2026 Volume 79 Issue 8 Pages e178-e186
Since 2021, Japan has maintained a surveillance program to preserve its officially tuberculosis-free status in cattle. In this program, intradermal tuberculin testing followed by an interferon-gamma (IFN-γ) assay is used to determine cell-mediated immune responses and identify potentially infected cattle. Nevertheless, in recent years, several cases have been confirmed in which nontuberculous mycobacteria (NTM) other than Mycobacterium bovis were isolated from cattle classified as suspected cases. The data has raised concerns regarding the specificity of the current diagnostic test, particularly in tuberculosis-free countries. In this study, we conducted genomic analysis of five NTM isolates obtained from three cattle suspected of bovine tuberculosis in Japan. Three of these strains were identified as belonging to the same mycobacterial species closely related to the M. terrae complex, which is generally found in environmental sources such as soil. To determine the impact of these isolates on the diagnosis of bovine tuberculosis, we conducted a sensitization experiment using guinea pigs and cattle. Results showed that sensitization with these NTM isolates induced nonspecific immune responses that cross-reacted with antigens derived from M. bovis, suggesting that environmental exposure to NTM may affect test outcomes. However, when the tuberculosis-specific peptide antigen cocktail PC-EC was incorporated into the IFN-γ assay, animals that demonstrated nonspecific positive responses to bovine purified protein derivative (B-PPD) were reclassified as negative. These results suggest that the combined use of PC-EC with conventional B-PPD in IFN-γ assays could improve the diagnostic specificity of bovine tuberculosis surveillance without compromising sensitivity, particularly in tuberculosis-free countries such as Japan.
