TOXINS, IMMUNITY, ETC. 435 



In primary plague pneumonia, from a consideration of the 

 anatomical changes and the clinical facts, the disease may be 

 said to be produced by the direct passage of the bacilli into the 

 respiratory passages. Nevertheless there must be certain factors, 

 still imperfectly understood, which determine the incidence of 

 this form ; as in some epidemics of the highest virulence plague 

 pneumonia has been practically absent, though opportunities for 

 infection by inhalation must have been present. On the other 

 hand, a case of plague pneumonia is of great infectivity in 

 producing other cases of plague pneumonia. If we except 

 infection through the respiratory passages in such cases, it may 

 be said that direct infection from patient to patient is relatively 

 uncommon. This is in accordance with the fact that in bubonic 

 plague the bacilli are not discharged from the unbroken surface 

 of the body, and are only present in the secretions in severe cases. 



The occurrence of the disease in rats was early recognised, and 

 there is no doubt that it plays a very important part in the 

 spread of epidemics. The disease in these animals has, in fact, 

 been the means of rapidly distributing infection over wide areas 

 of a town or district. This has been abundantly proved in 

 the case of Bombay, where observations have shown that the 

 migration of plague-infected rats to quarters comparatively free 

 from the disease, has been followed by extensive outbreaks in 

 these places. The facts stated above show how the disease is 

 spread among these animals by fleas, and how it is conveyed 

 by them to the human subject. 



Toxins, Immunity, etc. As is the case with most organisms 

 which extensively invade the tissues, the toxins in plague cultures 

 are chiefly contained in the bodies of the bacteria. Injection of 

 dead cultures in animals produces distinctly toxic effects ; post 

 mortem haemorrhage in the mucous membrane of the stomach, 

 areas of necrosis in the liver and at the site of inoculation, may 

 be present. The toxic substances are comparatively resistant to 

 heat, being unaffected by an exposure to 65 C. for an hour. By 

 the injection of dead cultures in suitable doses a certain degree 

 of immunity against the living virulent bacilli is obtained, arid, 

 as first shown by Yersin, Calmette, and Borrel, the serum of 

 such immunised animals confers a degree of protection on 

 small animals such as mice. On these facts the principles of 

 preventive inoculation and serum treatment, presently to be 

 described, depend. It may also be mentioned that the filtrate 

 of a plague culture possesses a very slight toxic action, and the 

 Indian Plague Commission found that such a filtrate has practi- 

 cally no effect in the direction of conferring immunity. 



