential uniform environmental patterns of the identified TBEV microfoci in Germany. The results are used to calculate probabilities for the present distribution of TBEV microfoci in Germany based on a geostatistical model. Methods: We aim to consider the specification of environmental characteristics of locations of TBEV microfoci detected in Germany using open access epidemiological, geographical and climatological data sources. We use a two-step geostatistical approach, where in a first step, the characteristics of a broad set of environmental variables between the 56 TBEV microfoci and a control or comparator set of 3575 sampling points covering Germany are compared using Fisher's Exact Test. In the second step, we select the most important variables, which are then used in a MaxEnt distribution model to calculate a high resolution (400 × 400 m) probability map for the presence of TBEV covering the entire area of Germany. Results: The findings from the MaxEnt prediction model indicate that multi annual actual evapotranspiration (27.0%) and multi annual hot days (22.5%) have the highest contribution to our model. These two variables are followed by four additional variables with a lower, but still important, explanatory influence: Land cover classes (19.6%), multi annual minimum air temperature (14.9%), multi annual sunshine duration (9.0%), and distance to coniferous and mixed forest border (7.0%). Conclusions: Our findings are based on defined TBEV microfoci with known histories of infection and the repeated confirmation of the virus in the last years, resulting in an in-depth high-resolution model/map of TBEV microfoci in Germany. Multi annual actual evapotranspiration (27%) and multi annual hot days (22.5%) have the most explanatory power in our model. The results may be used to tailor specific regional preventive measures and investigations.

Ticks, Hair Loss, and Non-Clinging Babies: A Novel Tick-Based Hypothesis for the Evolutionary Divergence of Humans and Chimpanzees

Human straight-legged bipedalism represents one of the earliest events in the evolutionary split between humans (Homo spp.) and chimpanzees (Pan spp.), although its selective basis is a mystery. A carrying-related hypothesis has recently been proposed in which hair loss within the hominin lineage resulted in the inability of babies to cling to their mothers, requiring mothers to walk upright to carry their babies. However, a question remains for this model: what drove the hair loss that resulted in upright walking? Observers since Darwin have suggested that hair loss in humans may represent an evolutionary strategy for defence against ticks. The aim of this review is to propose and evaluate a novel tick-based evolutionary hypothesis wherein forest fragmentation in hominin paleoenvironments created conditions that were favourable for tick proliferation, selecting for hair loss in hominins and grooming behaviour in chimpanzees as divergent anti-tick strategies. It is argued that these divergent anti-tick strategies resulted in different methods for carrying babies, driving the locomotor divergence of humans and chimpanzees.

Sero-molecular survey and risk factors of equine piroplasmosis in horses in Spain

BACKGROUND

Theileria equi and Babesia caballi cause equine piroplasmosis (EP), one of the most important tick-borne diseases of horses due to its high negative impact to the equine industry. Although infections with these parasites have been reported for decades in Spain, epidemiological studies have only been carried out in certain regions.

OBJECTIVES

To determine the (sero)prevalence of these parasites in asymptomatic horses nationwide in Spain and to identify potential individual and environmental factors associated with seropositivity to EP.

STUDY DESIGN

Sample size was calculated according to the horses registered in Spain in 2013 and by autonomous community using a random stratified sampling. A questionnaire was used to collect data on factors associated with EP seropositivity.

METHODS

Serological (cELISA and complement fixation test) and molecular (real-time PCR) analyses were carried out in 740 horses. Risk factors were identified computing two independent logistic regression models with the collated data.

RESULTS

Antibodies against EP were detected in 42.9% (95% CI 39.4-46.5) of horses, whereas 30.3% (95% CI 27.0-33.6) were EP positive by PCR. A substantial strength of agreement (k = 0.721) was estimated between serological tests. Exposure to T. equi was significantly higher than to B. caballi and the highest (sero)prevalence was detected in the northern communities. Increasing horse age, presence of ticks and contact with cows were factors related to EP seropositivity in the horses, whereas tetanus vaccination and fairs attendance were associated with lower seropositivity.

CONCLUSIONS

Almost half of the horses residing in Spain had antibodies against EP or circulating parasitaemia. Appropriate prevention measures and implementation of a EP surveillance programme should be considered in order to reduce and control the infection.

Enhanced threat of tick-borne infections within cities? Assessing public health risks due to ticks in urban green spaces in Helsinki, Finland

Most tick‐related studies in Europe have been conducted in nonurban areas, but ticks and tick‐borne pathogens also occur in urban green spaces. From a public health perspective, risks regarding tick‐borne infections should be studied in these urban areas, where contacts between infected ticks and humans may be more frequent than elsewhere, due to high human activity. We examined the risk of encountering an infected tick in urban green spaces in Helsinki, Finland. We collected ticks at nine sites throughout Helsinki, recorded the prevalence of several pathogens and identified areas with a high potential for contacts between infected ticks and humans. Moreover, we explored the relationship between the density of Borrelia burgdorferi sensu lato‐infected ticks and locally diagnosed cases of borreliosis and compared the potential for human‐tick encounters in Helsinki to those in nonurban areas in south‐western Finland. During 34.8 km of cloth dragging, 2,417 Ixodes ricinus were caught (402 adults, 1,399 nymphs and 616 larvae). From analysed nymphs, we found 11 distinct tick‐borne pathogens, with 31.5% of nymphs carrying at least one pathogen. Tick activity was highest in August and September, leading to the density of nymphs infected with B. burgdorferi s.l., and concurrently infection risk, to also be highest during this time. Nymph densities varied between the sampling sites, with obvious implications to spatial variation in infection risk. While ticks and tick‐borne pathogens were found in both Helsinki and nonurban areas in south‐western Finland, the estimates of human activity were generally higher in urban green spaces, leading to a higher potential for human‐tick contacts therein. The presence of ticks and tick‐borne pathogens and high local human activity in urban green spaces suggest that they form potential foci regarding the acquisition of tick‐borne infections. Risk areas within cities should be identified and knowledge regarding urban ticks increased.

