Parasites and wildlife in a changing world: The vector-host- pathogen interaction as a learning case

Rizzoli, Annapaola; Tagliapietra, Valentina; Cagnacci, Francesca; Marini, Giovanni; Arnoldi, Daniele; Rosso, Fausta; Rosà, Roberto

However, changes in climate and land use are not sufficient to explain changes in Ixodes ricinus temporal and spatial variation as well as in TBE hazard (Randolph, 2010; Medlock et al., 2013) . In fact, tick distribution, abundance and its infection rate with TBEV are also greatly affected by changes in other exogenous factors such as tick host abundance and distribution. Rodents for example are widespread in most ecological systems, have a small home range (max 2-3 ha) and are heavily parasitized by ticks. The fluctuations of rodent populations are well studied and often dramatic, although anthropogenic pressure can alter them substantially (Hansson and Henttonen, 1988; Rowe and Terry, 2014) . The large variation in rodents survival rates and population abundance affects in turn the tick feeding host availability (especially for the immature stage) and thus infection prevalence (Gern et al., 1998) . The relationship between tick demography and rodent density is complex. A negative effect of rodent density on the mean larval intensity per host has been previously observed (Kiffner et al., 2011; Perkins et al., 2003) , but in another study, an increase of the total number of feeding larvae on rodents recorded in a year was positively correlated with rodent density (Rosà et al., 2007) . Total larval density was higher in the years and trapping grids where rodent density was higher, while for nymphs no such effect was observed. The best explanatory variable for nymph density was the larval density in the previous year, confirming the discrete nature of tick demography (Rosà et al., 2007) . In contrast to rodents, other large-and meso-wild and domestic mammals play an important role as hosts sustaining tick populations, therefore indirectly affecting the spatial and temporal distribution of TBE foci and infection hazard Knap and Avšič-Županc, 2013; Jaenson et al., 2012 Jaenson et al., , 2018 . The European continent has been characterized by contrasting trends in ungulate distribution and abundance in the last decades. After a generalized decline mostly due to over-harvesting, after the second world war this negative trend has been reversed. Factors such as socio-economic changes due to traditional agriculture abandonment and climate change drove forest re-expansion (e.g. Lindner et al., 2014 ; see also https://www.eea.europa.eu/data-and-maps/indicators/forest-growth-2/assessment, accessed May 27, 2019). More recently, regulations for habitat conservation and wildlife harvest and management (Kerstin et al., 2008) , laid the seeds for a constant and solid re-growth of ungulate populations, especially deer, in terms of abundance and distribution (Apollonio et al., 2017) . Deer density is known to affect local tick abundance, hence it plays an important role on tick-borne diseases hazard. A number of studies focused on the evaluation of the effect of deer density on tick populations but the finding can change accordingly to the scale with an observed increase (Gilbert et al., 2012) or a decrease (Perkins et al., 2006) . For TBEV, an empirical study performed in several sites in Europe using a case-control approach , and substantiated by a deterministic framework , showed that TBEV occurrence starts to decrease at a threshold value of deer density, consequently decreasing the tick burden on rodents. However, an important aspect underlined by those suites of studies was the spatial scale at which hosts range at different densities, which in turn