PREVALENCE OF DIGITAL DERMATITIS IN EUROPEAN BISON (BISON BONASUS) IN SWITZERLAND AND REFERENCE TO OTHER EUROPEAN COUNTRIES.

In 2018, digital dermatitis (DD) associated with Treponema spp. was detected in a herd of European bison (EB, Bison bonasus) in Switzerland. This follow-up study was carried out in 2021-2022 to evaluate the DD prevalence in the Swiss EB population (n = 49) by taking photographs and biopsies of the interdigital cleft of 10 anesthetized and eight deceased animals from five out of the six Swiss EB herds. Further samples of captive animals were available from Germany (n = 4), France (n = 1), and Poland (n = 1), and of free-living individuals from Germany (n = 4) and Poland (n = 11). Except for the Polish specimens, of which only one front foot per animal was available, all four feet were examined. Overall, 120 feet from 39 animals were available. Biopsies were taken according to a standardized protocol and investigated by histopathology, Treponema spp. fluorescence in situ hybridization (FISH) and full-length 16S rRNA gene sequencing. In addition, a transmission electron microscopy (TEM) examination was performed on tissues of randomly selected animals (n = 7). For the Swiss population, a DD prevalence of 94.4% (72.7-99.9%) was determined. Overall, 89/120 (74.2%) feet presented with macroscopic lesions. Histopathologically, a perivascular and lymphoplasmacytic dermatitis was present in 86/119 (72.3%) of the feet. In TEM, helically coiled bacteria were identified within the tissue of four animals, whose morphology confirmed them to belong to the order Spirochaetales. FISH was positive in 60/116 (51.7%) samples. 16S rRNA gene sequencing revealed the presence of three Treponema phylotypes (PT1, PT12, and PT3), which were clearly distinct from previously reported Treponema spp. in cattle and other ruminant species. This study shows that DD associated with Treponema spp. is widespread in Swiss EB herds and in other European individuals. So far, the disease does not seem to have a significant clinical relevance for EB, as clinical signs, such as lameness were not evident.