Spatial risk analysis for the introduction and circulation of six arboviruses in the Netherlands

Background

Arboviruses are a growing public health concern in Europe, with both endemic and exotic arboviruses expected to spread further into novel areas in the next decades. Predicting where future outbreaks will occur is a major challenge, particularly for regions where these arboviruses are not endemic. Spatial modelling of ecological risk factors for arbovirus circulation can help identify areas of potential emergence. Moreover, combining hazard maps of different arboviruses may facilitate a cost-efficient, targeted multiplex-surveillance strategy in areas where virus transmission is most likely. Here, we developed predictive hazard maps for the introduction and/or establishment of six arboviruses that were previously prioritized for the Netherlands: West Nile virus, Japanese encephalitis virus, Rift Valley fever virus, tick-borne encephalitis virus, louping-ill virus and Crimean-Congo haemorrhagic fever virus.

Methods

Our spatial model included ecological risk factors that were identified as relevant for these arboviruses by an earlier systematic review, including abiotic conditions, vector abundance, and host availability. We used geographic information system (GIS)-based tools and geostatistical analyses to model spatially continuous datasets on these risk factors to identify regions in the Netherlands with suitable ecological conditions for arbovirus introduction and establishment.

Results

The resulting hazard maps show that there is spatial clustering of areas with either a relatively low or relatively high environmental suitability for arbovirus circulation. Moreover, there was some overlap in high-hazard areas for virus introduction and/or establishment, particularly in the southern part of the country.

Conclusions

The similarities in environmental suitability for some of the arboviruses provide opportunities for targeted sampling of vectors and/or sentinel hosts in these potential hotspots of emergence, thereby increasing the efficient use of limited resources for surveillance.

Pathogens, endosymbionts, and blood-meal sources of host-seeking ticks in the fast-changing Maasai Mara wildlife ecosystem

The role of questing ticks in the epidemiology of tick-borne diseases in Kenya's Maasai Mara National Reserve (MMNR), an ecosystem with intensified human-wildlife-livestock interactions, remains poorly understood. We surveyed the diversity of questing ticks, their blood-meal hosts, and tick-borne pathogens to understand potential effects on human and livestock health. By flagging and hand-picking from vegetation in 25 localities, we collected 1,465 host-seeking ticks, mostly Rhipicephalus and Amblyomma species identified by morphology and molecular analysis. We used PCR with high-resolution melting (HRM) analysis and sequencing to identify Anaplasma, Babesia, Coxiella, Ehrlichia, Rickettsia, and Theileria pathogens and blood-meal remnants in 231 tick pools. We detected blood-meals from humans, wildebeest, and African buffalo in Rh. appendiculatus, goat in Rh. evertsi, sheep in Am. gemma, and cattle in Am. variegatum. Rickettsia africae was detected in Am. gemma (MIR = 3.10) that had fed on sheep and in Am. variegatum (MIR = 250) that had fed on cattle. We found Rickettsia spp. in Am. gemma (MIR = 9.29) and Rh. evertsi (MIR = 200), Anaplasma ovis in Rh. appendiculatus (MIR = 0.89) and Rh. evertsi (MIR = 200), Anaplasma bovis in Rh. appendiculatus (MIR = 0.89), and Theileria parva in Rh. appendiculatus (MIR = 24). No Babesia, Ehrlichia, or Coxiella pathogens were detected. Unexpectedly, species-specific Coxiella sp. endosymbionts were detected in all tick genera (174/231 pools), which may affect tick physiology and vector competence. These findings show that ticks from the MMNR are infected with zoonotic R. africae and unclassified Rickettsia spp., demonstrating risk of African tick-bite fever and other spotted-fever group rickettsioses to locals and visitors. The protozoan pathogens identified may also pose risk to livestock production. The diverse vertebrate blood-meals of questing ticks in this ecosystem including humans, wildlife, and domestic animals, may amplify transmission of tick-borne zoonoses and livestock diseases.

Investigation of Ixodid ticks as vectors of Babesia caballi and Theileria equi (Protozoa: Apicomplexa) in central Italy

Babesia caballi and Theileria equi are widely recognized as causative agents of equine pirolasmosis (EP), an acute, sub-acute, and chronic disease of equines, with relevant economic impact on horse trade worldwide. Although several studies on EP prevalence from central Italy have been published, data on ticks responsible for its transmission are still lacking. In this study, we identified a potential competent vector, investigating main features of its ecology together with EP infection rates. A two-year sampling of questing ticks was carried out for the first time in Italy in an area known for high EP prevalence in horse sera, detecting the association between Rhipicephalus bursa and causative agents of EP. Most of the positive pools harbored a single infection (91.1%); mixed infections were also detected (8.9%). The infection rate for T. equi slightly decreased among years; B. caballi showed a lower, but increasing, infection rate. Tick phenology, climate variables, and peaks of EP prevalence indicated late May and second half of June as periods with the highest risk of new infections, especially during warm and dry days.

Ticks in the ecotone: the impact of agri-environment field margins on the presence and intensity of Ixodes ricinus ticks (Acari: Ixodidae) in farmland in southern England

The present study aimed to assess whether agri-environment field margins provide a habitat for the sheep/deer tick Ixodes ricinus. Field studies were conducted in arable farmland in southern England in both extant and newly constructed field margins. The presence and intensity (i.e. the mean number of nymphs per transect, excluding zeros) of questing nymphs and adult I. ricinus were compared between field margins with three adjacent habitats: woodland, hedgerow and arable land. The presence and intensity of ticks within a field margin was also compared between three ecozones: the ecotone, the margin and the crop. It was found that field margins do support I. ricinus, although the intensity of ticks was associated with field margins with adjacent woodland, with a higher tick intensity along the ecotonal ecozone, compared with the rest of the margin or the crop edge. The presence of a hedge also increased the likelihood of finding questing nymphs in a field margin compared with a margin adjacent to arable land. This effect, however, was less pronounced than in field margins with adjacent woodland. The provision of footpaths within the margin (at least 1-2 m from the ecotone), or on the edge of the crop where paths run next to woodland known to be an important tick habitat, could be promoted to minimize tick exposure. In addition, based on the results of the present study, raising awareness that walking alongside woodlands also constitutes a tick risk could be promoted.

