# Popular Science Monthly/Volume 69/September 1906/Immunity in Tuberculosis

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* IMMUNITY IN TUBERCULOSIS
By SIMON FLEXNER, M. D.
ROCKEFELLER INSTITUTE FOR MEDICAL RESEARCH, NEW YORK CITY
I CAN not begin this address without delaying a moment to testify to my sense of the great honor which has been conferred upon me by your invitation. Neither can I proceed with it until I have expressed to you my conviction that there are persons present in this audience whose scientific work on tuberculosis makes them far abler than I to discuss the complex problem of immunity in tuberculosis. My work in bacteriology in the past has not led me to an especial consideration of the highly important problem of the prevention and cure of tuberculosis, and I can therefore account in no other way for my selection to address you this evening than that you desired this topic presented to you from the point of view of one who has done some work in the general field of bacteriology.
The modern study of tuberculosis, as you know, begins with the generation which immediately preceded the epoch-making discoveries of Koch. It may, I think, be said with justice that this study was inaugurated by the first purposeful transmission by inoculation of the disease from animal to animal. For whatever may have been the speculations upon the infectious and transmissible character of the disease before this demonstration, yet the demonstration was necessary before further steps in the elucidation of the cause and prevention of the disease could be taken. Koch in his masterful monograph gives the credit of successful inoculation to Klencke, who in the year 1843 succeeded in inducing an extensive tuberculosis of the lungs and liver in rabbits by inoculation with portions of miliary and infiltrating tubercles from man. Klencke, after accomplishing this result, did not continue his investigations and they were consequently soon forgotten. In the meantime Villemin's experimental investigations were begun and pursued to a successful termination. He inoculated not only with tubercular material from human beings, but also from cases of bovine tuberculosis, and he seemed to have proved experimentally the identity of the latter disease with human tuberculosis. Villemin's researches, from the number of his experiments, the careful manner in which they were carried out and the employment of suitable control experiments, ​appeared to decide the question in favor of the infective theory of tuberculosis. The numerous workers who repeated Villemin's experiments, after the same or modified methods, arrived at very contradictory results. The opponents of the infective theory strove to prove that true tuberculosis could be induced by inoculation with non-tubercular material. To the decision of this question Cohnheim and Salomonsen contributed largely by selecting for inoculation the anterior chamber of a rabbit's eye. The great advantage which this method possesses over all others arises from the fact that the course of a successful tubercular inoculation can be watched throughout by the experimenter until the pathological process has advanced so far that the whole organism—the neighboring lymphatic glands, the lungs, spleen, liver and kidneys—becomes tuberculous. A further point in favor of this method of inoculation is that spontaneous tuberculosis of the eye has never been observed in rabbits. It was reserved for the genius of Robert Koch to discover nearly twenty years later, in 1882, by the employment first of an original staining method, the tubercle bacillus in sections of tuberculous organs, and next by the use of a special method of artificial cultivation, to secure growths of the bacillus free from all admixture with extraneous matter. With these pure cultivations he succeeded, as you well know, in reproducing in certain domestic animals all the characteristic appearances of tuberculosis in man. Furthermore, Koch's studies of this period convinced him of the unity of causation of the various tubercular affections met with in man and also of those met with in the common domestic animals. Refusing to be daunted by the fact that tuberculosis tends to appear under different aspects in each species, and directing his attention not upon the gross appearances of the disease, but focusing it upon the microscopical appearances of the primary tubercle, which as he said recurs with typical regularity in all the different processes in man, Koch recognized the essential identity of the apparently widely different forms of tuberculosis in the various species of animals. It does not detract from the immense value of his work that Koch failed to distinguish between the tubercle bacilli isolated from the tubercular tissue in fowls, cattle and man. This failure was by no means accidental, for the possibility of the existence of differences in nature of the cultures depending upon their origins was clearly in his mind. Many of you will recall the long list of cultures which is given in the paper on tuberculosis published in 1884. In regard to this list Koch says: "It may cause some surprise that so relatively large a number of cultures was set on foot when a few would have sufficed for observing the behavior of bacilli in cultures. It seemed to me, however, not improbable that though bacilli from varying forms of tuberculosis—perlsucht, lupus, phthisis, etc., presented no differences microscopically, yet, that in cultures differences might become ​apparent between bacilli from different sources. But although I devoted the greatest attention to this point, I could find nothing of the kind. In all the cultures, whether taken from miliary tubercles, lupus or perlsucht, the tubercle bacilli behaved exactly the same."
Our knowledge of the nature of the tubercle bacillus has been increased until at this time several distinct kinds are recognized. These may conveniently be classified according to their chief sources into human, bovine and avian tubercle bacilli, and into so-called tubercle bacilli of cold-blooded animals. This last group of bacilli, which will detain us only a short time, differs greatly from the other varieties, as can readily be seen when the fact is recalled that the high temperatures—temperatures approaching blood heat—which are required for the growth of the mammalian and avian bacilli, quite preclude their multiplication under conditions of ordinary external nature. Hence they are not adapted to a life outside the living body except as cultivated artificially at this relatively high temperature. In man's conflict with tuberculosis this fact is of the greatest service, since by reason of it he is enabled to disregard the danger of any increase in tubercle bacilli outside the animal body. The relatively low temperatures at which the tubercle bacilli of cold-blooded animals develop adapt them, indeed, to an independent existence; but, as they are wholly devoid of power to cause disease in warm-blooded animals and as they would appear to have a restricted dissemination even among cold-blooded species, they are of comparatively small importance.
