5 For data on which these statements are based, vide Phthisis, in a Series of Clinical Studies, by the author.
Age has a decided influence on the development of phthisis. Cases in which the ages of patients are between twenty and thirty years greatly preponderate over the number in any other decade of life. Next in order as to the number of cases are the ages between thirty and forty years. The form of tuberculous disease under present consideration is rare under ten years and also in advanced life. All that can be said with our present knowledge in explanation of the influence of age is, that either an existing diathetic condition tends intrinsically to the development of the disease or that the diathesis is likely to be acquired at certain periods of life more than at other periods. Of these two explanations the former is the more rational.
Statistics show that occupations which involve sedentary habits, confinement within doors, especially in small, illy-ventilated rooms, poor or insufficient food, and prolonged mental depression, increase the liability to phthisis. The disease is developed either during or shortly after gestation in a sufficient number of cases to show that pregnancy has a causative agency. Facts appear to show a less degree of prevalence of the disease in most cold and tropical climates than within the temperate zone. It is, however, true, as stated by Ruehle, that "there are regions in all zones which are free from the disease, and, on the other hand, there is no zone in which it is not very prevalent." The prevalence is less in high than in low altitudes. Humidity of the soil has been shown by Bowditch, Buchanan, and others to enter into the etiology. In order to determine how far purely climatic agencies exert an influence either for or against the prevalence of the disease, it is necessary to take into account other associated agencies, together with an innate predisposition; and the latter especially does not admit an exact estimation.
Certain general diseases seem to involve a liability to phthisis as a sequel. This is true of rubeola and pertussis. In cases of diabetes mellitus, phthisis is considered as occurring sufficiently often to show a causative connection. In my own clinical experience, however, phthisis has not been of frequent occurrence in that disease. Typhoid fever in some cases appears to favor the development of phthisis. Some, however, have contended for the reverse of this statement. Certain affections are apparently antagonistic in their influence. In this category are pulmonary emphysema and obstructive or regurgitant valvular lesions at the mitral orifice of the heart. The disease is rarely developed in chlorotic patients. Facts go to show that alcoholism opposes its development. In opposition to current belief, my clinical studies lead me to conclude that they who have had scrofulous disease of the cervical glands in early life are not likely to become phthisical in after years. Contraction of the chest from deformity diminishes the liability to the disease.
The communicability of phthisis is a doctrine dating as far backward as the history of medicine extends. Distinguished physicians in every age have held that the disease may be communicated under circumstances which involve close proximity, as from husband to wife or vice versâ, and from patients to nurses or attendants. The contagion is supposed to be contained in the expired breath. The clinical evidence in behalf of this doctrine is the number of instances which seem to be striking examples of communicability. It is easy to collect a considerable number of such examples. But in order to constitute clinical proof of the doctrine of communicability the number must be so large as not to be accounted for on the ground of mere coincidence. A collection of isolated instances gathered from medical literature or reports from different physicians does not establish the doctrine. Owing to the great frequency of phthisis, mere coincidence suffices to account for a certain number of instances. Moreover, long-continued proximity to cases of phthisis generally involves causative agencies other than a contagium—namely, confinement within doors and mental anxiety. In my collection of 670 recorded cases of phthisis, the number of instances in which there was room for the suspicion of the disease having been communicated either from the husband to the wife or from the wife to the husband amounted only to 5. In one of these instances, a wife, who became phthisical after her husband, had lost two sisters, one of whom was a twin sister, by the disease. It must be admitted that the analysis of these cases, without disproving the doctrine of communicability, fails to lend to it support, for the reason that in such a large collection of cases the number of examples of apparent communicability are so few.