Multi-scale patterns of tick occupancy and abundance across an agricultural landscape in southern Africa

Land use influences the prevalence and distribution of ticks due to the intimate relationship of ticks with their environment. This relationship occurs because land use alters two essential tick requirements: vertebrate hosts for blood meals and a suitable microclimate when off-host. Given the risks to human and animal health associated with pathogens transmitted by ticks, there is an ongoing need to understand the impact of environmental drivers on tick distributions. Here, we assessed how landscape features, neighborhood effects, and edges influenced tick occupancy and abundance across an agricultural landscape in southern Africa. We found that Rhipicephalus appendiculatus and Rhipicephalus simus increased in abundance closer to protected savanna, while Haemaphysalis elliptica increased in abundance closer to human habitation. The composition of the landscape surrounding savanna patches also differentially influenced the occupancy of each tick species; H. elliptica was more likely to be found in savanna patches surrounded by subsistence agriculture while R. appendiculatus and R. simus were more likely to be found in savanna surrounded by sugarcane monocultures. At the local scale we found that R. appendiculatus and R. simus avoided savanna edges. The availability of hosts and variation in vegetation structure between commercial agriculture, subsistence agriculture, and savanna likely drove the distribution of ticks at the landscape scale. Understanding how anthropogenic land use influences where ticks occur is useful for land use planning and for assessing public and animal health risks associated with ticks and tick-borne diseases.

Can Protected Areas Mitigate Lyme Disease Risk in Fennoscandia?

This Forum article synthesizes the current evidence on the links between predator-prey interactions, protected areas and spatial variations in Lyme disease risk in Fennoscandia. I suggest key research directions to better understand the role of protected areas in promoting the persistence of diverse predator guilds. Conserving predators could help reducing host populations and Lyme disease risk in northern Europe. There is an urgent need to find possible win-win solutions for biodiversity conservation and human health in ecosystems facing rapid global environmental change.

Risk factors associated with sustained circulation of six zoonotic arboviruses: a systematic review for selection of surveillance sites in non-endemic areas

Arboviruses represent a significant burden to public health and local economies due to their ability to cause unpredictable and widespread epidemics. To maximize early detection of arbovirus emergence in non-endemic areas, surveillance efforts should target areas where circulation is most likely. However, identifying such hotspots of potential emergence is a major challenge. The ecological conditions leading to arbovirus outbreaks are shaped by complex interactions between the virus, its vertebrate hosts, arthropod vector, and abiotic environment that are often poorly understood. Here, we systematically review the ecological risk factors associated with the circulation of six arboviruses that are of considerable concern to northwestern Europe. These include three mosquito-borne viruses (Japanese encephalitis virus, West Nile virus, Rift Valley fever virus) and three tick-borne viruses (Crimean-Congo hemorrhagic fever virus, tick-borne encephalitis virus, and louping-ill virus). We consider both intrinsic (e.g. vector and reservoir host competence) and extrinsic (e.g. temperature, precipitation, host densities, land use) risk factors, identify current knowledge gaps, and discuss future directions. Our systematic review provides baseline information for the identification of regions and habitats that have suitable ecological conditions for endemic circulation, and therefore may be used to target early warning surveillance programs aimed at detecting multi-virus and/or arbovirus emergence.

Sympatric Ixodes-tick species: pattern of distribution and pathogen transmission within wild rodent populations

The generalist tick Ixodes ricinus is the most important vector for tick-borne pathogens (TBP), including Borrelia burgdorferi sensu lato, in Europe. However, the involvement of other sympatric Ixodes ticks, such as the specialist vole tick I. trianguliceps, in the enzootic circulations of TBP remains unclear. We studied the distribution of I. ricinus and I. trianguliceps in Central Finland and estimated the TBP infection likelihood in the most common rodent host in relation with the abundance of the two tick species. Ixodes trianguliceps was encountered in all 16 study sites whereas I. ricinus was frequently observed only at a quarter of the study sites. The abundance of I. ricinus was positively associated with open water coverage and human population density around the study sites. Borrelia burgdorferi s. l.-infected rodents were found only in sites where I. ricinus was abundant, whereas the occurrence of other TBP was independent of I. ricinus presence. These results suggest that I. trianguliceps is not sufficient, at least alone, in maintaining the circulation of B. burgdorferi s. l. in wild hosts. In addition, anthropogenic factors might affect the distribution of I. ricinus ticks and, hence, their pathogens, thus shaping the landscape of tick-borne disease risk for humans.

A Predictive Model Has Identified Tick-Borne Encephalitis High-Risk Areas in Regions Where No Cases Were Reported Previously, Poland, 1999-2012

During 1999–2012, 77% of the cases of tick-borne encephalitis (TBE) were recorded in two out of 16 Polish provinces. However, historical data, mostly from national serosurveys, suggest that the disease could be undetected in many areas. The aim of this study was to identify which routinely-measured meteorological, environmental, and socio-economic factors are associated to TBE human risk across Poland, with a particular focus on areas reporting few cases, but where serosurveys suggest higher incidence. We fitted a zero-inflated Poisson model using data on TBE incidence recorded in 108 NUTS-5 administrative units in high-risk areas over the period 1999–2012. Subsequently we applied the best fitting model to all Polish municipalities. Keeping the remaining variables constant, the predicted rate increased with the increase of air temperature over the previous 10–20 days, precipitation over the previous 20–30 days, in forestation, forest edge density, forest road density, and unemployment. The predicted rate decreased with increasing distance from forests. The map of predicted rates was consistent with the established risk areas. It predicted, however, high rates in provinces considered TBE-free. We recommend raising awareness among physicians working in the predicted high-risk areas and considering routine use of household animal surveys for risk mapping.