Of far greater consequence is the question whether the disparity which exists between the several kinds of tubercle bacilli derived from warm-blooded animals is a wide one. This question, which at first sight may appear to be chiefly of academic interest, has, in reality, far-reaching practical significance. The close relationship which man bears to domestic animals makes every fact of animal disease of high value to him. And in the case of no animal disease are facts of greater moment than in tuberculosis. Not only is the human race, by reason of its dependence upon the animal kingdom for food, work, etc., exposed to the diseases of animals which are transmissible to man, but domestic animals are also exposed to diseases of human beings. This correlative susceptibility may, therefore, cooperate to produce a vicious circle of events by which infection or the dangers of infection are kept alive and threatening. Hence it is that an effective solution of the problem of limitation of tuberculosis, whether by suppression outright or by suppression through the induction of immunity, must take into account the degree to which tuberculous animals of different species, through direct or more remote association, are a source of danger to one another.
There is no longer any doubt that the avian tubercle bacillus departs considerably from the human and from the bovine types of bacilli. ​The early observations of the Italian investigators, Rivolta and Mafucci, have been confirmed and so extended as to give us a fairly comprehensive knowledge of the capacities for pathogenic action, upon different animal species, of the avian bacilli. At the same time painstaking studies of the degree to which birds are subject to inoculation with pure cultures of tubercle bacilli of human origin support the view of diversity in type of bacilli and susceptibility of species. And yet, while fowl react only with slight local lesions, as a rule, to inoculations of tubercle bacilli of human origin, certain mammals have proved themselves fairly subject to experimental inoculation with avian bacilli. While the guinea-pig, otherwise so sensitive to inoculation tuberculosis with the mammalian bacilli, is relatively resistant to the avian variety, the rabbit, which exhibits a marked degree of refractoriness to the human bacilli, succumbs quite readily to the avian bacilli. It is, however, worth noting that the reactions in the rabbit which avian tubercle bacilli call forth do not conform to those observed in tuberculosis in general; there is absence of typical tubercles and caseation, and the chief pathological alterations observed are found in connection with the enlarged spleen.
The literature on tuberculosis contains a small number of references to the cultivation from human subjects of the avian tubercle bacillus. From our present knowledge it may be postulated that avian tubercle bacilli occur rarely in man. Rabinowitsch has, indeed, recently emphasized the occasional occurrence of the avian bacilli in cattle, swine, horses and monkeys; but they constitute a small source of danger in the spread of tuberculous disease among mammals. The parrot, because of its use as a pet and of its susceptibility to the avian bacillus, on the one hand, and of the human bacillus, on the other, is a greater menace to public welfare.
The subject of bovine tuberculosis and of bovine tubercle bacilli is among the most important of all the questions relating to the suppression of tuberculosis. The admirable studies of Theobald Smith established the distinction in type subsisting between certain bacilli of human and of bovine origin. We have come now to regard these types as separate and not to be transmuted, at least not readily under artificial conditions of cultivation, into each other. Into the disputed questions of variation due to environment I can not afford to enter. But I would have you believe that transformations of avian, bovine and human bacilli into each other have probably not been accomplished by experimentation. The cultivation of one variety of bacilli in the body of an alien species has been said to alter profoundly the properties of the bacilli; but the observations upon this point are in my opinion far from convincing. The mere fact that avian and bovine varieties of bacilli preserve their peculiar properties when occurring naturally in ​the diseased body of an alien species—man, for example—tends to discredit the experimental transmutations referred to.
Bovine tubercle bacilli are characterized, as ascertained by Smith, by a greater degree of pathogenic power for mammals in general than human bacilli, with which fact is correlated certain peculiarities of cultural and physiological properties serving further to separate the bovine from the human bacilli. The bacilli of mammalian origin are, perhaps, closely related and less removed from each other by the sum of their properties than they are from the avian bacillus. With the few exceptions mentioned all forms of mammalian tuberculosis are caused by either the human or the bovine bacillus.
In view of the general fact that the bovine bacilli show a greater degree of pathogenic action for the lower mammals than the human bacilli, it was natural to assume that bovine bacilli would be powerfully pathogenic for man also. To test this probability directly by experiment is, of course, not permissible. But the belief that tuberculosis in cattle is a menace to man is expressed in the many regulations by which it is aimed to control and prevent the use as food of products derived from tuberculous animals. It was not until Koch's address was delivered in 1901 that any serious doubt existed in the minds of sanitarians and pathologists that tuberculous cattle offered a source of danger to man. The specific knowledge which has accumulated since that date has served to establish the transmissibility in some degree of bovine tuberculosis to the human subject. The inherent difficulty and tediousness of the investigation of the specific types of tubercle bacilli existing in human cases of tuberculosis necessarily limit the total number of instances in which it has been established, beyond peradventure, that the bovine type of bacillus does occur in tuberculous processes in man. In this country the responsibility of refuting the too general statement of Koch has fallen chiefly upon Ravenel and Theobald Smith, whose admirable studies in this direction are of a convincing nature.
