Bovine tuberculosis (bTB), a zoonosis that affects humans, among other species (1, 2, 3, 4), is difficult to control due to the lack of effective vaccine, the presence of wildlife reservoirs, and the lack of a diagnostic assay with sufficient sensitivity (Se) and specificity (Sp) to detect sick animals at all stages of infection (3, 5).
Therefore, the success of bTB eradication and control programs is based on early detection and removal of reactors from a herd (6). Routine testing and cull strategy have been applied globally (3). Therefore, screening-test accuracy is critical to eradication programs. The single intradermal cervical tuberculin (SCT) test or single intra-dermal comparative cervical tuberculin (CCT) test in Europe, the caudal fold tuberculin (CFT) test in North America, Australia and New Zealand (7), and the SCT and CFT tests in Brazil (6), are the prescribed test for international trade (8). From a practical point of view, the diagnostic performance, the feasibility of execution and practicality of each test, as well as the costs and associated biological risks, should be considered for better strategic use (7).
Since late 19th century, the Tuberculin Skin Test (TST) has been the primary antemortem test available to support bTB eradication campaigns (9, 10). Advantages of the TST and reasons for its wide use are low costs, high availability, long history of use and, for a long time, the lack of alternative methods to detect bTB (9, 10, 11, 12). On the other hand, this test has many known limitations, including difficulties in performance and result interpretation, need for a second-step visit, low degree of standardization, and reduced test accuracy (13).
Due to the TST limitations in terms of sensitivity and specificity, the credibility of the diagnosis is frequently questioned given the occurrence of false-positive and false-negative reactions, therefore, it is necessary to confirm reactive animals using other methods, ensuring the reliability of the diagnosis (5, 10, 6, 7). Research continues into the development of new, more accurate, more sensitive tests, less subject to the individual operative performance and subjective interpretation (14, 15, 16, 17, 18, 19). As a complementary technique to the intradermal test in the detection of antibodies to Mycobacterium bovis (M. bovis) in animals exposed to the agent, a serological test with good sensitivity and specificity would be a viable alternative (9).
Over the last few years, the potential for use of an antibody assays to detect M. bovis infection in cattle is being consolidated (16, 17, 18, 19, 20, 21, 22, 23, 24, 25). The ELISA have proven to be useful as ancillary serial (to enhance Sp) and parallel (to enhance Se) tests in several species (26, 27, 28, 29, 30). Moreover, a booster effect on the antibody response caused after injection of tuberculin has been reported and recommended as a strategic option to increase the sensitivity of serological assays (26, 27, 28). The ELISA using MPB83 and MPB70 antigens (IDEXX M. bovis Ab Test, IDEXX Laboratories, Westbrook, Maine, US) is an immune enzymatic assay promising sensitivity and specificities superior to most tuberculosis diagnoses for both primary and supplementary diagnosis in cases of inconclusive results of the diagnosis by simple or comparative cervical test (16, 31, 12).
The Brazilian guidelines for control and eradication of animal tuberculosis determines intradermal tuberculin testing as the standard method of diagnosing bTB (32). The primary screening is performed using the CFT test (beef only), and the SCT or the CCT test (dairy and beef). The CCT is also adopted as confirmatory test. Until 2019 no others ancillary indirect methods for bTB diagnosis were approved by the Brazilian Ministry of Agriculture, Livestock and Food Supply (MAPA/BRAZIL). Adoption of complementary field bTB diagnostic tests can improve the detection of infected animals and herds.
To the best of our knowledge, there is no in vivo study comparing CCT test with ELISA IDEXX™ to evaluate the last as supplemental test for bTB in beef cattle in Brazil. The objective of this study was to determine the sensitivity (Se) and specificity (Sp) of the CCT test and a commercial ELISA multiple test (ELISA IDEXX™), as ancillary tests in beef cattle, under field conditions, using a Bayesian approach in order to provide evidence-based suggestions to the improvement of national bTB eradication programs.