Temporal dynamics of the tick Ixodes ricinus in northern Europe: epidemiological implications

Background

Tick-borne pathogens pose an increasing threat to human and veterinary health across the northern hemisphere. While the seasonal activity of ticks is largely determined by climatic conditions, host-population dynamics are also likely to affect tick abundance. Consequently, abundance fluctuations of rodents in northern Europe are expected to be translated into tick dynamics, and can hence potentially affect the circulation of tick-borne pathogens. We quantified and explained the temporal dynamics of the tick Ixodes ricinus in the northernmost part of its European geographical range, by estimating (i) abundance in vegetation and (ii) infestation load in the most common rodent species in the study area, the bank vole Myodes glareolus.

Results

Ixodes ricinus nymphs and adult females, the life stages responsible for the most of tick bites in humans, peaked in May-June and August-September. Larvae and nymphs were simultaneously active in June and abundance of questing larvae and nymphs in the vegetation showed a positive association with bank vole abundance. Moreover, infesting larvae and nymphs were aggregated on bank voles, and the infestation of bank voles with I. ricinus larvae and nymphs was positively associated with bank vole abundance.

Conclusion

Our results indicate early summer and early autumn as periods of increased risk for humans to encounter I. ricinus ticks in boreal urban forests and suggest a 2 years life-cycle for I. ricinus with two cohorts of ticks during the same year. Moreover, we identified a simultaneous activity of larvae and nymphs which allows co-feeding on the rodent host, which in turn supports the transmission of several important zoonotic tick-borne pathogens. Finally, we showed that a high density of the rodent host may enhance the risk that ticks and, potentially, tick-borne pathogens pose to human health.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-017-2112-x) contains supplementary material, which is available to authorized users.

Spatial Analysis of Severe Fever with Thrombocytopenia Syndrome Virus in China Using a Geographically Weighted Logistic Regression Model

Severe fever with thrombocytopenia syndrome (SFTS) is caused by severe fever with thrombocytopenia syndrome virus (SFTSV), which has had a serious impact on public health in parts of Asia. There is no specific antiviral drug or vaccine for SFTSV and, therefore, it is important to determine the factors that influence the occurrence of SFTSV infections. This study aimed to explore the spatial associations between SFTSV infections and several potential determinants, and to predict the high-risk areas in mainland China. The analysis was carried out at the level of provinces in mainland China. The potential explanatory variables that were investigated consisted of meteorological factors (average temperature, average monthly precipitation and average relative humidity), the average proportion of rural population and the average proportion of primary industries over three years (2010-2012). We constructed a geographically weighted logistic regression (GWLR) model in order to explore the associations between the selected variables and confirmed cases of SFTSV. The study showed that: (1) meteorological factors have a strong influence on the SFTSV cover; (2) a GWLR model is suitable for exploring SFTSV cover in mainland China; (3) our findings can be used for predicting high-risk areas and highlighting when meteorological factors pose a risk in order to aid in the implementation of public health strategies.

Forest classes and tree cover gradient: tick habitat in encroached areas of southern Norway

Forest, in particular deciduous forest, is a key element in determining areas with a high probability of tick presence. The way forest is generally monitored may be ill suited to some landscapes where Ixodes ricinus is found, as forest is usually characterised using crisp land cover classes. However, tree vegetation can be found outside of forests and continuous gradations of tree density can be found in a variety of landscapes. In this paper we investigate the probability of tick presence in southern Norway using landscape description based both on land cover classes and continuous data describing the tree cover fraction. Both perspectives on the landscape are significant in the logistic model, indicating that the usual approach based solely on land cover classes may not be comprehensive enough in capturing tick habitat, and characterising the landscape with variables focused on single specific elements may be insufficient.

Scouts, forests, and ticks: Impact of landscapes on human-tick contacts

Just as with forest workers or people practicing outdoor recreational activities, scouts are at high risk for tick bites and tick-borne infections. The risk of a tick bite is shaped not only by environmental and climatic factors but also by land management. The aim of this study was to assess which environmental conditions favour scout-tick contacts, and thus to better understand how these factors and their interactions influence the two components of risk: hazard (related to vector and host ecology) and exposure of humans to disease vectors. A survey was conducted in the summer of 2009 on the incidence of tick bites in scout camps taking place in southern Belgium. Joint effects of landscape composition and configuration, weather, climate, forest and wildlife management were examined using a multiple gamma regression with a log link. The landscape was characterized by buffers of varying sizes around the camps using a detailed land use map, and accounting for climate and weather variables. Landscape composition and configuration had a significant influence on scout-tick contacts: the risk was high when the camp was surrounded by a low proportion of arable land and situated in a complex and fragmented landscape. The distance to the nearest forest patch, the composition of the forest ecotone as well as weather and climatic factors were all significantly associated with scout-tick contacts. Both hazard- and exposure-related variables significantly contributed to the frequency of scout-tick contact. Our results show that environmental conditions favour scout-tick contacts. For example, we emphasize the impact of accessibility of environments suitable for ticks on the risk of contact. We also highlight the significant effect of both hazard and exposure. Our results are consistent with current knowledge, but further investigations on the effect of forest management, e.g. through its impact on forest structure, on the tick-host-pathogen system, and on humans exposure, is required.

Methodological caveats in the environmental modelling and projections of climate niche for ticks, with examples for Ixodes ricinus (Ixodidae)

Interest is increasing in inferring the climate niche of health-threatening arthropods and projecting such inferences onto a territory. This approach is intended to predict the range of tick distribution and to elucidate tick responses to climate scenarios, using so-called correlative models. However, some methodological gaps might prevent achieving an adequate background for hypothesis testing. We explore, describe, and illustrate these procedural inaccuracies with examples focused on the tick Ixodes ricinus and examine how these factors might affect modelling outcomes. Our aim was to develop a backdrop of rules for developing reliable models for these parasites. The use of partial sets of tick occurrences might produce unreliable associations with climate because the algorithms cannot capture the complete niche with which the tick is associated. Reliability measures of the model cannot detect these inaccuracies, and undesirable estimations of the niche will prevail in the chain of further calculations. The use of inadequate environmental variables (covariates) may lead to inflation of the results of the model through two statistical processes, autocorrelation and colinearity. We demonstrate the high colinearity existing in climate products derived from interpolation of climate recording stations. Our explicit advice is to focus on the training of climate models with satellite-derived information of climate, from which colinearity of the time series has been removed through a harmonic regression. We also emphasize the high uncertainty if inference about the climate niche is expanded into different time slices, like projected climate scenarios.

