Mycobacterium avium serotype 4 infection of swine: the attempted transmission by contact and the sequence of morphological changes in inoculated pigs.

After 30 pigs were inoculated orally in groups at 4, 8, 12, 16 and 20 weeks of age with Mycobacterium avium serotype 4 and killed when approximately 210 lbs body weight, 24 pigs had gross lesions caused by M. avium and all had microscopic lesions. It was found that the group inoculated at 8 weeks of age had lesions that best reproduced those found in outbreaks of mycobacteriosis. Thirty-four contact pigs did not develop gross mycobacterial lesions and only 2 developed microscopic lesions. In a second experiment, 29 pigs were inoculated orally when 8 weeks of age and killed 6, 12, 18, 21 and 28 weeks later. Gross lesions caused by M. avium were present in 9 pigs and 17 pigs had microscopic lesions. Severe lesions were found in the pigs killed after 18 weeks. Gross lesions were enlargement or caseation of lymph nodes. Enlargement alone was not considered a reliable indicator of mycobacterial disease, as less than half the affected lymph nodes were histologically granulomatous. When a granulomatous lesion was present in enlarged lymph nodes, it was most likely to be an accumulation of epithelioid macrophages with areas of necrosis and caseation. The most common histological lesion found in grossly caseated lymph nodes was discrete granulomas with severe caseation and a thick fibrous capsule. Granulomas that consisted of epithelioid cells, or epithelioid cells and caseation were most numerous after 12 weeks or less. Multinucleate giant cells dominated the picture when the interval between inoculation and slaughter was 6 or 10.5 weeks. Encapsulated granulomas were the main type in animals inoculated 18 weeks or longer before slaughter. Caseation of epithelioid granulomas and the formation of multinucleate giant cells seemed to progress faster in the older inoculated animals. Microscopic granulomas were present in the tonsils and intestines of 8 pigs and 16 pigs, respectively, and most of these pigs were inoculated at 16 and 20 weeks of age and killed 10.5 and 6 weeks later. Lesions of the tonsil and intestine did not become encapsulated, and probably resolved. The conditions of our experiment should have allowed contact animals to be exposed to organisms shed from the tonsil and intestine of inoculated pigs, yet only a few microscopic lesions developed.