Utility of Serum Thymidine Kinase Activity Measurements for Cases of Bovine Leukosis with Difficult Clinical Diagnoses

This study evaluated the clinical usefulness of serum thymidine kinase (TK) activity for diagnosing bovine leukosis cases for which clinical diagnosis was difficult ('BL with difficult diagnosis'). Median TK activity values in 24 'BL with difficult diagnosis' and 36 cattle for which BL was clinically confirmed by cytology findings of enlarged superficial lymph nodes ('clinically confirmed BL') were 36.8 and 39.4 U/l, respectively (no significant difference). The percentage with positive TK activity (>5.4 U/l) was also similar in both groups (83.3% for 'BL with difficult diagnosis' and 97.2% for 'clinically confirmed BL'). TK activity was significantly higher in cows with 'BL with difficult diagnosis' compared to those with other tumors (N=13) and those with inflammatory diseases (N=14). Maximum TK activity in cows with other tumors and inflammatory diseases was not high (<10 U/l). Median TK activities in cows with other tumors and those with inflammatory diseases were 1.8 and 1.4 IU/l, respectively. Positive TK activity was found in a significantly higher percentage of cows with 'BL with difficult diagnosis' (83.3%) relative to the percentages of cows with other tumors (15.3%) and inflammatory diseases (21.4%). Thus, TK activity is an appropriate marker for detecting BL onset in cows with 'BL with difficult diagnosis' as well as 'clinically confirmed BL' group. While the specificity of TK activity required for BL diagnosis is not clear, simultaneous evaluation of serum lactate dehydrogenase activity may assist in the differential diagnoses of other tumors and inflammatory diseases from BL.