Influence of repeat SICCT testing on subsequent skin-tests. SICCT responses were measured in naturally infected cattle at approximately 60 day intervals (n = 23, 15, 23, 23 and 13 respectively). The comparative PPD response at each test is presented as the mean value for each group (± standard error). The line represents the significant decreasing linear regression with r² and p values indicated, and significant differences between D and subsequent tests is indicated by **(p < 0.01).