On the other hand, the disease is very rarely conveyed to sheep, even when they are kept for long periods with tuberculous cows. Moussu found that two years of close cohabitation were necessary for its development under these conditions.
Heredity is a factor of the highest importance in determining the causation of tuberculosis. At the present time a tendency exists to deny this, but such a view is erroneous.
Observation has clearly shown that tuberculosis is rarely conveyed from the mother to the fœtus, and that practically none of the calves borne by tuberculous mothers react to tuberculin (95 per cent.: Nocard and Bang); but even if this is absolutely correct, it only shows that great benefits might be derived if proper sanitary organisation and intelligent hygienic conditions in byres were found everywhere in the country. Unfortunately in practice this is far from being the case. These non-tuberculous calves are left in common contaminated byres, where they rapidly become infected and perpetuate the disease.
Physiologically these facts are easily explained. The placenta resists the passage of microbes, or at least only allows them to pass under quite exceptional conditions, and practically only when the blood-vessels are affected. As, on the other hand, tuberculosis of the ovaries, Fallopian tubes or uterus generally prevents pregnancy and causes sterility, there is nothing extraordinary in the fact that tuberculosis is not hereditary in the strict sense of the term. The influence of the sire has been invoked, but it has been proved that direct paternal infection is only possible where ulcerating tuberculous lesions of the testicle, prostate, or vesiculæ seminales exist. Such conditions seldom or never occur in the sires of domestic animals.
As a general rule, therefore, it may be said that tuberculosis is not hereditary. New-born animals become infected during the months following birth, either directly through the alimentary tract when the mothers are suffering from mammary tuberculosis, or, perhaps more frequently, through the respiratory and digestive tracts.
But although microbic infection is not hereditary, it by no means follows that the offspring of tuberculous subjects are as well prepared for the struggle of life as the descendants of healthy subjects. What is transmitted is a greater tendency to contract the disease.
This aptitude or predisposition is of such importance that in Moussu’s opinion it should be regarded as one of the essential factors in the development of tuberculosis. The cause of tuberculosis is Koch’s bacillus. It does not always produce its full effects in animals born of healthy parents; but in one that suffers from a tuberculous hereditary taint tuberculosis appears.
Physiological and pathological researches cast considerable light on this question. In tuberculous mothers the organism not only suffers from the infection, but from a permanent intoxication which interferes with normal metabolism in the vital organs and the exchanges between mother and fœtus. If the microbes remain confined to the system of the mother, their poisons are conveyed by the blood and pass through the placental barrier. In a greater or less degree they saturate the tissues of the little creature in process of development, and communicate to it a peculiar hereditary taint. The effects of this taint are often noticeable from the moment of birth, for comparative physiological and pathological investigations have shown that the tissues of tuberculous animals assimilate given foods less perfectly and are the seat of greater losses of all kinds than those of healthy subjects.
Although the disease itself, therefore, is not hereditary, it is otherwise with the organic taint which plays so important a part in its development. This organic taint consists in a special condition of the tissues or cells of the parents, which show a diminished power of resistance to the action of the germs of tuberculosis; it is therefore easy to understand how important a part these influences may play under certain conditions.
Without doubt, in the case of bovine animals, the predisposition could be neutralised in carefully managed studs by the immediate isolation of the new-born under conditions which shield them from tuberculous infection, and experiment has shown the benefits derived from such precautions; but it must not be forgotten that intelligently managed studs are the exception, and that for a long time to come we must in practice take cognisance of the actual conditions under which the disease develops.