PREVENTION 



233 



situated within the fine reticulum of the villi. The structural 

 alterations are everywhere proportional to the number of 

 bacilli, which indicates that, contrary to what is the case in 

 tuberculosis, the bacilli have little or no tendency to degener- 

 ate and disappear from the older lesions. The bacilli when 

 numerous are generally arranged in clumps or groups, and 

 these often form a very large part of the epithelioid areas. 



The lesions in the lymphatic glands have a similar his- 

 tology. They may be present either in the cortex or the 

 medulla, but they are not tuberculous in the anatomical sense. 

 A small number of giant cells may be present. 



The course of the disease varies from a few weeks to 

 several months. It seems to be fatal in most cases. 



§ 163. Differential diagnosis. This affection is to be 

 differentiated from tuberculosis and parasitic enteritis. The 

 non-virulence of the organism for guinea pigs is the most 

 reliable differential test between it and the bovine tubercle 

 bacteria. The finding of these acid- fast bacteria and the 

 absence of distinct lesions and parasites would distinguish it 

 from the other. As this disease is liable to occur in conjunc- 

 tion with genuine tuberculosis, great care must be exercised 

 in making the diagnosis. In some of these cases reported 

 there appears to have been such a mixed infection. 



§ 164. Prevention. Johne's disease must be regarded as 

 one which results from infection and from infection only. 

 M'Fadyean states that in all the cases which have come under 

 his observation there was a history of similar cases on the 

 farm, in several instances extending back over a period of 

 many years. During the advanced stages of the disease large 

 numbers of the bacilli must be voided with the feces, and in 

 all ordinary circumstances there are ample opportunities for 

 infection from this source. In this way both pasture and 

 other materials as well as drinking water may become seriously 

 contaminated. At the present moment there is no knowledge 

 with regard to the resistance of the bacilli outside the body. 



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