The Impact of Climate Trends on a Tick Affecting Public Health: A Retrospective Modeling Approach for Hyalomma marginatum (Ixodidae)

Abstract

The impact of climate trends during the period 1901-2009 on the life cycle of Hyalomma marginatum in Europe was modeled to assess changes in the physiological processes of this threat to public health. Monthly records of temperature and water vapour at a resolution of 0.5° and equations describing the life cycle processes of the tick were used. The climate in the target region affected the rates of the life cycle processes of H. marginatum: development rates increased, mortality rates in molting stages decreased, and the survival rates of questing ticks decreased in wide territories of the Mediterranean basin. The modeling framework indicated the existence of critical areas in the Balkans, central Europe, and the western coast of France, where the physiological processes of the tick improved to extents that are consistent with the persistence of populations if introduced. A spatially explicit risk assessment was performed to detect candidate areas where active surveys should be performed to monitor changes in tick density or persistence after a hypothetical introduction. We detected areas where the critical abiotic (climate) and biotic (host density) factors overlap, including most of the Iberian peninsula, the Mediterranean coast of France, eastern Turkey, and portions of the western Black Sea region. Wild ungulate densities are unavailable for large regions of the territory, a factor that might affect the outcome of the study. The risk of successfully establishing H. marginatum populations at northern latitudes of its current colonization range seems to be still low, even if the climate has improved the performance of the tick in these areas.

Fig 1. Definitions of the territory covered in this study.

(A) The target territory of this study overlaid with the spatial distribution of the points for which weather during the period 1901–2009 was used as an input for further modelling analyses. Historically, H. marginatum has been collected in the coloured territory. (B) The division of the territory into climate domains as obtained from the LANMAP framework. The study is restricted to Europe. Therefore the recorded distribution of the tick in northern Africa has been disregarded.

Fig 2. The annual change in developmental rates (DR), mortality rates of development (MRD), and survival rates of questing stages (SRQ) of H. marginatum in the period 1901–2009.

Data were calculated and disaggregated for the areas where the tick is known to have permanent populations (positive zone: PZ) or where the tick has not yet been collected (negative zone: NZ).

Fig 3. The annual change in developmental rates (DR), mortality rates of development (MRD), and survival rates of questing stages (SRQ) of H. marginatum in the period 1901–2009.

Data were disaggregated for the climate domains outlined in Fig 1B.

Fig 4. Classification of the target territory into categories of suitability for H. marginatum.

The five time slices are shown in A-E classified according to the criteria in Table 1. A spatial summary of the rates of change for the physiological processes of H. marginatum during the complete period is included in F, in which blue indicates areas that switched from categories 4 and 5 to category 3; yellow indicates a switch from category 3 to 2; and red indicates a switch from category 2 to 1. The blue areas are territories where introduction of the tick without further development of permanent populations could be sporadically detected. The yellow areas are territories where conditions have changed enough to allow a positive turnover of the tick, if introduced. The red areas must be interpreted as sites where the tick is already established and conditions have improved, allowing the greatest population turnover.

Fig 5. Evaluation of the target territory based on the trend of the change in the tick’s physiological processes over the period of study (exposure) and the density of domestic ruminants as hosts for adults (vulnerability).

The map is intended to indicate the candidate areas where active surveys should be performed to check for changes in the tick’s population status.