Opinions varied as to the frequency with which the transmission of tubercu losis from one species to the other oc curred. but practically never within the last eighteen years the possi bility and probability of such reciprocal infection. What are the grounds upon which we are asked to discard convic tions that appeared to rest on such a solid basis? He endeavors to state them briefly, as he understands Pro fessor Koch's train of reasoning: 1. The bacilli found in eases of bovine tuberculosis are much more virulent for cattle and other domestic quadru peds than the bacilli found in cases of human tuberculosis. 2. This differ ence is so marked and so constant that it may be relied upon as a means of distinguishing the bacilli of bovine tuberculosis from those of the human disease, even assuming that the former may occasionally be found as a cause of disease in man. 3. If bovine bacilli are capable of causing disease in man. there are abundant opportunities for the transference of the bacilli from the one species to the other, and cases of primary intestinal tuberculosis from the consumption of tuberculous milk ought to be of common occurrence. But post-mortem examination of hu man beings proves that cases of pri mary intestinal tuberculosis are ex tremely rare in man, and therefore it must be concluded that the human sub ject is immune against infection with the bovine bacilli, or is so slightly sus ceptible that it is not necessary to take any steps to counteract the risk of in fection in this way.
Now, it may be submitted that at least one of the premises contained in this argument is not well founded, that the others have little or no bearing on the question. and that there still remain reasonable grounds for regarding tuber culous cows' milk as distinctly danger ous to human beings.
It cannot be denied that what may be called bovine tubercle bacilli are, as a rule, distinctly more virulent for cattle and other domesticated animals than are human bacilli, or that the re sults of experiments indicate that in natural circumstances there is little danger of cattle becoming infected from human beings. But it cannot be ad mitted that the low virulence of human bacilli for cattle proves, or even makes it probable, that bovine bacilli have only a feeble pathogenic power for man. That might have been held to be prob able if it had been shown that bovine bacilli were very virulent only for cattle; but since it is well established that these bacilli are highly dangerous for such diverse species as the rabbit, horse, dog, pig, and sheep, and, in short, for almost every quadruped on which they have been tried, it appears to be highly probable that they are dangerous to man. At any rate, it is impossible to cite any ascertained fact relating to other bacterial diseases that makes the contrary conclusion probable. It is well known that the majority of the disease exciting bacteria are harmful to only one or two species, but all those that are common to all the domesticated ani mals are also pathogenic to man.
With regard to the view that the dif ference between human and bovine ba cilli in respect of virulence for cattle is of such a fixed and constant character that it may be relied upon to distinguish the one from the other, it need only be said that that is very far from being proved.
The evidence in favor of the view that the ingestion of tuberculous milk is one of the causes of human tuberculosis in cludes a number of recorded cases in which the relationship of cause and effect a ppeareu to be obvious, The inhalation of tubercle bacilli ex pelled from the bodies of human pa tients is doubtless the great cause of human tuberculosis, and every prac ticable means of preventing infection in that way ought to lie employed; but, at the same time, one ought not to con cede to the milkmen the right to sell to us tubercle bacilli, even if one were as sured that—like Professor Koeh's ex perimental pigs—one had nothing to fear beyond the development of "little nodules here and there in the lymphatic glands" of one's necks and "a few gray tubercles" in one's lungs. John Mc Fadyean (Lancet, Aug. 3, 1901).
It has been shown, in the most unmis takable way by many feeding experi ments conducted under the auspices of the Pennsylvania State Live-Stock Sani tary Board that, contrary to the early beliefs, animals fed tubercular materials may develop primary tuberculosis and, in some instances, fail to show lesions in any other organ. It is strange that Professor Koch failed to observe the im portance of this point, for in his address lie says: "Among the animals (swine) that have been fed with sputum no trace of tuberculosis was found, except here and there little nodules in the lymphatic glands of the neck and, in one case, a few gray nodules in the lungs. In these cases the animals were infected by feeding, but did not develop tuberculosis in the intestines first, or at all." A little later he says: "The ani mals that had eaten bacilli of bovine tuberculosis have, without exception (just as in the cattle experiment), se vere tubercular diseases, especially of the lymphatic glands of the neck and of the mesenteric glands, and also ex ten-sire tuberculosis of the lungs and spleen." Here, again, is pulmonary tuberculosis from feeding, complicated, it is true, by disease in other organs; but who could say, without knowledge of the history, by what channel the bacilli entered? The whole question, therefore, is strictly bacteriological, the question being: Is the bovine tubercle bacillus virulent for man? The work that has been done in this country shows very clearly that for ex perimental animals tubercle bacilli from cattle arc in all cases as virulent and usually very much more virulent than tubercle bacilli from man. The animals used in these comparisons include her bivora and carnivora and species that arc resistant, as the horse, goat, rabbit, and dog; and species that are vulner able, as the guinea-pig, cat, swine, and cattle.