Contagion is usually the result of cohabitation, although contact between diseased and healthy subjects for a period of some days or even weeks does not seem sufficient to produce the disease. Nocard has fixed a mean period of five to six months as necessary for the contraction of the disease by bovine animals, and Moussu has arrived at almost identical results by placing tuberculous and healthy cows together in a byre reserved for such researches. In this connection, however, very great differences of individual susceptibility exist, and these are difficult to appreciate in the present state of our knowledge. It thus happens that an animal of vigorous appearance and in good condition may easily contract tuberculosis, whilst a thinner and less vigorous one will resist it for a comparatively long time.
Speaking generally, it may be said that young animals contract tuberculosis by cohabitation in infected places more easily than adult or aged ones, and the fact that old animals contribute the larger number of cases is to some extent due to their having in the course of their lives been more exposed to continued or successive infection.
Contagion does not occur in byres unless as the result of the presence of animals with open tuberculous lesions, such as caverns in the lungs, tuberculous bronchitis with ulceration of the mucous membrane, tuberculous metritis, enteritis, etc. The virulent germs are expelled in the saliva, nasal discharge, excrement, etc., and are distributed over the forage, manure, litter, and in the drinking water; after desiccation they may be spread by currents of air.
The mangers, racks, drinking pails, and various stable utensils become permanently contaminated, the air of the cowsheds contains virulent dust, and the animals there confined are continually exposed to infection either through the respiratory or digestive passages.
Contamination through the respiratory tract is by far the most frequent cause of the evil, and recent experiments at Pouilly-le-Fort (1900) have shown how easy it is to convey the disease experimentally by inhalation.
Patients suffering from closed tuberculous lesions of the pleura, pericardium, spleen, peritoneum, etc., do not spread the bacilli. Healthy animals may remain in contact with them without danger, but it is well to remember that such cases are quite exceptional. As a rule the lesions are of a mixed character, and the general principle may be laid down that cohabitation of any duration with tuberculous subjects is dangerous.
Contagion spreads more easily, in proportion to the number of tuberculous subjects in a given byre, to the total number of animals in a herd, and to the neglect of cleanliness, good feeding, ventilation, etc.
Life in the open air and at grass greatly diminishes the chances of contagion. The virulent products are then disseminated in all directions and are soon destroyed by the general atmospheric conditions. Close confinement in ill-ventilated stables, on the contrary, strongly tends to the propagation and development of tuberculosis.
In calves infection may occur through the alimentary tract by means of tuberculous milk, whether such milk is obtained directly from the udder or out of a pail. The same may be true of young pigs fed with skimmed milk.
Goats contract tuberculosis somewhat readily by confinement in byres with tuberculous cows, and Moussu declares that contagion afterwards spreads just as rapidly among goats as among cows. The vaunted great resistance of goats to tuberculosis, formerly so often spoken of, and by some wrongly considered as a condition of immunity, is deceptive, and if tuberculosis is less frequently seen in goats, this is solely because goats enjoy the greatest liberty at all seasons.