If we pause for a moment to consider upon what data Koch based his statement of the independence and non-communicability of tuberculosis in cattle and man, we shall appreciate that, in so far as he dealt with established fact and not hypothesis, he had long been anticipated. That cattle are highly resistant to infection with tuberculous material and tubercle cultures obtained from human subjects can be concluded from the early experiments of Baumgarten, Sidney Martin, Frothingham and Dinwiddie. The most conclusive evidence upon this subject is contained in Theobald Smith's paper of 1898. in which he summarizes his experiments by stating that "putting all the facts obtained by experiments on cattle together, it would seem as though the sputum bacillus can not gain lodgment in cattle through the ordinary channels." ​In view of these facts, it is not surprising to find that Koch and Schütz later failed to produce marked or general tubercular infection of cattle by feeding or inoculating directly into the circulation tuberculous materials and cultures of tubercle bacilli of human origin. That this result does not dispose of the entire question at issue, but leaves open the important consideration of the implantation of the more virulent bovine bacilli upon man was, of course, present in Koch's mind, and was met by him by emphasizing the infrequency with which primary intestinal tuberculosis, which is the form of tuberculosis presumably arising from ingested virulent tubercle bacilli, is encountered in human beings. The reports which have appeared since have tended to show that primary tuberculosis of the abdominal viscera, especially in children, is not so infrequent as Koch believed it, and the researches inspired by Koch's address have brought out the important fact, now based upon actual observation under the microscope, that tubercle bacilli may pass through the intact intestinal wall and reach, by means of the lymph current, the mesenteric glands; and have made it seem probable, also, that by entering or being carried into the blood vessels in the intestine the bacilli may be carried to the lungs. When all the known facts of food infection in tuberculosis are assembled, they make quite an imposing array, for they indicate, quite in opposition to the exclusive view expressed by Koch, that tubercle bacilli entering the body with food may be implanted upon the mucous membrane of the mouth, from which, probably, chiefly in the region of the tonsils, they may be carried to the lymphatic glands of the neck and adjacent parts where they develop and produce tubercular disease; or they become implanted upon the intestinal mucosa and pass the epithelial barrier without first causing disease there, and set up lesions in the mesenteric lymph nodes or even be transported by the blood or lymph to the distant lungs; or they may first multiply in the intestine, cause tubercular disease there, and then migrate further, involving the abdominal and thoracic organs.
If I have seemed to tarry too long over this aspect of my subject, I will ask you to consider for a moment in how far the endeavor to limit the spread of tuberculosis among the human race must be influenced by the avenues of infection to which the race is exposed. If we side with Koch in the view expressed in 1901, and reiterated just the other day in his Nobel-prize address, that, as he says, human tuberculosis and tuberculosis in cattle are so distinct from each other that the latter is not to be feared as transmissible to man, at least, as his last utterance puts it, not in a form which comes in consideration in regard to tuberculosis as a 'Volkskrankheit' or race disease, then it is only necessary to direct efforts to the suppression of tubercle bacilli of human origin. For, if the danger of infection of surroundings and ​healthy individuals is limited to the expectoration of persons suffering from tuberculosis of the lungs and upper air passages, the problem before us, while still very large, is less by a considerable amount than if there must also be taken into account the widely prevalent disease among cattle, swine and other domestic animals. While I do not pretend to speak in terms of great authority, yet it would seem to me that the time is not yet ripe to disregard, in attempting to suppress tuberculosis, the disease in domestic animals. Greatly as I sympathize with the active propaganda which is being made by instruction and material help to protect tuberculous human beings from injuring themselves and others, and greatly as I hope to see promoted the means of caring for the tuberculous in sanatoria, etc., yet I hope that there may occur, at this time, no relaxation in the efforts being made to control the spread of tuberculosis among cattle and to prevent the consumption of infected milk and flesh by man and other animals. That, on the other hand, the suppression completely of tuberculosis among cattle would not be followed by a great reduction in the morbidity due to tuberculosis in man is shown by Kitasato's statistics from Japan. In that country the human disease prevailed with its usual activity at a time when the cattle disease, owing to the absence of cattle, was unknown, and milk formed no appreciable element in the food of children.
In dealing with the complex problem of tuberculosis—a problem whose difficulties enlarge with the continued growth in size of cities—we are materially assisted by the knowledge of the manner in which the virus of tubercle is separated from the diseased body, the conditions of its contamination of our environment, and the avenues through which it endeavors to enter the healthy body. Though it is, perhaps, scarcely to be hoped that a time will arrive when tuberculosis will have become, through precautions against infection, as rare as are to-day smallpox and typhus fever, yet it is a most hopeful result of the crusade against tuberculosis that a marked reduction in the mortality, and probably in the incidence of the disease, has been going on in some countries—as, for instance, in England—for forty years. In New York, the system organized by Biggs has brought about a reduction since 1886 of 35 per cent, in the mortality of the disease; and while in Prussia the mortality was stationary in the decade from 1876 to 1886, since that time a reduction of more than 30 per cent, has been noted. These figures show what may be accomplished in reducing the dangers of infection with tuberculosis by a régime of education, improved conditions of living for the poorer classes, and the segregation in hospitals and sanitoria of any considerable number of the infective tuberculous during the most dangerous period of the disease.