A new and strong impetus was given to the discussion of the doctrine by the discovery of the inoculability of tuberculous disease. Villemin in 1865 demonstrated the fact that this disease could be communicated to rabbits and guinea-pigs by inserting beneath the skin portions of the tuberculous product. The experiments of Villemin and many others have shown conclusively that the insertion of fresh undecomposed tuberculous matter beneath the skin or within the pleural and the peritoneal cavity, or in the anterior chamber of the eye, is followed by an eruption of tubercles in these animals within two or three weeks. If tuberculous matter taken from an animal in which the disease has been produced by inoculation be inserted in another animal, the disease is transmitted to the latter. These results of inoculation, which have been abundantly confirmed in all countries, prove indisputably the communicability, by that mode, of tuberculous disease in certain animals which have a peculiar susceptibility thereto. The fact that the disease is not readily communicated to dogs, cats, and other animals shows a peculiar susceptibility to be an important factor in the successful results of inoculation. The conclusion drawn by Villemin and others from these experiments is that the disease is communicated by means of a specific virus, a term implying the existence of a contagium.
Opposed to this conclusion are experiments which appear to prove that tubercles may be produced in rabbits by inoculating them with various kinds of non-tuberculous matter. By those who adopt the doctrine of a specific virus it is contended either that true tubercles are not produced in these experiments, or that, if followed by the development of true tubercles, the production of the latter is attributable to the derivation of the virus from the laboratories in which tuberculous animals had been confined or to a contagium received directly from these animals. The introduction of non-tuberculous matter was found by Cohnheim and Fraenkel never to be followed by tuberculous disease when the experiments were repeated in places where tuberculous animals had not been confined and the animals on whom the experiments were made were isolated from those affected with tuberculosis. Cohnheim states that inoculation with portions of indurated lung, or of the nodules resulting from peribronchitis, or of the contents of bronchiectasic cavities, will not give rise to true tubercles, for the reason that, although taken from phthisical lungs, they do not contain the tuberculous virus. This distinguished pathologist, at first an opponent of the doctrine of a specific virus, afterward became a strong advocate therefor. He was led to regard a successful inoculation as affording the only criterion and reliable test of tuberculous disease; that the etiology of tuberculous disease invariably involves the presence in the system of this virus; that it exists in a latent form whenever there is an innate predisposition to phthisis; and that it may enter the system in different directions—namely, with the inspired air into the lungs, and even within the skull through the foramen of the ethmoid bone, into the small intestine by deglutition, and into the uterus with the semen. Becoming developed in any situation, the virus may remain localized, or it may be disseminated more or less extensively by means of the lymph and blood. The behavior of the tuberculous virus, according to Cohnheim, corresponds closely to that of syphilis.
Experiments made by Gerlach, Bollinger, Aufrecht, Chaveau, Leisering, Harms, Gunthern and others, have shown that the disease may be communicated by incorporating tuberculous matter with food. Rabbits, guinea-pigs, dogs, calves, swine, sheep, and goats have been rendered tuberculous by these experiments. Klebs, Tappeiner, Parrot, and Puech claim to have communicated the disease by combining with the food the matter of expectoration from phthisical patients. Gerlach and Klebs have seen the disease in animals fed with milk from cows affected with the so-called pearl disease (perlsucht), which is considered to be identical with phthisis. Finally, the disease appears to have been produced by exposing animals to an atmosphere impregnated with fine particles of tuberculous matter by means of an atomizer, and by blowing into the trachea this matter reduced to a fine powder.6
6 For a summary of the experiments relating to the communicability of tuberculous disease by inoculation, by the ingestion of tuberculous matter, and by its inhalation, and for reference, the reader is referred to an article by Wm. P. Whitney in the Boston Medical and Surgical Journal, July 28, 1881; to the article on "Tuberculosis" by Frederick C. Shattuck in supplement to Ziemssen's Cyclopædia of the Practice of Medicine, 1881; to the "Cartwright Lectures," by William T. Belfield, M.D., published in the New York Medical Record in February and March, 1883; and to an article by Surgeon George M. Stemberg, U. S. Army, in the American Journal of Medical Sciences, January, 1885.
It is noteworthy that tuberculous disease may be produced by inoculating with the infiltrated product, with matter from miliary tubercles, or from scrofulous glands in the neck. The identity of these morbid products is thus made evident, assuming that the fact of communicability involves the existence of a specific virus.