INFECTIOUS DISEASES 219 



have claimed four varieties of tubercle bacilli, namely, 

 human, bovine, avian, and reptilian. The bovine and 

 human types have been encountered in swine. 



The tubercle bacilli are very resistant to external 

 injurious influences. This resistance is probably due to 

 the waxy coat they possess. Direct sunlight destroys 

 them when freely exposed for twenty to thirty minutes, 

 but they retain their vitality and virulence for from ten 

 to twenty days when exposed to diffuse daylight. When 

 desiccated and not exposed to light they may retain 

 their virulence for four months or even longer. In 

 decomposing tubercular tissues they remain virulent for 

 a long time. In milk they retain virulence for ten days. 

 Dry heat at 212 degrees Fahrenheit destroys them in one 

 hour; moist heat at the boiling temperature will destroy 

 them in fifteen minutes. They are also destroyed by 

 chemical disinfectants, the length of time for exposure 

 depending upon the concentration of solution and the 

 nature of the tissue or discharge incorporating the bacilli. 

 Gastric juice does not destroy the tubercle bacilli, and 

 bile has little or no injurious effect upon them. 



The inoculation of healthy, susceptible animals with 

 virulent bacilli produces tuberculosis, the extent of the 

 lesions produced depending upon the virulence of the 

 bacilli injected and upon the resistance of the inoculated 

 animal. 



Source Of infection. Tubercule bacilli are elimi- 

 nated from the affected animals, at least when there are 

 open lesions in organs, the secretions or excretions of 

 which pass to the outside world. Thus the tubercle 

 bacilli are eliminated in the feces from animals having 

 tubercular ulcers of the intestinal mucosa. Open tu- 

 berculous lesions of the respiratory tract would permit of 

 the elimination of the tubercle bacilli in the discharge 

 from the air tubes and also in the feces, for the chances 

 are that the pharynx would become contaminated by 

 the pulmonary or tracheal discharge and this infection 

 would be carried with food or drink and ultimately pass 

 from the body in the feces. Tubercle bacilli from lesions 



