Epidemiological and environmental risk factors of tick-borne diseases : combining hazard and exposure

Tick-borne diseases are infectious diseases transmitted by the bite of infected ticks. Usually circulating in animal populations but occasionally transmitted to humans, they are a complex system involving a pathogen, a vector, natural or accidental hosts and the environment. In order to understand disease dynamics and mitigate the risk, there is a crucial need for a better understanding the factors determining their spatial distribution. Focusing on Belgium, the main objectives of this thesis were: 1) to assess the risk of tick-borne diseases, and 2) to identify the epidemiological and environmental risk factors affecting the risk of tick-borne diseases. Assessing the risk of tick-borne diseases can be done by focusing on the vector, animal hosts or humans. Focusing of humans allows us to also highlight variables associated to the exposure part of the risk. Indeed, risk assessment is a combination of hazard (related to vector and host ecology) and exposure (possibility of contact between humans and infected vectors). This dissertation allows expanding the knowledge of Lyme disease in Belgium. For the first time, a comparison of the risk of infection with tick-borne diseases between populations with different risk profiles is achieved. A robust methodology is developed to identify environmental and individual risk factors by integrating both aspect linked to the hazard and the exposure part of the risk. This innovative methodology is applied to study the risk of tick bites in a population of Scouts and the seroprevalence of Borrelia burdgdorferi in two groups professionally exposed: Belgian veterinarians and farmers and forest rangers. This dissertation demonstrates that the exposure of people has a great impact on the risk. Therefore, prevention of tick bites both for population at risk through their professional or recreational activities is very important to reduce tick-borne diseases in Belgium.(SC - Sciences) -- UCL, 201

Potential of GIS in public health issues: environment and tick-borne diseases

Tick-borne diseases (TBD) are a public health issue worldwide. Understanding the spatial variation in TBD risk is essential for disease management and prevention. For TBD to circulate in a geographic area, five components of the system must overlap spatially and interact: the pathogen, a natural reservoir, a natural or accidental host, a vector, and a suitable biotope for vector development. Diverse factors determine the spatial distribution of those components: while tick distribution and activity are clearly influenced by ecological factors such as temperature and humidity, the exposure of humans may be more influenced by socio-cultural factors. As the risk of tick bite is notably shaped by environmental factors, the use of Geographical Information Systems (GIS) in the study of TBD is recommended. We use a diverse set of data sources and analysis methods to investigate how environmental factors, including those impacting human exposure, affect the spatial distribution of the risk of TBD. The incidence of tick bites is a major aspect of the risk of tick borne diseases, which finds itself at the crux of environments favorable to ticks and to humans. Understanding the spatial determinants of the risk of human-tick contacts is of public health interest because Ixodes ricinus, widely distributed in Belgium, is capable of transmitting several pathogens to humans and livestock, including Borrelia burgdorferi s.l., the causative agent of Lyme borreliosis (LB). Using data from a survey of the Firstly, a survey was conducted in the summer of 2009 on the incidence of tick bites in scout camps taking place in 2009 in southern Belgium, we show that landscape composition and configuration, in relation to camp sites, impact the incidence of tick bites. While the significant variables were in line with the current literature, effects related to human exposure, such as access to the forests, were highlighted. Joint effects of landscape composition and configuration, weather, climate, forest and wildlife management were examined using multiple gamma regression with a log link. The landscape was characterized in various sizes of buffers around camps using a detailed land use map, climate and weather variables. The seroprevalence of infection with B. burgdorferi in a group professionally at risk, veterinarians and farmers allows assessing the effect of environmental factors such as landscape composition on the incidence of infection at a broader scale. Like scouts, forest workers, hunters, or people practicing outdoor recreational activities, farmers and veterinarians can be identified as groups of land users at high risk of infection by tick bites. The aim of this study was to assess which environmental conditions can favour the presence of B. burgdorferi infection. Serological results for the 148 veterinarians and famers who participated to the study were analysed taking into account the municipality where they live. Using binary logistic, joint effects of landscape composition and configuration, forest and wildlife management were examined. Same datasets as the first study was used. While the ecology of ticks and the landscape favouring their abundance have been extensively studied, the environmental conditions favouring an intense contact with susceptible humans, including groups at risk, are sparse. The focus so far in research has often been on the ecological aspects of tick and pathogen distribution, or, at the other extreme on purely epidemiological aspects such as individual risk factors. However, understanding the distribution of cases and of risk requires looking into human-environment interactions. A geographic, spatially explicit perspective on these issues, assessing not only land cover but also land use and management is therefore highly relevant

Assessing the risk of vector-borne diseases as a combination of hazard and exposure in Wallonia

