6.S4 iNFECTiOtJR DISEASES. 



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 23ermanently contaminated, the air of the cow-sheds 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 experimen- 

 tally 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 con- 

 ditions. Close confinement in ill-ventilated stal)les, 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 obtahied 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 



