284 INFECTIOUS DISEASES 



visible symptoms duriiip; the early stage of the disease, are 

 responsible for this condition and the extensive infection of dairy 

 and breeding' herds. 



Prevalence of the Disease. — Tuberculosis is very prevalent 

 among cattle and swine in all countries where intensive agricul- 

 ture is practised. It is a rare disease among cattle of the steppes 

 of eastern Europe and the cattle ranges of the western portion 

 of the United States. In countries where dairying is an im- 

 portant industry, tuberculosis is a common disease of cattle and 

 hogs. The abattoir reports of Europe and the United States 

 show that tuberculosis is on the increase among domestic animals. 

 The Bureau of Animal Industry of the United States Depart- 

 ment of Agriculture reports that out of 400,008 cattle tested with 

 tuberculin 9.25 per cent reacted. jMelvin states that the annual 

 loss from tuberculosis in the United States is about $23,000,000. 

 In dairy herds in which the disease has existed for several years, it 

 is not uncommon to find from 25 to 75 per cent tubercular. 



The Direct Cause. — The direct cause of tuberculosis is Koch's 

 Bacillus tuberculosis. This is a slender, rod-shaped microorga- 

 nism (Eig. 88) occurring in the diseased tissues, faeces and milk 

 of a tubercular animal. It belongs to that small group knoA\Ti as 

 acid-fast bacteria. The tubercle bacillus is not really destroyed 

 by external influences, and it may retain its virulence for several 

 months in dried sputum if protected from the light. Its vitality 

 enables it to resist high temperatures, changes in temperature, 

 drying and putrefaction to a greater degree than most non-spore- 

 producing germs. Direct sunlight destroys the germ within a few 

 hours, but it may live in poorly lighted, filthy stables for months. 

 A temperature of 65° C. destroys it in a few minutes. 



Animals that have advanced or o]ien tuberculosis may dis- 

 seminate the germ of the disease in the discharge from the 

 mouth, nostrils, genital organs, in the intestinal excreta and milk. 

 The germs discharged from the mouth and nostrils are coughed 

 up from the lungs and may infect the feed. Milk is a common 