The discovery of the microbic agent of tuberculosis naturally awakened the hope that a specific means of treating and, possibly, of ​preventing tuberculosis might now be found. The early years following the cultivation of the tubercle bacillus saw no realization of this hope, and to-day we are still far from the desired goal. However, the prodigious labor which has been expended in the search for a means of protection against infection with the tubercle poison has not been wholly devoid of results.
In an address of this kind it is not practicable to deal with the separate contributions, in detail, which the many workers have made to the subject of immunity in tuberculosis. The most that can be accomplished is to bring together the more important results of all the workers and, after having assembled them, to judge of their value and to consider, possibly, in what important respects they are still imperfect. I can not do better, at the beginning, than to remind you that the successful point of departure has been the discovery that variations in type and in virulence exist among tubercle bacilli. The earlier view which taught that the tubercle bacillus is a micro-organism of uniform and fixed virulence has been shown to be erroneous, first by the discovery of variations according to certain origins, and second by a gradual decline in pathogenic power suffered by certain strains through long cultivation outside the animal body.
The animals which have been of special use for tests of immunity are rabbits, cattle and goats. The guinea-pig, which furnishes an almost ideal animal for the detection of tuberculosis, because of the sensitiveness of its reaction to inoculations with tubercular material, fails, for the same reason, to be a highly suitable animal in which to carry out tests of immunity; and yet it has been employed with some success.
The first important contribution to the subject of experimental immunity in tuberculosis was made by Koch in connection with his researches on tuberculin—a product of the growth in broth of tubercle bacilli, freed from the bacilli and concentrated. In spite of the failure of tuberculin to bring about a favorable issue in all cases of human tuberculosis in which it is administered, it still remains a useful, perhaps the most useful, strictly medicinal agent employed for the treatment of tuberculosis.-But the sum of its useful properties is not embraced in its employment as a therapeutic substance: it is also a diagnostic agent of high value, and its action upon the tuberculous organism is so specific and remarkable that it has proved itself of the greatest importance and aid in the effort to unravel the complicated series of biological phenomena which constitute the tubercular state.
It is possible to increase somewhat the resistance of animals to tubercular infection by previous treatment of tuberculin; but this increase is not remarkable. It is possible to bring about arrest of the tubercular process in the infected organism by means of tuberculin: ​and in some instances this arrest leads, through the changes induced in the tuberculous tissue by means of the tuberculin injections, directly to cure, or indirectly, through an increased power of resistance and attack on the part of the forces of the organism, to eventual cure. But a high and lasting degree of immunity has never been secured by the use of tuberculin. This fact, disappointing as it was at first, is now easily explicable. Tuberculin does not represent the entire series of forces contained in the bacilli which the body has to resist in preserving itself from infection with tubercular poison. The peculiar principles contained in tuberculin are, indeed, not highly toxic for the normal individual; and our experience in securing immunity to micro-parasites and their products has taught us that where no reaction or response to the introduction of the foreign poison is called forth, no degree of protection to larger doses or more virulent poisons of the same nature is to be expected. Toxic as is tuberculin to the tuberculous organism, it is almost innocuous to the tubercle-free body. It has been found, in keeping with this distinction, that the normal animal shows after tuberculin treatment evidence of the minimal production of the neutralizing or antibody for the tuberculin, which, were tuberculin a direct poison for the tissues, would probably be produced in larger amounts. On account of this absence of action on the normal organism it has been thought that the active principle in tuberculin does not exist in a free state, but occurs in some combination, from which the tuberculous, but not the non-tuberculous, organism can free it, and that the separation takes place in the tubercular foci upon which the specific action of the poison is directly exerted. If this view is correct then the failure of tuberculin to exercise any profound action on the healthy organism is easily grasped.
Increased knowledge of bacterial infection and immunity has taught us that in case of bacteria which invade the depth of the body and produce their peculiar effects by reason of their immediate presence, we can not expect to achieve marked immunity through the use of the soluble gross-products of the parasites. The reaction of the body to the invasion depends not upon the presence in the invader of one set of toxic principles, but of many, some of which are contained in the solid substance of the micro-parasite and do not go over into the fluids in which they multiply. Thus it has been found, in case of certain bacteria, that a degree of immunity or protection which it is impossible to obtain even after very prolonged treatment with the fluid portions of cultures, can be secured quickly when small quantities of the living or even dead microorganism are injected into the body. A high degree of bacterial immunity has been secured up to now for a small number of micro-organisms by vaccination—by the method introduced by Pasteur—for several animal diseases, notably anthrax or splenic fever, ​fowl-cholera and black-leg. In these cases the living attenuated microorganisms are employed.
