The objective of the present study was to investigate the prevalence of Theileria orientalis
infection and the influence of this disease on dairy grazing heifers in Kyushu by monitoring red blood cell (RBC) indexes, and to evaluate the efficacy of
diminazene diaceturate treatment of T. orientalis-infected animals. A monthly epidemiological survey was conducted for Holstein heifers, which were reared from 10 to 16 months of age on a large commercial dairy farm and grazed on eight independent grasslands from April to November, 2009. During the survey, a total of 2,803 blood samples were collected from the 891 grazing heifers, in which the prevalence of T. orientalis
infection was 52.4%. Compared with the heifers before
infection, heifers with high
parasitemia (more than 100 parasites per 104 RBC) had significantly decreased RBC indexes, such as RBC count,
hemoglobin concentration, and hematocrit (P<0.05), whereas heifers with low
parasitemia (less than 100 parasites per 104 RBC) had similar RBC indexes as those before
infection. Treatment with
diminazene diaceturate had lower efficacy in heifers with high
parasitemia than those with low
parasitemia (40.7% and 73.2% became negative, respectively, P<0.05). In summary, T. orientalis
infection is a potentially serious problem in Kyushu, and it is important to routinely implement control programs for heifers that are grazed on grasslands in this region.