Spatial environmental determinants of the risk of tick-borne diseases in Walloon pastures

Rousseau, Raphaël [UCL] Dion, Elise [SPW département de l'environnement et de l'eau] Vanwambeke, Sophie [UCL] et al.

Ticks present 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 life cycles 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. Tick borne diseases are infectious diseases affecting humans but also domestic animals. Bovines can be infected by tick-borne pathogens, such as Anaplasma phagocytophilum. This bacterium causes bovine anaplasmosis, a disease associated with influenza-like symptoms and decreases the production of milk. Knowing the environmental characteristics of a pasture that favors the presence of this disease is thus important for the agricultural sector. In 2010 and 2011, the ARSIA (Association Régionale de Santé et d’Identification Animale) tested one cow in 2,088 different Walloon herds for the presence of IgG antibodies to A. phagocytophilum. A positive test indicates that the cow has been in contact with the pathogen. However, a negative test does not necessarily indicate an absence of the pathogen in the entire herd. It was then impossible to consider this information as “presence/absence” data. Instead, we used the presence of a positive cow has an indication that the pasture occupied by the herd is located in a suitable environment for the tick-borne disease system. The different pastures are geolocated so it allows the creation of a spatial gradient of risk for each pasture in Wallonia, based on the pastures where known infected cattle graze. This gradient was used as the dependent variable in the assessment of the effects of environmental variables on the risk of bovine anaplasmosis in pastures. The results of this study were then compared with those from a previous study assessing the influence of landscape-level environmental variables on human-ticks contacts during scout summer camps in Wallonia (de Keukeleire et al., 2015). These two studies used different datasets but were analyzed according to the same set of independent variables extracted from the same source meaning that the results were directly comparable, unlike most of what is found in the literature because of the use of heterogeneous data sources on the landscape, with different semantic contents on land cover, different resolution, and diverse landscape indicators. The comparability between studies undergone in the same area can improve the assessment of the risk of tick-borne diseases.