Neither lasting nor marked immunity in tuberculosis can be obtained by the inoculation of cultures of tubercle bacilli killed by heat, sunlight or other agency. Dead tubercle bacilli are poisonous and bring out a striking reaction of the organism, but this reaction does not confer immunity to subsequent inoculations of the living germ. It may well be that the dead bacilli, especially if reduced to impalpable powder so as to facilitate absorption, may after injection raise the powers of resistance in the organic forces, although the height of the sustained forces is not sufficient to enable the body to throw off completely the living infecting organism. It is easy to prove that the animal organism is modified by the development within it of the tubercle bacilli; and merely disposing of dead bacilli increases its power of reaction against a second injection of dead tubercle bacilli; the second action being much more vigorous than the first. (Theobald Smith.) The experiments of Koch which immediately preceded the discovery of tuberculin clearly demonstrated that tuberculous guinea-pigs into which tubercle bacilli are reintroduced subcutaneously react in a very especial manner. An active inflammatory process develops about the site of second inoculation which eventually brings about the expulsion of bacilli with the exudations; a voluminous slough forms, which, when shed, carries with it a large number of bacilli; and this shedding is followed neither by the formation of a permanent ulcer nor hypertrophy of the neighboring glands, a regular result of the primary inoculation. The tubercular organism reacts in the same manner to dead as to living bacilli; the tuberculous animal has acquired immunity against reinfection or reintoxication by the tuberculous virus, which, however, in no way prevents the first inoculation from becoming generalized and setting up a tuberculosis of almost all the organs.
If we attempt an interpretation of these phenomena we can conclude that the organism, once it is poisoned with tubercle virus, becomes supersensitive to the tubercle poison. This supersensitiveness is displayed in the manner of reaction upon re-inoculation of the tuberculous organism to tuberculin and to dead and living tubercle bacilli. But the organism poisoned with dead tubercle bacilli is not in reality tuberculous; it is, however, sensitized. In keeping with this distinction, it can be said that while the tuberculous organism has acquired a degree of immunity to reinfection, the organism merely poisoned with tubercle bacilli has failed to develop this state of resistance.
The experimental results, which I shall relate to you, upon which are based our belief in the artificial production of immunity to tuberculosis, were all obtained by the use of living bacilli. It would, therefore, seem as if in the course of their residence and development within the ​body the immunizing organisms behave differently from those in artificial cultivations. This difference in behavior could be accounted for on the supposition that under conditions of parasitic life, surrounded as the bacilli are with complex fluids and more complex cells, they form, in their growth, products which either are distinct from those which are formed by them in cultures, or these products, in statu nascendi, are acted upon and modified by the active and labile ferments in the fluid and protoplasm of cells, with which the growth-products must come into immediate contact. Professor Welch, to whom this variation in behavior of bacteria under parasitic and saprophytic states of existence was fully apparent, endeavored a few years ago in his Huxley lecture to explain the difference in activity of bacteria growing within and outside the body by supposing that in the body they are induced to secrete substances the stimulus to the production of which is absent in the culture tube. However this may be, it is evident that the only form of immunity in tuberculosis which deserves the name has been obtained by the employment for inoculation of living cultures of the tubercle bacillus.
Although the earliest experiments which had for their object the production of immunity in small animals by means of previous inoculation of products of the growth and of attenuated cultures of the tubercle bacillus were published in 1890 (Martin and Grancher, Courmont and Dor), yet, I think, the first really promising, because successful, achievements of this end were made by Trudeau in 1902 and 1903 and by de Schweinitz in 1904.
Trudeau protected rabbits from virulent tubercle bacilli by first injecting them with a culture of bird tubercle bacilli, the subsequent injection of virulent mammalian bacilli being made into the anterior chamber of the eye. The rabbits to be protected were twice injected subcutaneously at intervals of 21 days with cultures of the avian bacilli. About one in four of the rabbits died within three months, profoundly emaciated, but without tubercular lesions. The remaining animals recovered and were apparently in good health, when, with an equal number of controls, they were inoculated in the eye with a culture of mammalian tubercle bacilli. The results are instructive: In the controls little or no irritation following the operation is observed and the eye remains quiescent or nearly so for about two weeks, when the changes described in the early parts of this address manifest themselves. After a few weeks general inflammation of the structures of the eye develops, the inoculation wound becomes cheesy and the eye is more or less completely destroyed. The disease, however, remains usually localized in the eye for many months, and may remain there permanently, depending upon the virulence and number of bacilli injected.
​In the vaccinated animals, on the contrary, the introduction of the mammalian bacilli at once gives rise to a marked degree of irritation. From the second to the fifth day the vessels of the conjunctiva become engorged, and evidences of marked inflammation appear in the anterior chamber and on the iris (reaction of immunity). However, at the end of the second to the third week, when the eyes of the controls begin to show progressive and steadily increasing evidence of inflammatory reaction, the irritation in those of the vaccinated animals begins slowly to subside and the eyes to mend. In from six to twelve weeks, in the successful cases, all irritation has disappeared and the eyes present only the evidences of traumatism and inflammation. This experiment leaves no doubt of the protective influence exerted by the first inoculations of the avian bacilli and clearly establishes that related cultures of tubercle bacilli of moderate virulence for an animal species, can afford protection to subsequent inoculation with special and more pathogenic strains of the bacillus. Notwithstanding the fact that, as Dr. Trudeau records, some of the protected animals slowly relapse and the disease resumes its progress, although by almost imperceptible stages, the experiment still shows that protection, not absolute immunity, from tuberculosis may be obtained in rabbits by a species of vaccination.
De Schweinitz in 1894 reported certain experiments which he made on guinea-pigs and cattle. He inoculated the former with a culture of tubercle bacilli of human origin cultivated for about twenty generations in broth. This culture was of a low grade of virulence for these animals, but it served to protect them to such an extent that when they were afterwards inoculated with tuberculous material from a cow they remained healthy, while control pigs injected with the same material became tuberculous and succumbed in about seven weeks. De Schweinitz injected large quantities of human tubercle bacilli into cattle—beneath the skin, into the peritoneal cavity and into the circulation—without injury.
