Evaluation of four ELISA assays to diagnose Mycobacterium tuberculosis complex infection in pigs

Abstract

[Introduction]: In countries in which bovine tuberculosis (bTB) is still prevalent or is re-emerging the contact among different animal species in extensive systems may contribute to the circulation of Mycobacterium bovis and other members of the Mycobacterium tuberculosis complex (MTC) and the spread of this disease. Thus, free-range pigs may be infected by MTC, developing subclinical infections, which are not detected until meat inspection procedures at slaughterhouse. Serodiagnosis has been recently proposed as a reliable screening tool for detecting infected herds. In this study four ELISA assays using different M. bovis peptides/proteins (MPB70+MPB83, INGENASA; treated bovine purified protein derivative, t-bPPD; bPPD1; and bPPD2 VACUNEK) as coating antigens were evaluated to diagnose MTC infection in pigs. [Materials and Methods]: Submandibular lymph nodes (SLN) and blood samples from 129 free-range pigs raised on Southern Spain farms with a history of condemnation due to tuberculosis-like lesions were sampled at slaughterhouse. SLN were tested by gross examination, histopathology, bacteriological culture and qPCR. Ninety-seven out of these animals were classified as bTB positive cases (compatible lesions and MTC detection by means of culture and qPCR) or bTB negative cases (absence of compatible lesions and negative MTC detection) and used as reference method. When necessary different cut-off values were evaluated. [Results]: All assays had a very good concordance between them (k ≥ 0.82). The MPB70+MPB83 based ELISA had the best sensitivity (Se) (78%, CI95 67.4%>88.5%) and a good concordance with the reference method (k=0.69). The t-bPPD and the bPPD1 in-house assays presented a slightly reduced Se (71.2%, CI95 59.6%>82.7%; and 66.1%, CI95 54%>78.2%; respectively) and a moderate concordance with the reference method (k=0.57 and 0.52, respectively). When the bPPD2 based ELISA was evaluated, similar Se to the previous ones was obtained using a cut-off of 0.35 (Se: 66.1%, CI95 54%>78.2%; k=0.52). Conclusion` +: These results suggest that despite the fact that MPB70+MPB83 ELISA presented the best results all four evaluated ELISA assays could be used as a screening tool to conduct TB surveillance in pigs at a population level. In addition, a cut-off of 0.35 is recommended for bPPD2 ELISA in order to obtain better diagnostic values.