Seasonal dynamics of brown bear parasites: opposite trends for nematodes and protozoa.

Zoonotic diseases caused by parasites of wildlife origin represent a global health problem. As a top mammalian predator, the brown bear (Ursus arctos) can spread various parasites, including those that are potentially hazardous to human health. However, data on brown bear parasite fauna in Europe, and especially its seasonal dynamics, are scarce. The aim of this study was to analyse brown bear gastrointestinal parasites (helminths and protozoa) and to investigate their seasonal dynamics. Brown bear scats were collected from the eastern part of Estonia during one year, from spring 2022 to spring 2023. At first, we performed genetic host identification and selected 148 scat samples for further analyses. Parasite eggs and oocysts were identified based on morphology. The results revealed that the endoparasite prevalence among brown bears of Estonia is one of the highest in Europe (FO = 75%). The most prevalent were nematodes (60%), followed by protozoa (16%), cestodes (7%), trematodes (4%), and a single finding of an acanthocephalan. Of all endoparasites, the bear nematode Baylisascaris transfuga had the highest prevalence (51%). Importantly, the prevalence of nematodes and protozoa was season-dependent: highest for nematodes in autumn and lowest in spring, whereas protozoa followed the opposite dynamics. The vast majority of identified parasite taxa were zoonotic and are thus potentially hazardous to humans. This highlights the importance of monitoring wildlife parasites as an essential part of the One Health approach.