I may, at this time, digress for a moment and leave the more strictly chronological method of presentation, to allude to the set of experiments on the protection of guinea-pigs from tuberculosis, which Trudeau reported to the National Tuberculosis Association at its last meeting. The special merit of this experiment is that it shows the existence of a connection between virulence and infectivity in the germ and its capacity to confer immunity. Unless the bacillus has the power to gain some foothold in the body it affords no protection; if on account of high pathogenic power or virulence, it easily gains a foothold, then it brings about infection. To choose a culture of tubercle bacilli of just the right grade of virulence is one of the conditions, apparently, of successful experiment, as it must also be, in view of this fact, one ​of the difficulties of the method. The same difficulty has been encountered in the practical carrying out of this method of immunization in cattle. Several series of guinea-pigs were inoculated with tubercle bacilli as follows: (a) with dead bacilli, (b) with living bacilli from cold-blooded animals, (c) with a culture of human bacilli cultivated artificially for more than twenty years which produces on inoculation no appreciable local lesions and never tends to generalize, and (d) another human culture cultivated artificially for more than fourteen years, which still causes in all the pigs slightly enlarged inguinal glands near the site of inoculation, and occasionally brings about slight caseation of the nodes with a tendency to partial generalization of the virus. The dead bacilli and the bacilli from cold-blooded species gave no protection; the second human culture, by reason of its greater invasive properties, protects better than the first, which is almost devoid of power to grow in the animal body. In no case, however, was the growth of the virulent bacilli wholly suppressed.
In man the question of acquired immunity has been answered by many authorities, as far as the main considerations go, in the negative. A large number of well-observed facts demonstrates that a person who has suffered from localized tuberculosis of the lymph glands—scrofula so-called—or other form of local tuberculosis, can not count upon an immunity from pulmonary tuberculosis. And yet it can, I think, be shown by reference to statistics that in man there exists a refractory condition which becomes increased after infection, since the number of persons who have been the victims, at some period of their life, of a tuberculous infection, is very large in comparison with the number who die of this disease, or the even larger number who develop severe forms of it. Hirsch gives the mortality of tuberculosis as compared with deaths from all other causes as 3:22, in other words, tuberculosis claims as victims of death 1 in every 7 persons. This proportion does not, however, express the morbidity from tuberculosis, which is. in reality, far greater than these figures indicate. It is difficult to secure by vital statistics reliable data of the incidence of tuberculosis; but trustworthy observations made at autopsies upon human beings indicate that as many as 90 per cent, of persons, dying from all causes, have at some period of their life been the victims of a tubercular infection. In far the greater number of instances the disease remains fixed in the bronchial or other lymphatic glands or the apex of the lungs and exerts no injurious effect upon the organism as a whole. We may, therefore, fairly conclude that the human organism possesses a strong inherent tendency to overcome infection with the tubercle bacillus. So much can be safely predicated. But whether the suppression of a local infection, such as I have described, gives an increased capacity for overcoming subsequently invading tubercle bacilli remains for the present an open ​question. It is certainly not disproved by the facts cited; and some authorities hold fast by the belief that a degree of immunity to tuberculosis may be acquired by man.
In the year 1901, on December 12, on the occasion of his acceptance of one of the Nobel prizes, Behring announced that he was engaged upon the study of artificial immunization of cattle to tuberculosis. In this address the claim was made that a method had been perfected whereby it was possible to vaccinate cattle successfully against tuberculosis. These experiments consisted in the endeavor to immunize cattle by means of tuberculin, other toxins, so-called, from the tubercle bacillus, dead tubercle bacilli, bacilli weakened with chemicals and living, active cultures of the tubercle bacillus. In the four years which have elapsed since this announcement was made a series of monographic papers bearing on this subject has appeared from Bearing's laboratory in Marburg. The plan of immunization has, in this time, undergone a number of modifications until now it consists in the inoculation intravenously of young cattle—calves twelve weeks old preferably—with a standard human culture, which is now furnished commercially. A second inoculation of an increased quantity of this culture is injected three months later. Cattle treated in this way are regarded as highly immune and are denominated by Behring as 'Jennerized.' If to them a dose of a virulent bovine culture of tubercle bacilli is given, no permanently bad results follow, although an equal dose of the virulent culture will cause, in an unvaccinated animal, the development of generalized tuberculosis leading, in a few weeks, to death.
In his endeavor to find a culture of the tubercle bacillus which would fulfill the requirement of producing a transient illness and leave protection behind, Behring discovered that not all tubercle bacilli of human origin were without danger to cattle inoculated with them. We were, indeed, not unprepared for this announcement, since, in the first place, we had learned that in some instances tubercle bacilli of the bovine type have been cultivated from examples of human tuberculosis, and, on the other, that not all the bacilli, of any type, exhibit equal degrees of virulence. The culture employed by Behring, although it has now been employed to inoculate several thousand cattle, is said never to have produced severe disturbances of health; even when animals already tuberculous are inoculated the results are not serious: fever lasting several days sets in, the animals may cough, and they may eat less and lose somewhat in weight, but even they return to what is for them the normal.