Shaping zoonosis risk: landscape ecology vs. landscape attractiveness for people, the case of tick-borne encephalitis in Sweden

BACKGROUND

In this paper, the hazard and exposure concepts from risk assessment are applied in an innovative approach to understand zoonotic disease risk. Hazard is here related to the landscape ecology determining where the hosts, vectors and pathogens are and, exposure is defined as the attractiveness and accessibility to hazardous areas. Tick-borne encephalitis in Sweden was used as a case study.

METHODS

Three boosted regression tree models are compared: a hazard model, an exposure model and a global model which combines the two approaches.

RESULTS

The global model offers the best predictive power and the most accurate modelling. The highest probabilities were found in easy-to-reach places with high landscape diversity, holiday houses, waterbodies and, well-connected forests of oak, birch or pine, with open-area in their ecotones, a complex shape, numerous clear-cuts and, a variation in tree height.

CONCLUSION

While conditions for access and use of hazardous areas are quite specific to Scandinavia, this study offers promising perspectives to improve our understanding of the distribution of zoonotic and vector-borne diseases in diverse contexts.

Monitoring human tick-borne disease risk and tick bite exposure in Europe: available tools and promising future methods

Ticks are the main vector for infectious disease pathogens in both humans and animals, and tick-borne diseases are currently spreading throughout Europe. Various surveillance methods have been developed to estimate the burden and risk of tick-borne diseases and host exposure to tick bites. The ultimate aims of these approaches are to determine the risk level of a tick-borne disease in a given area, determine its health priority, identify the at-risk population and propose specific countermeasures or complementary studies as needed. The purpose of this review is to present the current methods for monitoring the circulation of tick-borne diseases and to highlight the use of salivary antigens as original and recently developed serological tools that could be useful for tick bite risk assessment and could improve the current surveillance methods.

Epidemiologic features and environmental risk factors of severe fever with thrombocytopenia syndrome, Xinyang, China

Background

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease discovered in rural areas of Central China in 2009, caused by a novel bunyavirus, SFTS virus (SFTSV). The disease usually presents as fever, thrombocytopenia, and leukocytopenia, with case-fatality rates ranging from 2.5% to 30%. Haemaphysalis longicornis was suspected to be the most likely vector of SFTSV. By the end of 2012, the disease had expanded to 13 provinces of China. SFTS patients have been reported in Japan and South Korea, and a disease similar to SFTS has been reported in the United States.

Methodology/Principal Findings

We characterized the epidemiologic features of 504 confirmed SFTS cases in Xinyang Region, the most severely SFTS-afflicted region in China from 2011 to 2012, and assessed the environmental risk factors. All cases occurred during March to November, with the epidemic peaking from May to July. The patients' ages ranged from 7 to 87 years (median 61 years), and the annual incidence increased with age (χ² test for trend, P<0.001). The female-to-male ratio of cases was 1.58, and 97.0% of the cases were farmers who resided in the southern and western parts of the region. The Poisson regression analysis revealed that the spatial variations of SFTS incidence were significantly associated with the shrub, forest, and rain-fed cropland areas.

Conclusions

The distribution of SFTS showed highly significant temporal and spatial heterogeneity in Xinyang Region, with the majority of SFTS cases being elderly farmers who resided in the southern and western parts of the region, mostly acquiring infection between May and July when H. longicornis is highly active. The shrub, rain-fed, and rain-fed cropland areas were associated with high risk for this disease.

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease discovered in rural areas of Central China in 2009, caused by a novel bunyavirus, SFTS virus (SFTSV). The disease usually presents as fever, thrombocytopenia, and leukocytopenia, with case-fatality rates ranging from 2.5% to 30%. By the end of 2012, the disease had expanded to 13 provinces of China. SFTS patients have been reported in Japan and South Korea, and a disease similar to SFTS has been reported in the United States. Here we characterized the epidemiologic features of 504 confirmed SFTS cases in Xinyang, the most severely SFTS-affected region in China from 2011 to 2012, and identified the environmental risk factors. We found the distribution of SFTS cases showed highly significant temporal and spatial heterogeneity, with the majority of SFTS cases being elderly farmers who resided in the southern and western parts of the region, mostly acquiring infection between May and July when H. longicornis is highly active. The shrub, forest, and rain-fed cropland areas were strongly associated with high risk for SFTS.

Effects of environmental change on zoonotic disease risk: an ecological primer

Impacts of environmental changes on zoonotic disease risk are the subject of speculation, but lack a coherent framework for understanding environmental drivers of pathogen transmission from animal hosts to humans. We review how environmental factors affect the distributions of zoonotic agents and their transmission to humans, exploring the roles they play in zoonotic systems. We demonstrate the importance of capturing the distributional ecology of any species involved in pathogen transmission, defining the environmental conditions required, and the projection of that niche onto geography. We further review how environmental changes may alter the dispersal behaviour of populations of any component of zoonotic disease systems. Such changes can modify relative importance of different host species for pathogens, modifying contact rates with humans.

