No. 6. DEPARTMENT OF AGRICULTURE. 125 



bacillus of bovine type for all other animals, excepting, possibly, 

 for the extremely susceptible guinea pig'. 



Let us look for the cause of this difference. Every living object 

 is influenced by its environment; its habits of growth and its char- 

 acteristics are determined to a great extent by the conditions to 

 which it is subjected. When a living organism inhabits the tissues 

 on another living being, it is known as a parasite. The tubercle 

 bacillus is a parasite and has lived a parasitic existence so long- 

 that it is now incapable of growing under natural conditions outside 

 of the living body. If it is propagated, during a long period, from 

 one animal to another, of the same species, it must develop certain 

 characteristics expressive of the influence of its long continued 

 and unchanging environment. 



Tubercle bacilli as they affect mammals, are propagated chiefly 

 in the bodies of human beings and of cattle. While the disease 

 affects animals of other species and, indeed, no warm blooded ani- 

 mal is wholly exempt, tuberculosis is not propagated continuously 

 in animals of any other species than the tw r o just referred to. For 

 example, tuberculosis is in some regions very common among swine. 

 But it is always possible to show that the prevalence of tuberculosis 

 among swine is in proportion to the amount of milk they eat and 

 to the prevalence of tuberculosis among the cows that produce 

 this milk. Tuberculosis of swine is most prevalent where they are 

 fed on skimmed milk from creameries in districts where there is 

 most tuberculosis among the dairy herds. This disease is but rarely 

 transmitted from swine to swine. Tuberculosis of horses occurs 

 where there is much tuberculosis of cattle and where it is the prac- 

 tice, as was formerly the case in Denmark, and to a less extent in 

 England, to feed a certain amount of cow's milk to foals and to 

 horses out of condition. I know of no case where there has been 

 reason to believe that tuberculosis has been transmitted from one 

 horse to another. Tuberculosis of dogs and of cats is sometime,-; 

 contracted from cattle through feeding upon infectious milk or upon 

 the organs of animals afflicted with tuberculosis, as at a slaughter 

 house; or pet dogs and cats kept in the house, may contract tuber- 

 culosis from their consumptive masters. Tuberculosis of all other 

 mammals may likewise be traced to a bovine or human course. 



Thus it is, that there are two main branches or streams of mam- 

 malian tubercle bacilli, one following its course through the bodies 

 of consumptive people and the other through the bodies of consump- 

 tive cattle, and each giving off side branches to animals of other 

 species; but these secondary branches terminate within a genera- 

 tion or two after leaving one of the main stems, while the principal 

 currents continue to flow through the bodies of men and of cattle, 

 on and on, as they have done for centuries, leaving broad swaths 

 of dead and dying victims. This continuation of tubercle bacilli in 

 one line or the other has produced the definite characteristics that 

 have been mentioned as the distinguishing features of the human 

 and bovine type of this germ. 



The important question from a public standpoint is: Are the 

 germs of bovine tuberculosis capable of producing disease in man? 

 This question can now be approached in a new and enlightening way. 

 Formerly the attempt was constantly made to decide the question 

 as to the transmissibility from cattle to man by what might be 