It would appear that McFadyean is entitled to the credit of the discovery equally with Behring of the immunization of cattle from tuberculosis; and, indeed, there is reason to believe that his results ​even anticipated those of Behring. By using for injection first tuberculin and then in succession tuberculin and tuberculous material containing bovine and possibly human tubercle bacilli, McFadyean succeeded in increasing the resistance of several cattle to artificial tubercular infection.
Pearson and Gilliland, 1902, in this country early published accounts of some experiments which they carried out upon the immunization of cattle from tuberculosis. They employed a culture of human tubercle bacilli for producing immunity and found that subsequently the protected animals, as compared with the controls, which all succumbed to the virulent inoculation, either developed no lesions or very inconsiderable ones upon being given large quantities of highly pathogenic bovine cultures. As far as I know these experimenters are the only investigators who have endeavored to carry the principles of the method a step farther, so as to bring about arrest of the disease in cattle already tuberculous. While it is unlikely that such a therapeutic use of 'Vaccination' will ever be made in veterinary practise, the facts are of considerable theoretical interest, especially in view of the somewhat similar means employed to arrest tuberculosis in man.
The immense importance to scientific agriculture of the matter of immunization of cattle from tuberculosis and the even greater collateral interest which the subject has for man, as enlarging the possibilities of immunity even for him, have led to a discussion on the priority of the discovery between Neufeld, a pupil of Koch, and Behring. It would appear from Neufeld's writings that, while working under Koch's direction, he ascertained as early as 1900-1901 that large animals—donkeys chiefly, but cattle also—could be protected from artificial infection with virulent tubercle bacilli, always fatal to control animals, by previous treatment with tubercle vaccine, of which several different preparations were studied. It is not within the scope of this address to apportion the credit of priority; but in any case, assuming the facts to be as stated by the contestants, McFadyean should receive as great credit as either of the others, if not the chief credit. The principle which all the investigators employed is not new in experimental medicine, but has come to us from the genius of Pasteur. It may, however, be said that our knowledge of the tubercle bacillus and its varying activities had by the year 1900 become so much enlarged that the possibility of putting the facts of the newly discovered properties to a practical test of immunity occurred to the several independent workers in bacteriology. There can, I think, be no doubt that Behring deserves the credit of making the protection of cattle from tuberculosis a feasible, practical object of study. This alone is a merit of no small order.
From the mere fact that cattle have been successfully protected from infection by the tubercle bacillus, even under the severest ​conditions of laboratory experiment, it can not be concluded that they will be equally refractory when exposed to the natural sources and modes of infection. In the laboratory the virulent infectious agent is brought into the animal by injection, under the skin, into the serous cavities or into the circulation, which are avenues through which in the natural disease infection rarely if ever takes place. And while this mode of introduction of the virulent bacilli into the body may, theoretically, be more severe than their introduction into the lungs with inhaled air, or into the stomach through infected stalls and food, yet the profound differences in the defenses of the body with which the bacilli come into conflict, under these different circumstances, may, after all, determine the issue in a manner quite contrary to our expectations. It is, therefore, of the highest interest to learn that in their later tests Behring and his co-workers exposed vaccinated cattle to stalls and herds which were known to be badly infected, with the result that at the time of the report, they had apparently escaped infection. I am enabled through the courtesy of a private communication from Dr. Pearson to state that cattle vaccinated by himself and Gilliland which were kept for two years under natural conditions of infection have not contracted tuberculosis, while the control animals, exposed to the same conditions, have all developed the disease, some dying spontaneously by reason of the severity of the infection. Dr. Pearson also informs me that their experiments indicate that the degree of resistance bears a rather definite relation to the number of vaccinations given the cattle. No cattle vaccinated three times with their standard vaccine—a living culture of tubercle bacilli of human origin—have developed tuberculous lesions even after two years' severe exposure. In their experience, two injections of Behring's vaccine do not always suffice for such heavy exposure as they employed.
As regards the question of duration of the protection, it may be said that Behring, basing his views on results of vaccination made three years before, expressed the belief in 1901 that it would endure during the life of the animal. As young healthy cattle are vaccinated before they fall victims to infected stalls and herds, it would seem as if infected herds might therefore gradually be replaced by healthy ones. The gain, this being true, would be almost incalculable to agriculture.