Estimating hantavirus risk in southern Argentina: a GIS-based approach combining human cases and host distribution

We use a Species Distribution Modeling (SDM) approach along with Geographic Information Systems (GIS) techniques to examine the potential distribution of hantavirus pulmonary syndrome (HPS) caused by Andes virus (ANDV) in southern Argentina and, more precisely, define and estimate the area with the highest infection probability for humans, through the combination with the distribution map for the competent rodent host (Oligoryzomys longicaudatus). Sites with confirmed cases of HPS in the period 1995–2009 were mostly concentrated in a narrow strip (~90 km × 900 km) along the Andes range from northern Neuquén to central Chubut province. This area is characterized by high mean annual precipitation (~1,000 mm on average), but dry summers (less than 100 mm), very low percentages of bare soil (~10% on average) and low temperatures in the coldest month (minimum average temperature −1.5 °C), as compared to the HPS-free areas, features that coincide with sub-Antarctic forests and shrublands (especially those dominated by the invasive plant Rosa rubiginosa), where rodent host abundances and ANDV prevalences are known to be the highest. Through the combination of predictive distribution maps of the reservoir host and disease cases, we found that the area with the highest probability for HPS to occur overlaps only 28% with the most suitable habitat for O. longicaudatus. With this approach, we made a step forward in the understanding of the risk factors that need to be considered in the forecasting and mapping of risk at the regional/national scale. We propose the implementation and use of thematic maps, such as the one built here, as a basic tool allowing public health authorities to focus surveillance efforts and normally scarce resources for prevention and control actions in vast areas like southern Argentina.

Climate and environmental change drives Ixodes ricinus geographical expansion at the northern range margin

Background

Global environmental change is causing spatial and temporal shifts in the distribution of species and the associated diseases of humans, domesticated animals and wildlife. In the on-going debate on the influence of climate change on vectors and vector-borne diseases, there is a lack of a comprehensive interdisciplinary multi-factorial approach utilizing high quality spatial and temporal data.

Methods

We explored biotic and abiotic factors associated with the latitudinal and altitudinal shifts in the distribution of Ixodes ricinus observed during the last three decades in Norway using antibodies against Anaplasma phagocytophilum in sheep as indicators for tick presence. Samples obtained from 2963 sheep from 90 farms in 3 ecologically different districts during 1978 – 2008 were analysed. We modelled the presence of antibodies against A. phagocytophilum to climatic-, environmental and demographic variables, and abundance of wild cervids and domestic animals, using mixed effect logistic regressions.

Results

Significant predictors were large diurnal fluctuations in ground surface temperature, spring precipitation, duration of snow cover, abundance of red deer and farm animals and bush encroachment/ecotones. The length of the growth season, mean temperature and the abundance of roe deer were not significant in the model.

Conclusions

Our results highlight the need to consider climatic variables year-round to disentangle important seasonal variation, climatic threshold changes, climate variability and to consider the broader environmental change, including abiotic and biotic factors. The results offer novel insight in how tick and tick-borne disease distribution might be modified by future climate and environmental change.

Landscape structure influences avian malaria ecology in the Western Cape, South Africa

A central theme in landscape ecology is that of understanding the consequences of landscape heterogeneity for ecological processes. The effects of landscape heterogeneity on parasite communities are poorly understood, although it has been shown that anthropogenic impacts may contribute to outbreaks of both parasites and pathogens. We tested for effects of landcover type, composition, configuration, and urbanisation on avian diversity and avian malaria prevalence in 26 communities of wetland-associated passerines in the Western Cape of South Africa. We predicted that avian malaria prevalence would be influenced by the pattern of farmland and urban areas in the surrounding landscapes and the sizes of the wetlands in which birds were sampled. We quantified landscape pattern using a six-class simplification of the National Landcover data set at 35 × 35 m resolution and five extents of between 1 and 20 km from each wetland. The bird community was sampled using point counts and we collected blood samples from birds at each site. We screened these for malaria using PCR and molecular techniques. Passerine species richness and infection prevalence varied significantly between different landcover types. Host richness and parasite prevalence were highest in viticultural and cropping sites respectively and lowest in urban sites. Wetlands located in indigenous vegetation had intermediate numbers of bird species and intermediate parasite prevalence. Landscape composition and habitat type surrounding wetlands emerged as useful correlates of infection prevalence. Anthropogenic landscape modification appears to have both direct and indirect effects on avian communities and their associated parasite assemblages, with attendant consequences for avian health.

Driving forces for changes in geographical distribution of Ixodes ricinus ticks in Europe

Many factors are involved in determining the latitudinal and altitudinal spread of the important tick vector Ixodes ricinus (Acari: Ixodidae) in Europe, as well as in changes in the distribution within its prior endemic zones. This paper builds on published literature and unpublished expert opinion from the VBORNET network with the aim of reviewing the evidence for these changes in Europe and discusses the many climatic, ecological, landscape and anthropogenic drivers. These can be divided into those directly related to climatic change, contributing to an expansion in the tick's geographic range at extremes of altitude in central Europe, and at extremes of latitude in Scandinavia; those related to changes in the distribution of tick hosts, particularly roe deer and other cervids; other ecological changes such as habitat connectivity and changes in land management; and finally, anthropogenically induced changes. These factors are strongly interlinked and often not well quantified. Although a change in climate plays an important role in certain geographic regions, for much of Europe it is non-climatic factors that are becoming increasingly important. How we manage habitats on a landscape scale, and the changes in the distribution and abundance of tick hosts are important considerations during our assessment and management of the public health risks associated with ticks and tick-borne disease issues in 21(st) century Europe. Better understanding and mapping of the spread of I. ricinus (and changes in its abundance) is, however, essential to assess the risk of the spread of infections transmitted by this vector species. Enhanced tick surveillance with harmonized approaches for comparison of data enabling the follow-up of trends at EU level will improve the messages on risk related to tick-borne diseases to policy makers, other stake holders and to the general public.

A national case-control study identifies human socio-economic status and activities as risk factors for tick-borne encephalitis in Poland

BACKGROUND

Tick-borne encephalitis (TBE) is endemic to Europe and medically highly significant. This study, focused on Poland, investigated individual risk factors for TBE symptomatic infection.

