Simulation Tool for Mapping Acarological Risk in Agricultural Landscapes – OSCAR

Field work is conducted in 24 sites located in 4 different landscape types in each of the two agricultural zones («Zone Atelier Armorique« and «Vallons et Coteaux de Gascogne«). To estimate density and infection prevalence, questing ticks are sampled by flagging, where a white cloth is dragged across the vegetation in a standardised fashion. Small mammal assemblages and relative abundances will be investigated using a live line trap technique with baited traps. Roe deer local abundance will be assessed in each LTER site using line transects. In the Gascogne LTER site, individual roe deer are marked with GPS or VHF collars allowing us to estimate their movements in the landscape. Roe deer density will be estimated by capture mark recapture. The three pathogenic agents (Anaplasma spp, Borrelia spp et Babesia spp) are detected using molecular biology tools based on DNA extractions from ticks or from blood samples collected from the trapped mammals.

Tick sampling of the spring and automn 2012 in the 2 study sites revealed variations between 1.6 to 30.1 ticks/100m2. A drastic decline in the number of trapped small mammals in one the two sites (ZAA) was observed in fall 2012 and in spring 2013 (e.g., 286 animals caught in spring 2012 versus 22 in spring 2013). This sharp decline in host numbers for questing ticks may result in a reduction of nymphal densities in fall 2013 and in spring 2014. The three studied pathogens show prevalence levels varying from 3,2 à 5,5%.

A database allowing one to enter (1) data collected in the field studies (2) results from molecular analysis in the lab and (3) Geographical Information System (GIS) of the two sites has been built. The effort will now focus on the analysis of these data to investigate the role of landscape in the epidemiology of tick-borne-diseases. This information will then be incorporated into a general model that will enable one to simulate exposure risk to tick-borne pathogens in different agro-landscapes.

Several articles have dealt with the OSCAR project in 2013 (Le Monde, Libération, Actu-Environnement, Ouest France, Le Télégramme de Brest). Two articles (development of tick molecular markers & review of tick host specialisation and its consequences) are accepted for publication.

In agricultural landscapes, where livestock production occupies a large proportion of the surface area, pastures often adjoin different natural ecosystems (forests, hedges...). This type of landscape mosaic means that areas exploited by livestock are also frequently used by a diverse range of wild fauna. Many parasites and pathogenic agents are shared among these animal species and, even in the absence of direct contact, pathogenic agents may be transmitted between agricultural and natural systems via common arthropod vectors. In Europe, ticks are major vectors for both human (e.g., Lyme disease) and livestock disease (granulocytic anaplasmosis, bovine babesiosis...), with Ixodes ricinus being the most common species. In agricultural lands, the localisation and utilisation of a given parcel will determine its use by wild fauna and thus shape local host-tick interactions. Breeding practices associated with livestock circulation among pastures within a given exploitation will also influence exposure risk to ticks and to the pathogenic agents that they transmit. Agricultural landscapes therefore constitute a mosaic of habitat types that are more or less favourable to tick survival and more or less frequented by different host types. This habitat heterogeneity and its use will condition the dynamics and functioning of these host-vector-pathogen systems and thus, the acarological risk posed to humans and their livestock. However, these mosaics are not temporally fixed and we are currently witnessing modifications due to the influence of global changes, and particularly those associated with alterations in land-use (i.e., relative proportions of breeding/crop surfaces, forest or hedge fragmentation, etc). Climate change will also have an impact on this system, notably through its affect on tick populations, whose dynamics are tightly linked to temperature and humidity regimes.
The goal of the OSCAR project is to explore the consequences of these landscape-level changes on acarological risk (density of infected ticks) using a mapping simulation tool based on a spatially-explicit model of tick population dynamics. This study will be carried out in two agricultural zones that are part of the international long-term ecological research Network (ILTER) and encompass the intrinsic diversity of agricultural landscape structures. Landscape heterogeneity in these zones has been previously mapped using Geographic Information System (GIS) tools, and their local vertebrate communities have been well-characterised. From this solid basis, the OSCAR project consists of three main tasks. The initial task consists of mapping the distribution of ticks, pathogens and the principal domestic (cattle) and wild (deer, small mammals) hosts, based on field sampling and molecular analyses of ticks. This task will be completed with the construction of a georeferenced database. We will then carry out statistical analyses on these data to identify the principal biotic and abiotic factors that explain these distributions. The movement of these organisms will be explicitly considered using both direct (e.g., GPS collars on deer) and indirect methods (population genetic structure of ticks and pathogens). Finally, a spatially-explicit model of tick population dynamics that integrates host use and movements will be developed. With the addition of pathogen prevalence data, this model will permit us to simulate the evolution of acarological risk at the scale of the agricultural landscape in relation to different scenarios of change in land use and landscape structure. Although the behaviour of the socio-economic actors present in agricultural landscapes is not explicitly taken into account, our simulation tool for mapping acarological risk will provide essential information for adapting agricultural practices when confronted with environmental change such that the impact of tick and tick-borne disease can be minimised.