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state; we have seen that tubercle bacilli in milk are transferred to the cream, butter, and cheese made from it, and may occur in these products in greater concentration than in the milk from which they are derived; we have seen that an excellent medium for the preservation of the life and virulence of tubercle bacilli is found in butter by reason of its moist, bland, opaque character; we have been told that the medical profession is well-nigh unanimous in the view that tubercle bacilli from the bovine source in dairy products are a serious menace to public health; and we have seen that, in our fight for the suppression and eventual eradication of tuberculosis, we must seek to make harmless all the sources from which tubercle bacilli are expelled. Add to this that the available evidence regarding different types of tubercle bacilli shows that bovine types have been found in human lesions and human types in bovine lesions; that transition forms connect bovine types directly with human types; that the most variable feature about a tubercle bacillus is the character that is used to classify it as a special type; that tubercle bacilli of human types have been converted into bovine types and those of bovine types into human types; and that tubercle bacilli of the so-called bovine type are, as a general rule, more virulent than those of the human type for all animals, including man-like apes; and the conclusion is almost forced upon us that the tuberculous dairy cow is, to say the very least, one of the most important sources of tubercle bacilli with which we have to deal.
The commoner occurrence of tuberculosis in the lung than in other parts of the body should not encourage us to undervalue tubercle bacilli concealed in articles of food, as it has been shown that infection may penetrate to the lung as easily by way of the intestine as directly through the trachea and bronchi; in fact, a critical consideration of the two modes of infection-inhalation and ingestionshows that the latter is in better harmony with known facts than the former.
The normal channel through which solid material from without enters the body is the digestive canal. It has been shown by Nicolas and Descos, by Ravenel, by Schloszmann and Engle, by Calmette and his associates, and by other bacteriologists and pathologists too numerous to mention, that tubercle bacilli may penetrate rapidly through the healthy walls of the intestines and reach the great thoracic lymph duct. The thoracic duct empties its contents into one of the large veins that communicate with the heart; mixed with the blood in this vein the material from the duct enters the heart and is pumped directly to the lung, where it is filtered through the lung capillaries, which are the finest and most complex capillaries of the body. If we recall that the careful anatomical examinations made by Aufrecht and by Calmette and his associates proved that the
tuberculous processes in the lungs have their beginning in the finer lung capillaries and not in the finer air tubes, we are in a position to conclude that infected food, much more than infected air, is to be dreaded as a cause of tuberculosis.
Tuberculosis among dairy cows is so common and widespread that we can not hope to clean all dairy herds of the disease for some time to come; hence it is necessary, for the protection of health, to avail ourselves of the one expedient which is immediately at hand, and that is pasteurization. And pasteurization should not be restricted to milk, but all milk, cream, etc., used in the manufacture of butter, cheese, and other dairy products should be pasteurized unless it is obtained from healthy, nontuberculous cows that are stabled under hygienic conditions in an environment wholly free from tuberculous infection.
The elimination of tuberculosis from the dairy herd is urgently recommended, not only because the protection of public health requires it, but also because tuberculosis among cattle is a serious cause of pecuniary loss, so serious indeed that from the strictly economic point of view it must be regarded as the most important problem that those interested in animal husbandry can undertake to solve.
THE CAUSATION AND CHARACTER OF ANIMAL TUBERCULOSIS, AND FEDERAL MEASURES FOR ITS REPRESSION."
By JOHN R. MOHLER, V. M. D.,
Chief of the Pathological Division.
There is probably no disease of animal or man which is at present receiving more consideration from the practitioner, the sanitarian, and the economist than tuberculosis. Furthermore, it is one of the most prevalent diseases, and is responsible for more deaths among people and greater financial loss to stock owners than any other affection. Because of this widespread interest in tuberculosis and on account of its ravages, which extend to all the mammalia as well as birds and reptiles, it has seemed advisable to present a brief discussion of the character, causation, and prevalence of tuberculosis.
THE VARIOUS METHODS OF TUBERCULOUS INFECTION.
In the study of tuberculosis the names of two scientists stand out prominently. The first is Villemin, who in 1865 demonstrated by animal experiments that tuberculosis was an infectious disease; the second is Koch, who in 1882 isolated the causative factor of the disease, Bacillus tuberculosis. This bacillus is the only cause of tuberculosis, and is always derived from a preexisting case of the disease, whether in man or in animal. Its presence is readily demonstrated in the lesions of the affected parts as well as in certain discharges and secretions. Its peculiar ability to hold tenaciously the stain after once taking up the color despite the action of acids, and the difficulty in obtaining a growth of the organism on culture media, are facts familiar to bacteriologists.
The chief method by which the tubercle bacillus enters and obtains lodgment in the animal body is not so well known, however, and today there are two principal theories on this subject, both of which have many adherents among leading scientists. One opinion which has been adhered to for years is that the principal mode of infection in tuberculosis is by the inhalation of bacilli-laden air, thus permitting the almost direct lodgment of tubercle bacilli within the lungs, with subsequent development of pulmonary lesions, which are generally the most pronounced alterations present. The opposite
" Presented at the annual convention of the American Veterinary Medical Association, Philadelphia, Pa., September 10, 1908, as a part of the report of the committee on diseases.
view, while admitting that pulmonary tuberculosis is by far the most common form of the disease, holds that the lungs become diseased indirectly as a result of the tubercle bacilli entering the system by the mouth (ingestion), after which they are swallowed, taken up by the intestinal lacteals without any injury to the intestinal mucous membrane, pass into the thoracic duct, thence into the venous circulation, and finally are filtered out of the blood by the lungs. That this latter opinion is correct in a great number of instances is supported by numerous careful experiments, and this is probably the chief method of infection, especially in animal tuberculosis.
That there are other channels of entrance for tubercle bacilli in addition to the digestive and respiratory tracts is evident by a study of such localized lesions as the tuberculous genitals of a bull, or a local tuberculous arthritis of the hock joint, indicating, respectively, infection by way of the genital tract and by direct inoculation through the skin. Hereditary transmission, or congenital tuberculosis in the offspring, is evidently more frequent in cattle than in man, but nevertheless it must be considered as a rare form of infection. In the few cases of fetal tuberculosis that have come under the writer's observation, lesions of the maternal placenta were always in evidence. However, infection could occur by means of the semen, although such cases are believed to be exceedingly rare.
Hereditary transmission is therefore not to be regarded as an important cause of tuberculosis; the cause is rather to be found in some form of exposure to infection after birth has taken place. Predisposing causes which arise from insanitary conditions, lowered vitality, exposure, forced development, etc., only are responsible for giving the true cause (Bacillus tuberculosis) an opportunity for lodgment and development, or better facilities for propagation in case the lesions are already present.
COMPARATIVE STUDY OF TUBERCLE BACILLI.
The greatest interest attaches to the relationship of tubercle bacilli as found in their several hosts. This relationship between the tubercle bacilli recovered from the various species of mammals, birds, fishes, and reptiles is certainly very intimate. It may be that the slight differences which may now be demonstrated between the different types of tubercle bacilli have been of slow development, and due to their environment, to differences in the temperatures at which they have been forced to live, and to differences in the amount and quality of the nutrition with which they have been supplied. Whatever the variations between the types, they are not great enough to prevent the successful interchange of tubercle bacilli by means of inoculations between representative hosts of the several types.