Why account for biodiversity for mitigating tick-borne disease risk? Insights and perspectives from eastern Italian Alps

The biodiversity crisis is one of the most critical environmental issues of our time. Species losses affect ecosystem functioning by depriving those services - such as carbon sequestration, nutrient cycling and resistance to drought - on which also humans rely. Among these, control of infectious diseases is a highly valuable service provided by biodiversity with implications for human health. Habitat degradation and the resulting biodiversity loss, alter the ratio among generalist species and specialist, thus increasing the risk of zoonotic emergence since many generalist species are competent reservoirs. Currently, scientific studies considered either the role of anthropogenic changes (e.g., climate and land-use) on disease emergence or how biodiversity affects dynamics of endemic diseases. Indeed, a multitude of studies performed in the Eastern Italian Alps (Autonomous Province of Trento, Italy) unveiled what are the environmental and climatic factors driving risk of tick-borne zoonoses emergence. In particular, findings from a systematic review evidenced that temperature-dependent variables were negatively associated to the variation of Tick borne encephalitis (TBE) across Europe. Further, habitat diversity seemed to also dilute TBE virus (TBEv) incidence. Conversely, covariates related to the presence of competent hosts (i.e., rodents) and forest cover had a positive role in TBEv circulation, while incompetent hosts (i.e., deer) showed a hump-shaped effect. These patterns were confirmed also through empirical studies. Specifically, a study found that TBEv detection was related with number of co-feeding ticks carried by rodents, which in turn is negatively correlated with autumnal cooling rate and increased with tick burden on rodents, deer and rodent abundance. Therefore, a specific combination of climatic conditions and certain rodent and deer densities resulted the principal determinants of the TBEv incidence. Similarly, another study highlighted the strong relation between TBEv cases and air pollen abundance, a proxy for seed production. This suggested the major role of rodents as amplifying hosts, being favored by the availability of seeds and thus by pollen quantities. Similar insights were observed in experimental research evaluating the role of artificial ungulate feeding sites in disease spreading. Here, food availability promoted the density of mice, while favoring revisitation patterns of roe deer and interspecific competition among rodents’ species. Co-occurrence of roe deer and mice at these sites affected infectious pathogen transmission, with lower prevalence of tick-borne pathogens (Borrelia burgdorferi s.l., Babesia microti, Anaplasma phagocytophylum and Hepatozoon spp.), while the opposite was found for rodent-borne ones (hantaviruses). Beyond hosts, also land-use plays a crucial role in driving disease circulation. In this sense, a study investigating the effects of land-use type on acarological hazard for Ixodes ricinus and on its infection rate found a positive effect of climate on relative abundance of questing nymphs, while no differences were observed among land-use categories. However, when considering the density of infected nymphs (DIN), the effect of land-use emerged with the natural habitats having the highest DIN values for B. burgdorferi s.l., the urban habitats the highest ones for A. phagocytophilum, while for Rickettsia spp., no differences were observed. Overall, this provides evidence for complex ecological factors to consider for predicting microbial hazard. However, so far, few studies have tried to reconcile the relationship between biodiversity, anthropogenic change, circulation of endemic pathogens and risk of disease emergence. By considering the cascading eco-epidemiological processes occurring in ecosystems with different degrees of naturalness we aim to identify the anthropogenic impacts on biodiversity, and the role of the latter in driving infectious disease circulation and emergence.