Tuberculosis

It is clear, theref ore, that, in densely populated countries, tuberculous infection is far more widespread than tuberculous disease. In sparsely populated and isolated countries, on the other hand, neither tuberculous infection nor tuberculous disease is common. Borrel, for instance, found that only 3% of the Sene galese soldiers summoned to Europe for the World War gave a positive response to the tuberculin skin-test on arrival in France, while numerous investigations carried out in the African and Asiatic colonies and dependencies of the European Powers show that primitive tribes living under their natural conditions are almost free from tuberculosis.

It has frequently been shown, however, that the members of these primitive communities, on quitting their native surroundings and coming into contact with the "tuberculized" populations of Europe, tend to show a marked susceptibility to tuberculosis ; the clinical course, in these cases, being of a rapidly progressive and usually fatal type.

It would seem, in fact, that, where there has been little or no previous exposure to infection, the individual remains completely unprotected against tuberculous disease, while, on the other hand, there is manifest in the members of communities in which tuber culous infection is widespread, a considerable power of resisting the development of the tubercle bacillus and of rendering latent the active foci of infection.

Clinical Manifestations.

The primary clinical manifesta tions of tuberculosis tend to vary with the portal of entry of the bacillus. In uninfected subjects, the germs can pass through the mucous membranes of the respiratory and alimentary tracts, without causing, at the moment, any marked local disturbance, and their transit thence is easy along lymphatic channels to the nearest lymphatic glands. In these glandular filters, the tubercle bacilli tend to be arrested and there they pullulate and set up the first foci of disease.

The formation of secondary foci of infection depends upon the spread of tubercle bacilli from their primary foci in the lymphatic glands to other tissues and organs. The presence of secondary foci thus implies a generalization of the infection chiefly by way of the blood stream. This generalization can take place much more easily in susceptible persons than in those who are more resistant, and in the young than the old. Thus the tendency to widely disseminated tuberculosis is much more marked amongst those groups of individuals in which the "tuberculin test" shows the highest proportion of negative results.

In the infants and young children of European and American communities, the proportion of bone and joint tuberculosis and of tuberculous meningitis is far higher than in adults ; while, in the adults of "primitive" communities, brought for the first time into contact with infection, the same tendency to wide dissemination of lesions is always found.

The reactive intolerance implied in a positive response to the tuberculin test which is only acquired as the result of a preceding infection, constitutes a factor of supreme importance in limiting the spread of tubercle bacilli within the body, by causing local inflammation, cell-proliferation and finally fibrosis, with arrest of the wandering germs and their confinement within a fibrous or calcified area. And this reactive intolerance is manifested, not only toward endogenous re-infection from pre-existing foci but to exogenous re-infection from outside the body.

Pulmonary Disease.

With this conception in mind, it is not difficult to understand why the tuberculosis of adult life in civil ized and "tuberculized" communities usually takes the form of pulmonary disease. In the generalization of infection through the blood stream from the primary lymph-gland foci, the bacillary emboli must travel along the veins from the glands and since all the venous blood must necessarily pass through the capillary bed of the lungs before being re-distributed throughout the body, the lung tissue presents the first filter to be encountered by the blood borne bacilli. It is into the lungs, too, that all contaminations are drawn in respiration. While both blood-borne and air-borne tubercle bacilli may pass unchecked through the "virgin" lung tissue of children and non-infected adults, they tend, where "re active intolerance" exists, to be arrested in the pulmonary tissue and to set up the inflammatory and ulcerative phenomena of phthisis.

Changes in Clinical Type.

The statement is frequently made that the cases of pulmonary tuberculosis encountered 3o or so years ago were much more acute than those seen to-day, and a general impression exists amongst clinicians that phthisis is more acute even now in parts of Ireland, Scotland and Wales than in London and the big industrial centres. This clinical impression receives strong support from the statistical enquiries of J. Brown lee (M.R.C. Special Report Series No. 18, 1918), who has shown that, in males especially and to a less extent in females, the age period of maximum death-rate has steadily receded from the "young adult" group to the "middle age" group in the succeeding decennia from 1851-6o to the present time.