Simple SummaryThe prevalence of parasitic infection exhibits seasonal variation and is affected by several factors, including environmental and host conditions. In this study, we investigated the seasonal variation in and factors affectingTrypanosoma theileriLaveran, 1902, infection in wild sika deer (Ezo sika deer)Cervus nippon yesoensis(Heude, 1884) in Eastern Hokkaido, Japan. The seasonal prevalence of trypanosome infection ranged from 0 to 41% (mean: 13%) based on hematocrit concentrations and from 17 to 89% (mean: 46%) based on PCR results. The factors identified as significantly affecting trypanosome infection included host age and sampling season, as per multiple logistic regression analysis. This is the first study to investigate the seasonal variation of and risk factors affecting trypanosome infection in wild deer.AbstractTrypanosoma(Megatrypanum) spp. are isolated from domestic and wild ruminants, including deer, worldwide. The prevalence of trypanosomes in mammals is influenced by a number of factors such as host age and vector abundance. However, the seasonal variation of and factors affecting trypanosome infection in the wild deer population remain elusive. In this study, we analyzed the seasonal variation in trypanosome prevalence and the factors that affectTrypanosoma theileriLaveran, 1902, infection in wild sika deer (Ezo sika deer)Cervus nippon yesoensis(Heude, 1884) in Eastern Hokkaido through a two-year survey. Seasonal variation in the prevalence of trypanosome infection in the deer population ranged from 0 to 41% as per hematocrit concentration and 17 to 89% as per PCR results. In general, the prevalence ofT. theileriby PCR in 2020 was higher than that in 2019. Moreover, the prevalence was significantly higher in the aged population than among the younger population. These findings may explain why individual conditions and sampling season were associated with trypanosome prevalence. This is the first study to investigate the seasonal variation in and risk factors affecting trypanosome infection in wild deer.