Constancy of Virulence of Bacilli. 629 



bacilli from one species of animal (at least in exceptional 

 eases) may produce local or even generalized tuberculosis in 

 other species, all point to the conclusion that tubercle bacilli 

 obtained from various sources all belong to the same species, 

 bacillus tuberculosis, and only represent varieties of "habitat" 

 which differ from each other in unimportant characteristics. 

 These characteristics are evidently the result of prolonged 

 growth, generation after generation, in different hosts and may, 

 under favorable conditions, again be obliterated. 



The pathogenic action of the bacilli of human tuberculosis, 

 pearl disease and tuberculosis of fowls is always most pro- 

 nounced for the species of animal from which they have been 

 respectively obtained, but none of them are harmless for other 

 species. In addition to guinea pigs and rabbits the virus of 

 human tuberculosis is pathogenic, in general, for all carnivora, 

 slightly so for swine and for many species of birds (birds like 

 canaries and parrots kept as pets in dwellings, in particular). 

 The bacillus of bovine tuberculosis is quite virulent for swine 

 and is, in general, more pathogenic for other domestic animals 

 than the human type. The bovine bacillus is also occasionally 

 the cause of fatal generalized tuberculosis in human beings, 

 especially in children. The bacillus of avian tuberculosis is, 

 in general, only slightly pathogenic for mammals, but may oc- 

 casionally produce tuberculous disease in man, in the ox and 

 the horse. 



In view of the experimentally demonstrated possibility of 

 the transmission of tuberculosis from one species to the other, 

 including man, the fact that this occurs in part only occasionally 

 is of subordinate importance. Neither does the fact that bovine 

 bacilli that have been the cause of tuberculous lesions in the 

 human body (or avian virus found in lesions of the horse, or 

 human virus found in parrots inhabiting the dwellings of human 

 beings) have retained for a time their original characteristics, 

 detract from the force of the argument. As a matter of fact, 

 the vast majority of medical and veterinary medical investiga- 

 tors admit the etiological entirety of tuberculosis in general 

 and the specific identity of the various forms of tubercle bacilli 

 and to this day accept Koch's original point of view, viz. : ''In 

 spite of the anatomical and clinical differences observed in 

 the course of the disease, tuberculosis of animals, and particu- 

 larly bovine tuberculosis, must be regarded as identical with 

 tuberculosis of man on account of the identity of the parasites 

 that are the cause of the disease. 



For practical purposes the recognition of a typus humanus, a 

 typus bovinus and a typus gallinaceus may be justified in order to 

 designate the morphological, cultural and pathogenic characteristics 

 of the different groups of bacilli, providing the fact is kept in mind 

 that these types represent merely habitat varieties of the same species 

 and that the occurrence of transition forms be kept in mind. 



