| Summary: | The objective of the present study was to investigate the effects of <i>Theileria orientalis</i> on the severity of anemia, the prevalence of disease within 21 days after calving and productivity in cows raised inside barns. This longitudinal observational study, which was conducted on a commercial dairy farm in Japan, involved 627 Holstein cows subjected to PCR analysis for <i>T. orientalis</i>. In study 1, we collected blood samples from 156 sick cows within 21 days after calving, and we found the prevalence of <i>T. orientalis</i> infection to be 65.4%. In study 2, we randomly selected 471 cows during the dry period and collected blood samples to conduct PCR analysis for <i>T. orientalis</i> and determined the prevalence of <i>T. orientalis</i> infection to be 69.0%. Compared with the values for the <i>T. orientalis</i>-uninfected group, the <i>T. orientalis</i>-infected cows had significantly decreased hemoglobin concentrations and hematocrit, but there were no differences in the other complete blood count indexes between the two groups. In addition, there were no differences in productivity and the prevalence of major diseases between the <i>T. orientalis</i>-infected and uninfected cows. In summary, <i>T. orientalis</i> had few effects on anemia, productivity and the health of cows raised inside a barn.
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