334 



SWINE PRACTICE 



Lesions. — The typical initial lesion of tuberculosis is the tubercle 

 which has been aptly defined as a nonvascular nodule composed of 

 leukocytes, endothelial, giant and connective tissue cells, with a ten- 

 dency for the central part of the nodule to undergo necrosis. There 

 may be a slight variation in the tubercle in different animals, due to 

 variation of susceptibility of the animal and variation of the virulence 

 of the infecting bacilli. The typical lesion may also be materially 

 modified by secondary lesions. Tuberculous lesions may occur in any 

 tissue or organ, but lymphoid tissue is most frequently affected. 



The tubercle bacilli and their products are the causative factors in 

 the formation of a tubercle. Having lodged in a tissue favorable for 

 their growth and development, the bacilli begin to multiply and to 

 eliminate products that stimulate the surrounding connective tissue 

 and endothelial cells to increase in number, and at the same time 

 exert a positive chemotactic action upon the leukocytes. In swine 

 there is a very marked chemotactic action and limited stimulation of 



Pig. 98. TUBERCiTLosis OF THE SKi.x. A, the skin showing caseo-calcareous 

 lesions; B. subcutaneous fat. C, muscle. 



connective tissue. The typical porcine tuberculosis tubercle is largely 

 composed of leukocytes and has little fibrous tissue. 



Tubercles grow because of the increased accumulation of leukocytes 

 and the formation of a small quantity of fibrous tissue. The tubercle 

 contains no blood vessels; that is, it is nonvascular, although in the 

 early stages there may be a hypermic zone around the tubercle. This 

 liyperomic zone is the result of irritation in the tissue, the tubercle 

 formation causing a tissue reaction. As the tubercle becomes larger, 

 due to the accumulation of leukocytes on the margin, the central part 

 begins to degenerate. 



Porcine tubercular lesions are characterized by enlargement of 



