Distribution of lesions in cattle infected with Mycobacterium bovis

Detailed postmortem examinations were conducted on 30 cattle from a dairy herd with bovine tuberculosis to determine the distribution of lesions in Mycobacterium bovis-infected cattle. Twenty-four different tissue specimens from each animal were examined for gross lesions and collected for bacteriologic culturing and histologic examination. Tuberculosis was confirmed in 15 cattle with evidence of infection in 1 or more of the following tissues: medial retropharyngeal, parotid, tracheobronchial, mediastinal, caudal deep cervical, and subiliac lymph nodes; palatine tonsil; and lung. Gross and histologic lesions were present most frequently in lymph nodes of the thoracic region. Mycobacterium bovis was isolated from 3 cattle that had no gross lesions of tuberculosis. One animal had lesions only in the subiliac lymph node, which is not routinely examined during slaughter surveillance. Results of this study indicate that not all cattle infected with M. bovis have visible lesions of tuberculosis in sites that are routinely inspected. These findings are important because detection of gross lesions of tuberculosis during inspection of carcasses at slaughter is the primary method for detection of tuberculous cattle and herds in the United States.