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.

Flaviviruses West Nile (WNV), Usutu (USUV) and Bagaza (BAGV) virus and avian malaria parasites are vector borne pathogens that circulate naturally between avian and mosquito hosts. WNV and USUV and potentially also BAGV constitute zoonoses. Temporal and spatial co-circulation and co-infection with Plasmodium spp., and West Nile virus has been documented in birds and mosquito vectors, and fatally USUV infected passerines coinfected with Plasmodium spp. had more severe lesions. Also, WNV, USUV and BAGV have been found to co-circulate. Yet little is known about the interaction of BAGV and malaria parasites during consecutive or co-infections of avian hosts. Here we report mortality of free-living red-legged partridges in a hunting estate in Southern Spain due to coinfection with BAGV and Plasmodium spp. The outbreak occurred in the area where BAGV first emerged in Europe in 2010 and where co-circulation of BAGV, USUV and WNV was confirmed in 2011 and 2013. Partridges were found dead in early October 2019. Birds had mottled locally pale pectoral muscles, enlarged, congestive greenish-black tinged livers and enlarged kidneys. Microscopically congestion and predominantly mononuclear inflammatory infiltrates were evident and Plasmodium phanerozoites were present in the liver, spleen, kidneys, muscle and skin. Molecular testing and sequencing detected Plasmodium spp. and BAGV in different tissues of the partridges, and immunohistochemistry confirmed the presence and co-localization of both pathogens in the liver and spleen. Although Plasmodium spp. are known to be highly prevalent in red-legged partridges, this is the first account of mortality caused by co-infection with BAGV and Plasmodium sp. Due to the importance of the red-legged partridge in the ecosystem of the Iberian Peninsula and as driver of regional economy such mortalities are of concern. Also, they may reflect climate change related changes in host, vector and pathogen ecology and interactions that could emerge similarly in other pathogens.