METHODS AND FINDINGS

In a nation-wide population-based case-control study, of the 351 TBE cases reported to local health departments in Poland in 2009, 178 were included in the analysis. For controls, of 2704 subjects (matched to cases by age, sex, district of residence) selected at random from the national population register, two were interviewed for each case and a total of 327 were suitable for the analysis. Questionnaires yielded information on potential exposure to ticks during the six weeks (maximum incubation period) preceding disease onset in each case. Independent associations between disease and socio-economic factors and occupational or recreational exposure were assessed by conditional logistic regression, stratified according to residence in known endemic and non-endemic areas. Adjusted population attributable fractions (PAF) were computed for significant variables. In endemic areas, highest TBE risk was associated with spending ≥10 hours/week in mixed forests and harvesting forest foods (adjusted odds ratio 19.19 [95% CI: 1.72-214.32]; PAF 0.127 [0.064-0.193]), being unemployed (11.51 [2.84-46.59]; 0.109 [0.046-0.174]), or employed as a forester (8.96 [1.58-50.77]; 0.053 [0.011-0.100]) or non-specialized worker (5.39 [2.21-13.16]; 0.202 [0.090-0.282]). Other activities (swimming, camping and travel to non-endemic regions) reduced risk. Outside TBE endemic areas, risk was greater for those who spent ≥10 hours/week on recreation in mixed forests (7.18 [1.90-27.08]; 0.191 [0.065-0.304]) and visited known TBE endemic areas (4.65 [0.59-36.50]; 0.058 [-0.007-0.144]), while travel to other non-endemic areas reduced risk.

CONCLUSIONS

These socio-economic factors and associated human activities identified as risk factors for symptomatic TBE in Poland are consistent with results from previous correlational studies across eastern Europe, and allow public health interventions to be targeted at particularly vulnerable sections of the population.

Consequences of landscape fragmentation on Lyme disease risk: a cellular automata approach

The abundance of infected Ixodid ticks is an important component of human risk of Lyme disease, and various empirical studies have shown that this is associated, at least in part, to landscape fragmentation. In this study, we aimed at exploring how varying woodland fragmentation patterns affect the risk of Lyme disease, through infected tick abundance. A cellular automata model was developed, incorporating a heterogeneous landscape with three interactive components: an age-structured tick population, a classical disease transmission function, and hosts. A set of simplifying assumptions were adopted with respect to the study objective and field data limitations. In the model, the landscape influences both tick survival and host movement. The validation of the model was performed with an empirical study. Scenarios of various landscape configurations (focusing on woodland fragmentation) were simulated and compared. Lyme disease risk indices (density and infection prevalence of nymphs) differed considerably between scenarios: (i) the risk could be higher in highly fragmented woodlands, which is supported by a number of recently published empirical studies, and (ii) grassland could reduce the risk in adjacent woodland, which suggests landscape fragmentation studies of zoonotic diseases should not focus on the patch-level woodland patterns only, but also on landscape-level adjacent land cover patterns. Further analysis of the simulation results indicated strong correlations between Lyme disease risk indices and the density, shape and aggregation level of woodland patches. These findings highlight the strong effect of the spatial patterns of local host population and movement on the spatial dynamics of Lyme disease risks, which can be shaped by woodland fragmentation. In conclusion, using a cellular automata approach is beneficial for modelling complex zoonotic transmission systems as it can be combined with either real world landscapes for exploring direct spatial effects or artificial representations for outlining possible empirical investigations.

Prevalence, distribution and risk associated with tick infestation of dogs in Great Britain

Current concerns over the potential impacts of climate change and the increased movement between countries of people and companion animals on the distribution of ectoparasites, highlight the need for accurate understanding of existing prevalence patterns. Without these future changes will not be detected. Here, the distribution and prevalence of tick infestations of domestic dogs in Great Britain were examined. A total of 173 veterinary practices were recruited to monitor tick attachment to dogs in their local areas between March and October 2009. Practices selected five dogs at random each week from those brought to the surgery and undertook a thorough, standardized examination for ticks. Each veterinary practice participated for 3 months before being replaced. Any ticks identified were collected and a sample sent to the investigators for identification, along with a clinical history of the dog. A total of 3534 dogs were examined; 810 dogs were found to be carrying at least one tick. Ixodes ricinus (Linnaeus) (Acari: Ixodidae) was identified in 72.1% of cases, Ixodes hexagonus Leach in 21.7% and Ixodes canisuga Johnston in 5.6% of cases. Five samples of Dermacentor reticulatus (Fabricius) (Acari: Ixodidae) were also found, adding to the growing evidence that an established population of D. reticulatus now exists in south-eastern England. Almost all the ticks found were adults. Overall, 19.2% of the veterinary practices reported no tick detections, 50% reported that ≥14.9% of the dogs seen were infested and 14.6% reported that >50% of the dogs inspected carried ticks. The estimated incidence of tick attachment was 0.013 per day in March (lowest) and 0.096 per day in June (highest). A number of risk factors affected the likelihood of tick attachment on dogs. Gundog, terrier and pastoral breed groups were more likely to carry ticks, as were non-neutered dogs. Dogs with shorter hair were less likely to have ticks, and dogs were most likely to carry a tick in June. This study is of value because, unusually, it presents the results of a randomized sample of dogs and gives a prevalence which is higher than those previously recorded in Great Britain.

Predictive mapping of human risk for West Nile virus (WNV) based on environmental and socioeconomic factors

A West Nile virus (WNV) human risk map was developed for Suffolk County, New York utilizing a case-control approach to explore the association between the risk of vector-borne WNV and habitat, landscape, virus activity, and socioeconomic variables derived from publically available datasets. Results of logistic regression modeling for the time period between 2000 and 2004 revealed that higher proportion of population with college education, increased habitat fragmentation, and proximity to WNV positive mosquito pools were strongly associated with WNV human risk. Similar to previous investigations from north-central US, this study identified middle class suburban neighborhoods as the areas with the highest WNV human risk. These results contrast with similar studies from the southern and western US, where the highest WNV risk was associated with low income areas. This discrepancy may be due to regional differences in vector ecology, urban environment, or human behavior. Geographic Information Systems (GIS) analytical tools were used to integrate the risk factors in the 2000–2004 logistic regression model generating WNV human risk map. In 2005–2010, 41 out of 46 (89%) of WNV human cases occurred either inside of (30 cases) or in close proximity (11 cases) to the WNV high risk areas predicted by the 2000–2004 model. The novel approach employed by this study may be implemented by other municipal, local, or state public health agencies to improve geographic risk estimates for vector-borne diseases based on a small number of acute human cases.

