430 CHRONIC INFECTIOUS DISEASES 



Modes of Infection. — ^Tubercle bacilli are taken into the 

 body: (a) Via digestive tract with contaminated food and 

 water; (b) via respiratory tract by the inhalation of tuber- 

 cular spray ejected by coughing or lowing infected cattle; 

 (c) via genital organs during coitus; (d) via udder through 

 teat canal; (e) via wounds (very rare; may follow castra- 

 tion), and (/) congenital tuberculosis has been noted only in 

 isolated cases. 



(a) Calves and swine are commonly infected through milk 

 from creameries, especially skimmed milk obtained by centrif- 

 ugal separation. The ingestion of cattle excrement by swine 

 is a pregnant source of infection, especially in America, where 

 the practise of allowing hogs to follow cattle is much in vogue. 

 The feeding of the offal of slaughter houses to swine is likewise 

 dangerous, as tubercular lungs, livers, lymph glands, gastro- 

 intestinal contents, etc., are consumed. 



The bronchial exudate of tubercular cattle, coughed up or 

 otherwise raised from the lungs, may mix with the saliva, 

 and thus tubercle bacilli are carried to feed or watering 

 troughs, bedding, etc. As most of the exudate is swallowed 

 by the patient (not expectorated as in man), the feces become 

 polluted, scattering bacilli wherever dropped. Susceptible 

 animals (hogs and cattle) eating or drinking substances con- 

 taminated by such discharges become infected. The same 

 would apply to any other secretion or excretion containing 

 tubercle bacilli (vaginal discharge, urine, etc.). 



(b) Infection through the respiratory organs comes from 

 the inhalation of either globules of bronchial exudate, 

 mixed with mucus and saliva, which are coughed out or 

 otherwise forcibly ejected from the nose and mouth of tuber- 

 cular cattle; or of tubercle bacilli which have become 

 partially dried and are adhering to dust particles floating in 

 the air. In the former case cattle immediately next to an 

 "open" case of tuberculosis acquire the infection by close 

 contact, and in the latter, which is far less frequent, by 

 breathing in the dried bacilli which contaminate the inspired 

 air. 



(c) Infection by coitus may occur provided the genital 

 organs of the bull (penis, prostates, testes), or cow (vagina. 



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