290 THE TUBERCLE BACILLUS 



of animal man and bovine respectively in which it finds its normal habitat 

 may nevertheless infect either of them, and though the bovine bacillus appears 

 to be more virulent than the human bacillus the latter, according to de Jong, 

 may by passage through goats be made as virulent for bovine animals as 

 the bovine bacillus itself. 



[The findings of the English Commission l are not altogether in agreement 

 with the statements contained in the preceding paragraph. 



[In the opinion of this Commission the human and bovine types are not 

 identical but "varieties of the same bacillus." They point out that since 

 the human and the bovine tubercle bacilli are " morphologically indistin- 

 guishable " the question of their identity or non-identity resolves itself into 

 a consideration of their cultural and pathogenic differences or similarities. 



[With regard to the former, the human type consistently grows more 

 luxuriantly in culture than the bovine type and this difference in cultural 

 characteristics is quite definite though " the gap which separates the human 

 type from those strains of the bovine type which grow most abundantly is 

 not wide." 



[A study of their pathogenic resemblances and differences shows on the 

 one hand that the disease produced in certain species of animals such as 

 guinea-pigs and monkeys by the two types is " histologically and anatomi- 

 cally identical " and on the other hand that in man fatal tuberculosis due 

 to infection with bacilli of the bovine type is identical with that caused by 

 the human type. 



[That the bovine bacillus can infect man is certain. Many cases of tubercu- 

 losis in children and a few in adults investigated by Cobbett and A. S. Griffith 

 (working for the English Commission) were shown to be caused solely by the 

 bovine tubercle bacillus. An infection of the bovine species by the human 

 tubercle bacillus on the other hand did not occur : the human tubercle 

 bacillus was in fact incapable of producing in cattle anything but a slight 

 and non-progressive tuberculosis, however large the dose. 



[Neither did the human type of bacillus cause anything more than a slight 

 non-progressive tuberculosis in goats, pigs and, with rare exceptions, in rabbits, 

 while the bovine bacillus readily caused a fatal tuberculosis in these animals 

 as well as in cattle. 



[Certain tubercle bacilli isolated during the investigations of the Com- 

 mission from cases of lupus and equine tuberculosis had the cultural charac- 

 teristics of the bovine bacillus but were only slightly virulent for calves and 

 rabbits (the animals usually relied on for differential tests) and were of 

 relatively low virulence also for monkeys and guinea-pigs. These bacilli, 

 it would seem, in no way bridge the gap between the two types ; for while 

 they approach the human tubercle bacillus in their low degree of virulence 

 for calves and rabbits, they recede from it in virulence for monkeys and 

 guinea-pigs (A. S. Griffith). At the same time, as the Commissioners point 

 out, " the discovery of these exceptional bacilli makes it impossible to regard 

 differences of virulence for the calf and rabbit as sufficient to establish the 

 non-identity of the human and bovine types." Several of these attenuated 

 bacilli isolated from human (lupus) and equine sources were raised to the 

 full virulence of a typigal bovine bacillus by passage through calves and 

 rabbits. 



[To establish the complete identity of the two types it would appear to be 

 necessary to demonstrate that both cultural and pathogenic differences were 

 unstable, i.e. that the transmutation of the human type of bacillus into the 



[ x The references to the " English Commission " in this chapter are to the Reports 

 and Appendices thereto of the Royal Commission on Tuberculosis appointed in 1901.] 



