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Crimean-Congo haemorrhagic fever (CCHF) is an emerging tick-borne human disease in Spain. Understanding the spatiotemporal dynamics and exposure risk determinants of CCHF virus (CCHFV) in animal models is essential to predict the time and areas of highest transmission risk. With this goal, we designed a longitudinal survey of two wild ungulate species, the red deer (Cervus elaphus) and the Eurasian wild boar (Sus scrofa), in Doñana National Park, a protected Mediterranean biodiversity hotspot with high ungulate and CCHFV vector abundance, and which is also one of the main stopover sites for migratory birds between Africa and western Europe. Both ungulates are hosts to the principal CCHFV vector in Spain, Hyalomma lusitanicum. We sampled wild ungulates annually from 2005 to 2020 and analysed the frequency of exposure to CCHFV by a double-antigen ELISA. The annual exposure risk was modelled as a function of environmental traits in an approach to understanding exposure risk determinants that allow us to predict the most likely places and years for CCHFV transmission.The main findings show that H. lusitanicum abundance is a fundamental driver of the fine-scale spatial CCHFV transmission risk, while inter-annual risk variation is conditioned by virus/vector hosts, host community structure and weather variations. The most relevant conclusion of the study is that the emergence of CCHF in Spain might be associated with recent wild ungulate population changes promoting higher vector abundance. This work provides relevant insights into the transmission dynamics of CCHFV in enzootic scenarios that would allow deepening the understanding of the ecology of CCHFV and its major determinants.

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Background Available methods to census exophilic tick populations have limitations in estimating true population size due to their inability to capture a high proportion of the actual tick population. We currently ignore the efficacy of these methods to capture questing Hyalomma spp. ticks, vectors of the Crimean-Congo hemorrhagic fever virus. To address the need to accurately estimate questing densities of Hyalomma spp. we designed a field experiment to test the efficacy of blanket dragging, blanket flagging, CO2-baited traps, and and ad hoc designed method, absolute surface counts, in capturing adult Hyalomma lusitanicum ticks. Methods The experiment was designed in two stages to estimate the point (one-day sampling) and cumulative (three-day serial sampling) efficacy of the methods under varying sampling effort and habitat. Tick survival, host interference, and weather effects on efficacy were controlled for in multiple regression models. Results There was high variability in method efficacy for capturing ticks, which was also modulated by effort and habitat. The most effective method was absolute surface counts for both point estimates (39%) and cumulative efficacy (83%). CO2-baited traps reached a maximum efficacy of 37%, while blanket dragging and blanket flagging captured around 8% of the marked ticks. Conclusions Our results reveal the strengths and weaknesses of the different tick capture methods applied to study the demography of H. lusitanicum, and lay the groundwork for more accurate inferences about the true size of exophilic tick populations.

Crimean-Congo haemorrhagic fever (CCHF) is an emerging tick-borne human disease in Spain. Understanding the spatiotemporal dynamics and exposure risk determinants of CCHF virus (CCHFV) in animal models is essential to predict the time and areas of highest transmission risk. With this goal, we designed a longitudinal survey in two wild ungulate species, the red deer ( Cervus elaphus) and the Eurasian wild boar ( Sus scrofa), in Doñana National Park, a protected Mediterranean biodiversity hotspot with high ungulate and CCHFV vector abundance, and which is also one of the main stopover sites for migratory birds between Africa and western Europe. Both ungulates are hosts to the main CCHFV vector in Spain, Hyalomma lusitanicum. We sampled wild ungulates annually from 2005 to 2020 and analysed the frequency of exposure to CCHFV by a double-antigen ELISA. The annual exposure risk was modelled as a function of environmental traits in an approach to understand exposure risk determinants that allow us to predict the most likely places and years for CCHFV transmission. The main findings show that H. lusitanicum abundance is a major driver of the fine-scale spatial CCHFV transmission risk, while inter-annual variations in the risk are conditioned by virus/vector hosts, by host community structure and by weather variations. The most relevant conclusion of the study is that the emergence of CCHF in Spain might have been associated with recent wild ungulate population changes promoting higher vector abundance. Decreasing wild ungulate population densities could reduce vector abundance and thus virus prevalence and the risk of CCHFV transmission to humans.