Dermacentor ticks and their human pathogens in various ecosystems of eastern France

Dermacentor is Europe's second most important tick genus. It comprises two species: Dermacentor reticulatus, which has a more northerly range, and Dermacentor marginatus, which is typically found in Mediterranean regions. In France, especially in the east, the presence of Dermacentor remains understudied. Leveraging projects on Ixodes ricinus, we identified high-risk ecosystems for Dermacentor ticks. Both species were present, but D. reticulatus was dominant (96.3 %), always coexisting with Ixodes, at densities reaching 30.3 questing adults/100 m² in Bas-Rhin. This species was found to thrive in wet meadows, alluvial forests, and lakes, where wild boars (Sus scrofa) are common. Among domestic animals, dogs and horses were the preferred hosts. Regarding human pathogens, we focused on the circulation of rickettsiae responsible for tick-borne lymphadenopathy. Acarological risk varied by region, with Rickettsia raoultii being the most prevalent bacterial species in all the departments, whereas Rickettsia slovaca was rarer and present only in 2 Bas-Rhin nature reserves. At the peak of tick activity, up to 18.7 % of Dermacentor adult ticks carried R. raoultii. In the studied region, D. reticulatus is the only known vector of these potentially pathogenic microorganisms.