Patterns of intestinal parasite prevalence in brown bears (Ursus arctos) revealed by a 3-year survey on the Shiretoko peninsula, Hokkaido, Japan

This study examined the parasite fauna of wild brown bears and differences in the likelihood of parasite detection by season (summer vs autumn), year, and host factors (sex and age class). From June 2022 to November 2024, 334 fecal samples were collected from the Shiretoko Peninsula, Hokkaido, Japan, and examined for parasites using the centrifugal flotation technique. Fecal DNA analysis and data from a long-term field monitoring survey led to the identification of 49 individuals, which were further classified based on sex and age. Parasites detected in the feces included <i>Uncinaria</i> sp. (35.0% of total samples), <i>Baylisascaris transfuga</i> (13.5%), <i>Dibothriocephalus nihonkaiensis</i> (8.4%), Strongylida (2.7%), Capillariidae (0.6%), and coccidia (0.6%). The prevalence of detection for <i>D. nihonkaiensis</i> tended to be higher in autumn (September-November) than in summer (May-August). The likelihood of <i>B. transfuga</i> and <i>Uncinaria</i> sp. detection varied by season, year, and bear age class. The likelihood of <i>B. transfuga</i> detection was significantly higher in autumn than in summer, and tended to be higher in young bears (0-2 years) than in subadult/adult (≥3 years) bears, whereas that of <i>Uncinaria</i> sp. was significantly higher in summer than in autumn and in subadult/adult than in young bears. Egg shedding by these three parasites tended to disappear before or during hibernation. These results suggest that the likelihood of parasite detection in brown bear reflects the interactions of environmental and host factors, including seasonal and/or annual changes in diet, winter hibernation, and host growth.