Outlines of dairy bacteriology, 10th edition / A concise manual for the use of students in dairying

by H. L. Russell; E. G. Hastings

Tuberculosis. This disease is by far the most important bacterial malady that affects man and beast. In man, it assumes a wide variety of phases, ranging from consumption, tuberculosis of the lungs, which is by far the most common type, to scrofulous glands in the neck, cold abscesses, hip-joint, and bone diseases, as well as affection of the bowels. These various manifestations are all produced by the inroads of the specific organism, Bacillus tuberculosis. The bovine, as well as swine, fowls, and other warm-blooded animals, are also affected with similar diseases. In man, the importance of the malady is recognized when it appears that fully one-seventh of the human race die of this scourge. In cattle, the disease is equally widespread, particularly in those countries where live stock has been intensively developed. In the northern countries of Europe, such as Denmark, Germany, England, France, and the Netherlands, as well as in Canada, and this country, this disease has been most widely disseminated. This has been occasioned, in large measure, because of the exceedingly insidious nature of the disease in cattle, thereby permitting interchange of such diseased stock without the disease being recognized. Tuberculosis is found more abundantly in this country in dairy than in beef stock. Dairy cattle are, however, not more susceptible, but the closer environment in which milch cattle are kept, and the fact that there has been greater activity in the matter of introducing improved strains, accounts for the larger percentage of affected animals.

It has been a disputed question for some years whether the organisms producing bovine and human tuberculosis are identical or from the practical standpoint, whether the bovine type of disease is transmitted under natural conditions to man. The bacteriologist can readily detect differences in appearance, in growth of cultures, and in disease-producing properties between the two strains. Of the two, the bovine is much the more virulent when inoculated into experimental animals. In a considerable number of cases, record of accidental infection from cattle to man has been observed. These have occurred in persons making postmortem examination on tuberculous animals, and the tubercular nature of the wound proven by excision and inoculation.

More recently, since the agitation by Robert Koch of Germany, a number of scientific commissions have studied particularly the problem of transmission. It is now estimated that perhaps seven per cent of the tuberculosis in man is of bovine origin. This is almost wholly confined to children. The portions of the body that become diseased, when the infection has resulted from the use of milk, are the glands of the neck and of the abdomen.

Manner of infection in man. In the main, the source of the malady may be traced either to air infection or to the food, if one disregards the comparatively small number of cases of wound infection. Air is frequently a medium by which the germ is transferred from one person to another. The sputum is exceedingly rich in tubercle bacilli and since this material is carelessly distributed by tubercular people, the air of the cities, villages and public buildings will frequently contain tubercle organisms. Some of the organisms in the air find their way into the lungs, there to develop and produce consumption. The organisms in the air may be deposited in the nasal passages and throat, and ultimately find their way into the tissues of the body by penetrating the walls of the throat or of the intestine. It is probable that the tubercle bacilli thus introduced may find their way to the lungs and there develop without leaving any trace of their path.

Food may also possibly serve as a medium of infection. The contamination of solid food from flies and other sources is, of course, a possibility, but tuberculous meat from cattle and swine is much more likely to occur, although it must be said that the processes of preparing such food for use (roasting, frying, and boiling) are sufficient to destroy the vitality of the causal organism. The fact that most food products of this character are now inspected renders this possibility less likely to occur.

Unquestionably, the likelihood of ingesting tubercle organisms is much greater with milk than with any other food supply, as milk is consumed usually in an uncooked state, and as microscopic and physiologic tests indicate that not infrequently milk from tuberculous animals contains these organisms.

Distribution of the disease in animals. As practically any organ of the body may be affected with tuberculosis, it naturally follows that the lesions of this disease are widely distributed. The disease germ is introduced, in the main, through the lymph and not the blood system; consequently, in the initial stages the evidence of tuberculosis is often comparatively slight, and the lesion is restricted in its development. Where such a condition obtains, it is known as "closed," in contradistinction to "open" tuberculosis, where the diseased tissue is more or less broken down and is discharging into the circulation, or elsewhere. Manifestly, the danger of spreading not only in the affected animal itself, but to the outside, is much greater in the case of the open lesion. Especially is this true where the disease is present in the lungs or organs that have an exterior opening so that the material containing the organisms is discharged from the body in the sputum, manure, urine or milk. The intestines themselves are rarely affected, but the lymph glands associated with the intestinal tract are not infrequently involved.

Infection of milk with tubercle bacilli. In a small percentage of cases, the udder itself becomes involved. Where this condition obtains, one or more hard lumps are formed, which slowly increase in size, usually being restricted to one quarter of the udder. Sometimes the affected quarter may develop to an enormous size, producing a hard, painless tumor. Not often does the affected tissue break down into pus; consequently, no abnormal appearance is to be noted in the milk secretion until the disease has made very extended progress, in which case the percentage of fat generally diminishes. Whenever the udder shows physical manifestation of this disease, the milk almost invariably is rich in tubercle bacilli.

Tubercle organisms may also appear in milk of animals in which no physical symptoms of the disease are to be found. This fact has been demonstrated by microscopic and animal experiments, but it is also abundantly confirmed by the frequent contraction of the disease by calves and hogs when fed on factory by-products. This latter class of animals is particularly dangerous, because there is no way in which the danger can be recognized.