INl^ECTlOlJS DISEASES '-^oO 



lymphatic glands, tlie formation of tubercles of variable sizes in or 

 upon serous membranes and within the substance of glandular organs, 

 bones, and other connective tissues. The tubercles produce increased 

 density of invaded soft tissues and are, therefore, easily detected ex- 

 cept in very recent infection. The tubercular nodules in the early 

 stages present about the same color as the surface of the tissue invaded. 

 In sectioning the tubercle the central portion is caseous and slightly 

 yellow, or fibrous and white. Sometimes there is a combination of 

 both conditions, and occasionally the tubercles contain calcareous 

 granules. The central portion of porcine tubercles rarely contains 

 liquefied necrotic tissue. 



Microscopically, porcine tubercular lesions are always cellular in 

 the beginning, the cellular tubercles being fairly constant in structure, 

 regardless of the tissue in which they occur. The center is at first rep- 

 resented by a* mass of lymphoid cells, the other cellular elements occur- 

 ring as the tubercle develops. 



Necrosis, or fibrosis, succeeds the cellular stage in the porcine tuber- 

 cular lesion. Necrosis of tubercles is probably the result of the activ- 

 ity of virulent bacteria or the low resistance of the infected animal. 

 The necrotic center may be surrounded by a cellular zone (lymphoid 

 and endothelial cells), or it may be surrounded by fibroblasts. The 

 necrotic material is invariably caseated and later becomes calcified. 



Fibrous lesions vary from the formation of small quantities of 

 fibrous tissue to dense fibrous centers. They are probably produced 

 by bacteria of low virulence, or occur in animals having a marked 

 resistance. The central portion of the fibrous lesion may become 

 calcified. 



The so-called arbor vitte gland is a fibrous center in which the 

 fibrous tissue is arranged similarly to the trunk and branches of a 

 tree, hence the name. In the hog, this lesion is' observed in the cervical 

 lymph nodes. The Bacterium tuberculosis has been demonstrated in 

 about thirty per cent of arbor vitse glands. 



The lesions of porcine tuberculosis are in brief either cellular, 

 necrotic, and calcified tubercles, or cellular, fibrous and calcified tuber- 

 cles. The lesion is always nonvascular, as in other animals. 



Tubercular lesions are most common in the digestive viscera. The 

 liver and spleen frequently contain caseous nodules. 



Pulmonary porcine tuberculosis manifests itself either by a few or 

 many cheesy foci or by a few large centers. There may be a general 

 caseous pneumonia associated with more or less pleurisy. 