Ticks are a major threat for both human and animal health because they transmit diseases that affect these populations. Risk of vector-borne diseases results from the combination of hazard and exposure. Hazard represents the strength of the zoonotic transmission cycle, and is determined by the ecological conditions that influence the lifecycles of the pathogens, the vectors and the hosts. Exposure represents the intensity of contacts that susceptible human or animal populations have with places where infected ticks are present, in relation to their activities and preferences. It is largely determined by land use, for example the accessibility and attractiveness of places where infected hosts/vectors are found. Landscape has thus an influence on both hazard and exposure. Distinguishing the effects of some landscape variables on hazard and on exposure can be challenging, especially as many variable implementation use proxies of tick habitat suitability or of landscape attractiveness. This challenge is further compounded by the data used to understand vector-borne disease data. Only data on infectious vector abundance can represent hazard, but due to the difficulty of collecting such data in a large number of places, other data sets are often used. Epidemiological data, in the form of human or domestic animal disease cases, is found attractive in this context, as they tend to correspond to systematic, spatially exhaustive reporting. However, such data does not allow a straightforward distinction between hazard and exposure as a disease case is the result of both. Still, a number of landscape factors are not exclusively attributable to one or the other components of risk, and interpretations found in the literature are often considering primarily hazard. Comparisons of empirical studies across the literature is further complicated by the use of heterogeneous data sources on the landscape, with different semantic contents on land cover, different resolution, and diverse landscape indicators. In this study, we use a single set of environmental data to analyze different indicators of risk. Two datasets were analyzed according to one set of independent variables extracted from the same source, meaning that results are directly comparable, unlike most of what is found in the literature. The first data source assesses Scouts-ticks contacts during summer camps in Wallonia. Because of their numerous outdoor recreational activities, children participating in scouting activities are particularly exposed to a risk of contact with ticks and tick-borne diseases. These contacts are affected by both hazard, as rural and forested environments are highly suitable for ticks, and exposure, depending on whether these are attractive and accessible to the scouts. A survey was carried out in 2009 in 27 scout camps, during which volunteering groups inspected themselves for tick bites and recorded them in a customized notebook. The incidence of bites (per 1000 person-day) was computed for each camp. We assessed the effect of landscape-level environmental variables related to hazard and exposure on tick-bite incidence. The second data set concerns infection with Anaplasma phagocytophilum, for Walloon cattle in 2010 and 2011. A. phagocytophilum is a tick-borne bacterium that causes bovine ehrlichiosis in cattle, which is associated with influenza-like symptoms and decreases the production of milk. 2089 herds were tested for the presence of IgG antibodies to Anaplasma phagocytophilum. The pastures used by the farm are geolocated. It was then possible to analyze the environments where cattle graze using the same set of independent variables as used for the Scout-tick contacts dataset. While the environments most favourable to ticks are fairly well known, the heterogeneity of data sources used in the literature, both for the dependent and independent variables, makes comparison across study cases and regions challenging. This is especially true for habitats of intermediate quality, for which results are less coherent. By using various indicators and a single set of independent variable, we identified habitats favourable to ticks and to tick-borne pathogens circulation, and landscape variables associated to exposure to infected ticks. Both hazard and exposure are important factors in understanding and predicting the risk of contacts between ticks and susceptible populations of humans or animals. This can also help to manage risks for humans and cattle more efficiently

Seroprevalence of Borrelia burgdorferi in Belgian forestry workers and associated risk factors.

BACKGROUND: As forest is the preferred environment for ticks, forestry workers are exposed to tick bites and tick-borne diseases. We assessed the seroprevalence of anti-Borrelia burgdorferi (Bb) antibodies and investigated, using an integrated landscape approach, the individual and environmental factors associated with the seroprevalence of Bb in Belgian forestry workers, a high-risk group in Belgium. METHODS: A group of 310 Belgian forest workers was examined to assess the seroprevalence of anti-Borrelia IgG antibodies. Using principal component analysis and binary logistic regression, the joint effects of individual characteristics and environmental characteristics were examined. RESULTS: Sixty-seven of the 310 workers were seropositive for Lyme disease (LD), leading to a seroprevalence of 21.6%. The seroprevalence was higher among forest workers visiting forests more frequently (P = 0.003) or who reported over 100 tick bites (P-value < 0.001). The intensity of tick bites and the use of protection measures against tick bites have a positive impact on LD seroprevalence while the quantity of shadow from trees at ground level had a negative one. CONCLUSIONS: This study showed that forest workers are a population at risk for LD and, by extension, at risk for various tick-borne diseases. In addition to the role of the environment, our results also showed the importance of considering exposure when predicting the risk of infection by Bb