Experimental infection with Mycobacterium avium, serotype 2, 2. Oral infection with large doses of M. avium.

Twelve pigs were inoculated orally with Mycobacterium avium. The doses used were 0.5, 2 or 10 mg daily for 5 days, or 10, 50 or 180 mg once (1 mg = 37 × 10 viable units). Two pigs were used per dose, 1 of which was sacrificed 3 days, the other 28/31 days after the last inoculation (Table 1). Three days after inoculation, M. avium was found in the tonsils and in the intestinal mucosa of all 6 pigs, and in the mesenteric lymph nodes of 4. Viable unit counts for tonsils and intestinal mucosa were highest in pigs inoculated with 180 mg×1 and 10 mg×5. Histopathologically these pigs showed activation of the lymphoid tissue in the tonsils, Peyer patches and mesenteric lymph nodes. Twenty-eight/31 days after inoculation a spreading of the infection had taken place in all pigs, most often to the liver, less frequently to the spleen and the lungs. The kidneys and the musculature were not infected (Table 4). A correlation was apparent between the size of dose and the number of viable organisms in the tissues. Divided doses gave about 10 times higher viable counts than a single dose with the same total number of organisms (Table 5). No gross lesions were found 28/31 days after inoculation. Microscopic granulomatous lesions were found in the tonsils of 6 pigs, in the intestinal mucosa of 4 pigs, in the mandibular and mesenteric lymph nodes of 6 pigs, in the retropharyngeal lymph nodes of 3 pigs, and less frequently in the parotid and hepatic lymph nodes (Table 3). Five of 6 pigs were weakly sensitive to avian tuberculin PPD, 1000 t.u. per dose, when tested 22/25 days after inoculation; 1 of these pigs cross-reacted to human tuberculin (Table 2).