Another important problem, one regarding hereditary immunity, is already nearer solution. I believe I am warranted in saying that, as a rule, the immunity against tuberculosis is not transmitted by the cow to her calf during intra-uterine life. One can readily be deceived by cases of apparent hereditary immunity. For example, in examining the fourteen-day-old calf of a highly immunized cow, I found the calf to possess a comparatively high degree of immunity against perlsucht bacilli. Without my previous experience with infantile immunity to guide me, I might easily have been led into the error of assuming this to be a case of hereditary immunity. Comparative tests, however, have made it very probable that the immunity, undoubtedly possessed by this calf, was derived from the milk of the immune cow with which the calf was nourished.
It will still be some time before I can present any experimental evidence to show whether a cow, less highly immunized than the one just mentioned, also produces a milk which possesses protective immunity, and if so, how far it is possible to increase this property; whether such a milk can be used to combat human tuberculosis; and finally, whether it can be transported long distances without losing this property. I have, to be sure, every reason to hope that we are on the right track when we believe that immune milk constitutes a remedy for tuberculosis with which no other remedy can even remotely compare. You will, I am sure, believe that I shall leave nothing undone which will lead to a realization of these hopes.
Right here, perhaps, it is well to say that, for the purpose of making very careful therapeutic experiments, I am delivering immune milk to only a few of my medical friends. I cannot, therefore, meet the many demands for this remedy coming from medical and other sources. They are so numerous that I cannot even answer them all. When, as a result of these clinical experiments, the conditions and restrictions are determined under which immune milk exerts its therapeutic action, I shall make public all the facts in regard to this remedy, and thus make it accessible to every one.
In the mean time I believe it will serve a very useful purpose if I publish in readily comprehensible form the scientific facts on which my plans for the suppression of tuberculosis rest. With this, of course, will come a discussion of controversial points in the field of tuberculosis investigation which have attracted the attention not only of the medical profession, but also of the laity.
I shall commence with the question as to what relations exist between human and bovine tubercle bacilli. Two years ago, in London, Koch said that the excitants of bovine perlsucht have nothing to do with human consumption. If this be true, then I must at once admit that my hopes regarding the utility of cattle immunization for human therapeutic purposes have very little foundation. It is well known that the protection afforded by immune bodies excited by the injection of any virus extends only to the same species of virus, not to that of different species. Nor, in our entire investigations in immunity, do we possess a single example showing the possibility of protecting animals or man by means of one infectious agent against the action of another of a different species. All this is now so well known that I need not discuss it. When, therefore, the researches at the Koch Institute show that it is possible to protect goats, donkeys, and cattle against perlsucht by inoculating them with human tubercle bacilli, does it not show that in these diseases there is no difference of species?
According to the recently expressed opinion of his co-workers, Schütz and Neufeld, Koch in his London address did not maintain that there is a difference in species. He merely maintained that the virus of human consumption is not identical with the virus of bovine perlsucht, and that, likewise, bovine tuberculosis is not identical with human tuberculosis. That, to be sure, would be something quite different from what the rest of the world understood. Human pulmonary consumption and bovine perlsucht are anatomically so dissimilar that it required extraordinarily clever researches and keen reasoning to lead to the discovery of a common etiological factor in these diseases. But one can easily go further than this. One can maintain without exaggeration that tubercular lupus, tubercular scrofula, and other human tuberculosis localized in joints, serous surfaces, etc., are just as little identical with pulmonary consumption as are the cases of bovine perlsucht. The term identical does not even fit all cases of consumption. Strictly speaking, a process or thing is identical only with itself. Certainly not all tubercle bacilli are identical, nor yet all kinds of tubercle bacilli. Koch’s assertion that there is an essential difference between human and bovine tubercle bacilli, and that these differences are not bridged by any connecting links, provoked the strongest opposition. This assertion has since called forth observations from all over the world which positively demonstrate the existence of intermediary stages in the virulence of tubercle bacilli derived from mammals. Generally, tubercle bacilli derived from cattle are more virulent for all animal species, thus far examined, than are human tubercle bacilli. And the opinion is constantly gaining ground that bovine tubercle bacilli are also more virulent for man.
In the scientific controversy as to whether the virus of consumption and that of perlsucht belong to the same species or not, various misunderstandings have prevented an agreement. Very often the terms “similar species” and “different species” have been used in a different sense. It is, of course, wholly a matter of individual judgment where to draw the line limiting the membership of what, in our minds, constitutes a species. Even if we apply Darwin’s definition and include under the term “species” only “organisms of similar origin,” the term is still very elastic. However, in the light of our present biological knowledge, based on Darwin’s theory of evolution, we are under no circumstances permitted to take as a criterion for like and unlike species such an easily influenced factor as the degree of virulence, i.e., the capacity for giving rise to pathological changes in another organism. Nowadays all botanists and zoologists are agreed on this, that membership in a species is determined by the origin of the organism. An anthrax culture, which has been made entirely avirulent by means of Pasteur’s procedure for reducing virulence, surely belongs to the same species as does the virulent variety from which it was derived. And in this case the difference in the degree of virulence is far greater than it ever is between human and bovine tubercle bacilli. If we keep in mind all the functional differences that have experimentally been shown to exist between tubercle bacilli which have long lived in a human body and those which have long lived in a bovine body, we shall surely not go wrong when we assume that with a little patience and expert knowledge we shall be able to make these two varieties absolutely similar again, even in respect to their virulence.
Passing now to the discussion of the real subject of my lecture I shall try to give you an idea of the distribution of tuberculosis in the human species. Although the number of persons dying of consumption is frightfully large, the number of tubercular, and tubercularly infected, persons is much larger. Not until recent years have we secured tolerably reliable data to enable us to judge of the distribution of tuberculosis in civilized countries.
If we classify as tuberculous every case in which there are pathological changes due to tubercle bacilli, then surely the oft-quoted saying of the noted Greifswald physician is true, “We are all a bit tuberculous.” It has not been easy to demonstrate this fact scientifically, and it would have been impossible of demonstration without the two epoch-making discoveries of Robert Koch, that of tubercle bacilli, and of tuberculin.
Not even a man of the scientific attainments of Virchow was able to judge what constitutes tuberculosis, although during fifty years he had endeavored to formulate data for the differential diagnosis of post-mortem findings. Since we have learned that Koch’s bacillus is the bond of union between all the various tubercular diseases, we see that many inflammatory processes which Virchow described as entirely distinct and different are nothing else than special stages in the tubercular infection. The anatomical unit of the tubercular process is the tiny, translucent gray nodule of millet-seed size described by Laennec and Virchow, the miliary tubercle. But what may develop from this may take the form of most widely divergent pathological lesions, and it was not granted to Virchow to puzzle out the endless variety of things etiologically the same. Yet it is etiologically that the vital processes must be analyzed if we wish to have a firm basis for our struggle against preventable diseases. This has more and more become the opinion of modern physicians. The progress that has been made, first in surgery and then in internal medicine, progress that only fifty years ago was declared by the most eminent representatives of medicine in those days to be impossible, has been possible only through the etiological investigations developed by Pasteur and by Koch.