I am in the fortunate position of being able to bring before you a critical summary of the subjects just presented by one wholly conversant with its practical as well as its theoretical aspects. Through the courtesy of Dr. Leonard Pearson I have been enabled to read the advance sheets of a review on immunization in tuberculosis which will soon be issued from the Phipp's Institute. Dr. Pearson concludes that there appears to be no doubt that different cultures of human bacilli have different immunizing values. Some can not be used at all because ​they are of too high and others, possibly, because they are of too low, virulence for cattle. There is also need for comparison in immunizing value of fresh cultures and cultures that have been dried in vacuum and reduced to powder. Some observations appear clearly and strongly to indicate that the fresh cultures are preferable. Although it has been shown that vaccination can be practised so as to be entirely harmless to the animals, yet, on the other hand, it is not always unattended with danger. What is the shortest and most economical procedure for the protection of cattle on a large scale is still to be established. Only prolonged observation of carefully recorded results of vaccinations practised on a large scale can settle this point. The question of duration of immunity is still an open one. It has been shown that the immunity endures a year. To say, at the present stage of the studies, that it will last during the entire life of an animal is to make a statement for which there is no experimental proof. Modes of vaccination, as illustrated by the intervals between the successive injections, differ greatly. Behring recommends an interval of three months, while others have obtained a high degree of immunity by repeated injection at short intervals. As artificial immunity is relative and not absolute it need not excite surprise that the immunity to the tubercle bacilli can be overcome by the injection of large quantities of active bacilli. What is desired in practise is a degree of immunity that will suffice to protect animals from acquiring the disease under natural, and consequently highly variable, conditions. In some herds, where the natural disease prevails in a mild form, a lower degree of immunity may suffice than in other herds in which the disease is more severe and wide-spread. We are, therefore, at the beginning of this complex and highly important subject. These are Dr. Pearson's conclusions.
There is another aspect of this subject which demands attention. When it is recalled that immunity in cattle is obtained by the injection of living human tubercle bacilli the question arises whether this procedure is wholly free from danger to the consumers later of the flesh and milk of these cattle. It would appear that the human bacilli do not excite in cattle the tubercular lesions, in which doubtless the bacilli are so enclosed as to be, to a considerable degree, protected from perishing. It is equally true that as the living micro-organism can not be replaced by dead ones in bringing about immunity, the immunizing process is in some way bound up with their survival and even, possibly, with a restricted multiplication. Hence it is necessary that we ascertain, first, how long the human bacilli survive in the organs of the vaccinated animals, and second, whether they are ever eliminated with the milk of cows. The observations already made upon these points are so few as at present not to be useful for any scientific deductions. But before the method is too implicitly relied upon these questions should be answered.
​It is an interesting subject of speculation as to what the result will be when cattle in general, and possibly, man later, shall have been immunized to tuberculosis. Will the race of tubercle bacilli disappear in large measure from the world? This would indeed be a beneficent result. But Dr. Smith has pointed out in a recently delivered address that doubtless host and parasite eventually come to hold a kind of equilibrium to each other, and hence an increased degree of resistance in the former might tend to bring about that selection among the parasites through which races of greatly augmented power for invasion would be produced. If this were true, and he even suggests that the natural process of weeding out the weaker among the human race tends to this result, the parasite would try to keep up with the host as his resistance increased until a point was reached beyond which further enhancement of power was impossible. Would the higher animal or the lower vegetable organism finally claim the victory? We need perhaps at this moment not to relax our efforts to achieve a practical immunity for man as well as for animals because of this future danger. I am not aware that the smallpox germ has increased measurably in virulence since vaccination became general, but I would also add that a century is a small period of time in the life history of any living organism.
Before closing this address I should like to refer briefly to the new interest which has been excited in the use of tuberculin in the treatment of human tuberculosis by reason of the application to the study of tuberculosis of a method introduced by E. A. Wright, of London, whereby it is held that the exact effect of the tuberculin injection can be measured and controlled. The method consists in the determination of the capacity of the blood leucocytes to take up tubercle bacilli when the blood and the bacilli are brought together outside the body in a test tube. Wright and his pupils have worked out the normal power of the blood to cause the englobing of the bacilli; and they have noted a diminution of this capacity in the blood of many persons suffering from tuberculosis. They speak of this englobing capacity of the blood as 'opsonic index,' from the word meaning to prepare—to cater for; since the bacilli must first be prepared by substances in the blood serum before they can be ingested by leucocytes. The injection of tuberculin, when cautiously done, tends to bring about a rise in the tuberculous, of the 'opsonic index' which Wright believes is a measure of the good done, as an increase in immunizing substances in the blood is the cause of the rise. He also discovered that time is required for the occurrence of the rise and that the immediate result of the injection is a fall of the index—so-called negative phase. This latter must be permitted to pass away and be succeeded by the positive phase before another injection is given. Gradually the 'opsonic index' is driven up in the cases that are favorable to the treatment.
​I do not intend to discuss the value to the clinician of this interesting method and Wright's observations based upon it. The subject appears to me to be one of great intricacy and therefore to be approached in a spirit of proper criticism despite its evident allurements. My purpose in mentioning it at all is to bring again to your attention a method of exciting the tuberculous body to put forth an effort at self immunization which is sometimes efficient to a marked degree. It is not the injected tuberculin that accomplishes directly the changes in the condition of the patient, for there already exists, doubtless, an excess of similar poisons in the tuberculous foci in the body. The healthy body, indeed, does not react in this manner and is not to be protected, enduringly, from tuberculous infection by a previous treatment with tuberculin. As Koch's phenomenon shows the tuberculous organism to have developed defenses against subsequent tuberculous infection which the normal body does not possess in equal degree, the employment of tuberculin indicates that the diseased body can be aroused artificially to put forth a stronger effort than its unaided natural forces enable it to make, in order that the disease may be overcome. Herein resides a great principle, an immense power for good, and, consequently, a great hope for future progress in the rational and specific treatment of tuberculosis in man. Efficient efforts at suppression of the causes of tuberculosis, deeper knowledge of the principles of bacterial immunity, are the two forces which in time may stay the ravages of the 'White Death.'