Exposure of red deer (Cervus elaphus Linnaeus, 1758) to the endosymbiont Midichloria mitochondrii uncovers tick presence in underexplored Alpine areas

The hard tick Ixodes ricinus , a vector of pathogens important in human and veterinary medicine that harbors the bacterial endosymbiont Midichloria mitochondrii , is expanding across Europe, including Alpine regions. Hosts such as red deer ( Cervus elaphus ), whose populations have increased markedly in the Alps, contribute to this spread. Wildlife surveillance provides opportunities to assess vector distribution and the role of hosts in tick expansion. It has been widely described that vertebrates parasitized by I. ricinus develop antibodies against M. mitochondrii antigens, offering a tool to infer exposure to tick bites. We therefore investigated indirect exposure to I. ricinus in red deer of the Stelvio National Park (central Italian Alps) for which no data on tick presence are available. During two culling seasons (2017–2019), sera were collected from 201 red deer, alongside sex, age, weight, Kidney Fat Index, and geographic coordinates of the culling sites. Antibodies against the M. mitochondrii flagellar protein FliD were detected by ELISA, and variation in optical density was modeled using deer age class and spatial trend surfaces of coordinates. Analyses indicated a significantly lower probability of exposure in calves than in older animals, together with differences between the southernmost and northernmost areas of the park, indicating spatial heterogeneity in tick distribution. The use of the endosymbiont as a marker of tick bites was intended as a broader-scale tool to identify potential areas at risk for tick presence in contexts where information on these parasites is lacking; this could guide further surveys using conventional methods for the detection of ticks and tick-borne pathogens, and support more targeted surveillance strategies.