Experimental infection with Mycobacterium avium, serotype 2, in pigs. 3. Oral infection with small doses of M. avium.

Pigs were inoculated orally with Mycobacterium avium in doses ranging from 15.6 × 10 to 15.6 × 10 viable units daily for 15 days (Table 1). The animals were necropsied 31–32 days after the last inoculation. Pigs given doses of 15.6 × 10 and 15.6 × 10 viable units showed delayed hypersensitivity to avian tuberculin 24 days after the last inoculation (Table 2). The pig inoculated with 15.6 × 10 viable units showed macro- and/or microscopic lesions of the intestines and the liver, and of the mandibular, mesenteric and hepatic lymph nodes. Cultures showed growth of M. avium from the same tissues and from the spleen and the left tracheobronchial lymph node. The pig inoculated with 15.6 × 10 viable units showed a pronounced granulomatous infiltration in the tonsils and the mesenteric lymph nodes. Growth of M. avium was obtained from the tonsils, the intestinal mucosa (Peyer patch) and the mandibular and mesenteric lymph nodes. Viable unit counts were high in the tonsils and in the mesenteric lymph nodes (Tables 3 and 4). Lower doses gave rise to a minimal tissue reaction and/or very low viable unit counts, and are not considered to be capable of producing a progressive tuberculosis.