| Summary: | The seasonal dynamics of the prevalence, abundance, and mean intensity of egg excretion by the nematode parasite Baylisascaris transfuga in the European brown bear (Ursus arctos) were monitored relative to environmental factors (mean temperature, humidity, and temperature) over three years. The prevalence, abundance, and mean intensity of egg excretion tended to increase from spring to autumn throughout the monitoring period. The seasonal prevalence (84.2–90.6%), abundance (470.2–545.3 eggs per gramme (EPG) of faeces), mean intensity of excretion (558.3–602.1 EPG), and number of eggs (1150 EPG) were highest in autumn. The prevalence of eggs (up to 48.5%), abundance (37.8–60.6 EPG), and mean intensity of excretion (94.4–125.0 EPG) were high in winter, despite the period of hibernation. The seasonal dynamics of B. transfuga abundance in bears, the mean temperature between spring and autumn, and the seasonal trend of increase in intensity of egg excretion with temperature from winter to summer were interrelated. Abundance differed significantly between winter and autumn, spring and autumn, and summer and autumn (p < 0.0001) in all years and between spring and summer in 2016 (p < 0.005), 2017 (p < 0.05). B. transfuga abundance differed significantly between the seasons over the three years only in spring (p ≤ 0.0001).
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