Economic downturn results in tick-borne disease upsurge

BACKGROUND

The emergence of zoonoses is due both to changes in human activities and to changes in their natural wildlife cycles. One of the most significant vector-borne zoonoses in Europe, tick-borne encephalitis (TBE), doubled in incidence in 1993, largely as a consequence of the socio-economic transition from communism to capitalism and associated environmental changes.

METHODS

To test the effect of the current economic recession, unemployment in 2009 and various socio-economic indices were compared to weather indices (derived from principal component analyses) as predictors for the change in TBE case numbers in 2009 relative to 2004-08, for 14 European countries.

RESULTS

Greatest increases in TBE incidence occurred in Latvia, Lithuania and Poland (91, 79 and 45%, respectively). The weather was rejected as an explanatory variable. Indicators of high background levels of poverty, e.g. percent of household expenditure on food, were significant predictors. The increase in unemployment in 2009 relative to 2008 together with 'in-work risk of poverty' is the only case in which a multivariate model has a second significant term.

CONCLUSION

Background socio-economic conditions determine susceptibility to risk of TBE, while increased unemployment triggered a sudden increase in risk. Mechanisms behind this result may include reduced resistance to infection through stress; reduced uptake of costly vaccination; and more exposure of people to infected ticks in their forest habitat as they make greater use of wild forest foods, especially in those countries, Lithuania and Poland, with major marketing opportunities in such products. Recognition of these risk factors could allow more effective protection through education and a vaccination programme targeted at the economically most vulnerable.

Forecasting risk of tick-borne encephalitis (TBE): using data from wildlife and climate to predict next year's number of human victims

BACKGROUND

Over the past quarter century, the incidence of tick-borne encephalitis (TBE) has increased in most European nations. However, the number of humans stricken by the disease varies from year to year. A method for predicting major increases and decreases is needed.

METHODS

We assembled a 25-y database (1984-2008) of the number of human TBE victims and wildlife and climate data for the Stockholm region of Sweden, and used it to create easy-to-use mathematical models that predict increases and decreases in the number of humans stricken by TBE.

RESULTS

Our best model, which uses December precipitation and mink (Neovison vison, formerly Mustela vison) bagging figures, successfully predicted every major increase or decrease in TBE during the past quarter century, with a minimum of false alarms. However, this model was not efficient in predicting small increases and decreases.

CONCLUSIONS

Predictions from our models can be used to determine when preventive and adaptive programmes should be implemented. For example, in years when the frequency of TBE in humans is predicted to be high, vector control could be intensified where infested ticks have a higher probability of encountering humans, such as at playgrounds, bathing lakes, barbecue areas and camping facilities. Because our models use only wildlife and climate data, they can be used even when the human population is vaccinated. Another advantage is that because our models employ data from previously-established databases, no additional funding for surveillance is required.

Spatially disaggregated disease transmission risk: land cover, land use and risk of dengue transmission on the island of Oahu

Vector-borne diseases persist in transmission systems that usually comprise heterogeneously distributed vectors and hosts leading to a highly heterogeneous case distribution. In this study, we build on principles of classical mathematical epidemiology to investigate spatial heterogeneity of disease risk for vector-borne diseases. Land cover delineates habitat suitability for vectors, and land use determines the spatial distribution of humans. We focus on the risk of exposure for dengue transmission on the Hawaiian island of Oahu, where the vector Aedes albopictus is well established and areas of dense human population exist. In Hawai'i, dengue virus is generally absent, but occasionally flares up when introduced. It is therefore relevant to investigate risk, but difficult to do based on disease incidence data. Based on publicly available data (land cover, land use, census data, surveillance mosquito trapping), we map the spatial distribution of vectors and human hosts and finally overlay them to produce a vector-to-host ratio map. The resulting high-resolution maps indicate a high spatial variability in vector-to-host ratio suggesting that risk of exposure is spatially heterogeneous and varies according to land cover and land use.

Pathogenic landscapes: interactions between land, people, disease vectors, and their animal hosts

BACKGROUND

Landscape attributes influence spatial variations in disease risk or incidence. We present a review of the key findings from eight case studies that we conducted in Europe and West Africa on the impact of land changes on emerging or re-emerging vector-borne diseases and/or zoonoses. The case studies concern West Nile virus transmission in Senegal, tick-borne encephalitis incidence in Latvia, sandfly abundance in the French Pyrenees, Rift Valley Fever in the Ferlo (Senegal), West Nile Fever and the risk of malaria re-emergence in the Camargue, and rodent-borne Puumala hantavirus and Lyme borreliosis in Belgium.

RESULTS

We identified general principles governing landscape epidemiology in these diverse disease systems and geographic regions. We formulated ten propositions that are related to landscape attributes, spatial patterns and habitat connectivity, pathways of pathogen transmission between vectors and hosts, scale issues, land use and ownership, and human behaviour associated with transmission cycles.

CONCLUSIONS

A static view of the "pathogenecity" of landscapes overlays maps of the spatial distribution of vectors and their habitats, animal hosts carrying specific pathogens and their habitat, and susceptible human hosts and their land use. A more dynamic view emphasizing the spatial and temporal interactions between these agents at multiple scales is more appropriate. We also highlight the complementarity of the modelling approaches used in our case studies. Integrated analyses at the landscape scale allows a better understanding of interactions between changes in ecosystems and climate, land use and human behaviour, and the ecology of vectors and animal hosts of infectious agents.