COMPARATIVE EFFICACY OF DIFFERENT TESTS FOR ANTE-MORTEM DIAGNOSIS OF BOVINE TUBERCULOSIS
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Date
2022-11-07
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COLLEGE OF VETERINARY AND ANIMAL SCIENCES, MANNUTHY, THRISSUR, KERALA VETERINARY AND ANIMAL SCIENCES UNIVERSITY
Abstract
Blood and nasal swab samples, 50 each and 17 milk samples were collected from animals with symptoms suggestive of bTB, presented for slaughter, at Thrissur corporation slaughterhouse, Kuriachira. The samples were subjected to detection by ZN staining, IS6110 insertion element specific PCR to detect MTBC, 12.7 kb fragment specific mPCR to differentiate M. bovis and M. tuberculosis, real-time PCR specific for a region upstream of p34 gene to detect the presence of M. bovisand M. tuberculosis, bovine gamma interferon immuno assay of blood and isolation and further characterisation. Histopathology was also performed on post-mortem tissue samples. The presence of drug resistance genes (viz., rpoB for rifampicin, pncA for pyrazinamide and katG for isoniazid) were detected in the isolates obtained using mPCR. Further, comparison of the results of different ante-mortem tests to that of isolation from the tissue samples, which being the gold-standard for confirmative diagnosis of TB, was made and analysed statistically. Finally, in order to confirm efficacy of the various ante-mortem diagnostic methods assessed in the study, 118 apparently healthy dairy cattle, maintained in organised farms and households in and around Thrissur were screened for TB. In the present study, ZN staining on nasal swab/milk/tissue samples was found to be highly specific, but showed poor sensitivity. The sensitivity of the PCR (nasal swab), real-time PCR (nasal swab), isolation (nasal swab) and gamma interferon assay was 100, 100, 100 and 50 per cent, respectively. The specificity of PCR (nasal swab), PCR (blood), real-time PCR (nasal swab), isolation (nasal swab) and gamma interferon assay was 95.83, 85.42, 97.92, 100 and 91.67 per cent, respectively. The sensitivity and specificity of PCR and real-time PCR from tissue samples was 100. None of the molecular methods could detect mycobacteria in any of the milk samples. Visible lesions were absent in the tissue samples. The HP of PCR positive tissue samples revealed multiple areas of mononuclear infiltration, epitheloid cells and mildchanges in lung samples. Present study revealed that, nasal swabs were found to be a better choice for ante-mortem diagnosis of bTB, than milk. Isolation of bacteria from nasal swab and PCR of the nasal swab were found to have maximum sensitivity and specificity for diagnosing bTB. Screening of 118 cattle maintained in organised and unorganised farms in Thrissur district, revealed a positivity rate of 2.54, 7.63 and 2.54 respectively for PCR, gamma interferon immuno assay and isolation, respectively. Drug resistance genes were detected in five of the isolates obtained, of which, all the M. bovis isolates amplified all three genes namely rpoB, pncA and katG. However, the M. tuberculosis isolates showed discordant results and failed to amplify all genes. In conclusion, a single method alone did not specifically detect mycobacteria in the present study. Therefore, a combination of tests could be used for screening and accurate diagnosis of bTB, considering other factors like, availability of samples, cost of the test and diagnostic facilities available in the laboratory.