Spatial pattern of serology against Borrelia and tick-borne encephalitis virus in roe deer (Capreolus capreolus) , Northern France, 2019-2022

Summary Background Lyme borreliosis and tick-borne encephalitis (TBE) are tick-borne zoonoses of public health importance, transmitted in Western Europe by the same tick vector Ixodes ricinus . Roe deer ( Capreolus capreolus ) is a widespread feeding host for I. ricinus stages. Here, we explored the spatial extent of both pathogens in the wild in Northern France by assessing serological status in roe deer. Methods We collected 3,147 blood samples (2019-2022), and searched antibodies against both pathogens using ELISA tests. We fitted logistic models to investigate exposure to both or either pathogens, and explored spatial patterns using Kulldorf clustering scan tests. Results The overall ELISA IgG prevalence was 23.24% (95%CI [21.77-34.72]) for Borrelia and 8.64% (95%CI [7.45-9.86]) for TBEV. The median Borrelia seroprevalence varied from 4% to 42% between departments, with two high-risk clusters of exposure identified. TBEV seroprevalence did not show any spatial pattern, and positives by seroneutralisation tests were reported in eight departments. Discussion Our findings suggest the presence of both pathogens over large areas. Wider spatial sampling including areas at lower risk and improved serological methods would be beneficial to further validate those estimates.