Could the Type and Severity of Gross Lesions in Pig Lymph Nodes Play a Role in the Detection of Mycobacterium Avium?

The Mycobacterium avium complex (MAC) comprises a widespread group of slowly-growing bacteria from the Mycobacteriaceae. These bacteria are responsible for opportunistic infections in humans and animals, including farm animals. The aim of the study was to determine whether it is possible to predict the presence of M. avium in pig lymph nodes based on the size and type of lesions found during post-mortem examination at a slaughterhouse. Lymph nodes were collected from 10,600 pigs subjected to such post-mortem examination. The nodes were classified with regard to their quality, and the number of tuberculosis-like lesions; following this, 86 mandibular lymph nodes with lesions and 113 without visible macroscopic lesions were selected for further study. Cultures were established on Löwenstein-Jensen and Stonebrink media, and a commercial GenoType Mycobacterium CM test was used to identify and differentiate M. avium species. The prevalence of M. avium was 56.98% in the lymph nodes with lesions and 19.47% in the unchanged ones. Statistical analysis indicated that visual assessment of lesions in the mandibular lymph nodes, in particular the number of tuberculous lesions, is a highly-efficient diagnostic tool. Similar results were obtained for estimated percentage area affected by the lesion, i.e. the ratio of the changed area of the lymph node in cross-section to the total cross-sectional area of the lymph node; however, this method is more laborious and its usefulness in slaughterhouse conditions is limited. By incising the lymph nodes and assessing the number of tuberculosis-like lesions, it is possible to limit the inclusion of meat from pigs infected with M. avium into the human food chain.