Cervids and raccoon dogs as wildlife sentinels for the geographical distribution of tick-borne encephalitis virus and Francisella tularensis in Denmark

Collecting and testing ticks for rare tick-borne pathogen surveillance can be labour-intensive and true absence of rare pathogens in ticks is difficult to determine. Nevertheless, knowledge of the distribution of tick-borne pathogens is relevant for risk assessment and diagnosis. Tick-borne encephalitis and tularemia are rare human tick-borne diseases. Knowledge of the distribution of the pathogens of these diseases in Denmark is limited. The aim of this study was to assess the geographical distribution of tick-borne encephalitis virus (TBEV) and Francisella tularensis using wildlife as sentinels. Wildlife serum and plasma from 717 cervids (305 roe deer (Capreolus capreolus), 291 red deer (Cervus elaphus), 107 fallow deer (Dama dama), 14 sika deer (Cervus nippon)) and 64 raccoon dogs (Nyctereutes procyonoides) from all Danish regions, collected from 2017 to 2023, were screened for IgG antibodies against TBEV by using a commercial enzyme-linked immunosorbent assay (ELISA). Positive samples were subsequently tested in a virus neutralisation test (NT). A total of 294 cervids and 67 raccoon dogs were examined for antibodies against F. tularensis using a commercial quick agglutination assay. We detected a TBEV NT seroprevalence of 0.6 % in cervids and 1.6 % in raccoon dogs and an F. tularensis seroprevalence of 6.8 % in cervids and 16.4 % in raccoon dogs. We confirmed TBEV presence in several areas of Denmark and we detected antibodies in new areas of the island of Zealand. Furthermore, this is the first study to indirectly explore the geographical distribution of F. tularensis in Denmark and we identified areas with probable presence of F. tularensis.