Seasonal changes in hemograms and Theileria orientalis infection rates among Holstein cattle pastured in the mountains in the Republic of Korea

- We assessed the seasonal changes in RBC values and infection rates of a tick-borne pathogen between grazing and indoor cattle.
- The Theileria orientalis infection rates for pastured cattle were increased after grazing.
- RBC values showed a dramatic decrease after grazing, leading to a remarkable increase of ticks in mountainous areas in the summer.
- These hematological changes may be associated with T. orientalis infection caused by tick-biting.

sIn the current study, we compared seasonal changes in complete blood counts (CBCs) and rates of infection with a tick-borne pathogen between Holstein cattle housed indoors and those maintained outside on pasture. There were differences in white blood cell (WBC) parameters, but the changes were not associated with seasons or the housing type. Analysis of red blood cell (RBC) parameters showed lower values in August and November versus March, and in the cattle maintained on pasture versus the housed cattle. In comparison with the RBC count of the housed cattle in March (10.1 M/μL), the RBC counts of the pastured cattle were significantly lower in August (7.8 M/μL; p < 0.01) and November (7.5 M/μL; p < 0.01). The hematocrit (HCT) also showed a decrease in March (33.5%), August (30.0%, p < 0.01) and November (28.5%, p < 0.01). According to PCR analysis, the Theileria infection rate among the pastured cattle in March was only 11%, but this rate increased to 22% and 60% in August and November, respectively. The RBC count (7.4 M/μL) and HCT (27.7%) values in Theileria-positive pastured cattle in November showed a dramatic decrease compared to those of cattle examined in March. Phylogenetic analysis revealed that these Theileria isolates correspond to T. orientalis. These results suggest that a remarkable increase in tick infestation in mountainous areas in the summer may cause increased rates of infection with T. orientalis, leading to significant changes in the RBC profile after grazing. Therefore, these hematological changes may be associated with T. orientalis infection caused by tick-biting; thus, additional studies on the pathogenicity of T. orientalis are needed.