BACTERIOLOGY. 191 



to mucus production that our pneumonic-plague strains in any 

 way differ from the bubonic strains. 



VIRULENCE. 



The organism seems to have retained a maximum virulence 

 throughout the epidemic, at least all of the cultures isolated and 

 studied by inoculation into animals possess this very high degree 

 of virulence. Cultures isolated near the close of the epidemic 

 showed an equally high virulence to those isolated near its 

 beginning. However, the idea that this epidemic of pneumonic 

 plague was due to the fact that the strain possessed an abnormally 

 high virulence — much greater than that possessed by the or- 

 ganism of bubonic plague — and that this accounted for the very 

 high mortality during the epidemic appears to be erroneous; 

 The very. acute course of the disease, the very high death rate 

 during the epidemic as compared with that of bubonic plague, 

 and the apparently increased virulence of this pneumonic strain 

 may be satisfactorily explained by the fact that the portal 

 of entry of the organism and the location of the primary points 

 of infection in pneumonic plague and in bubonic plague are dif- 

 ferent. The plague organism finds in the pulmonary tissues a 

 much more favorable and extensive medium for its multiplica- 

 tion and diffusion than it does in the lymphatic glands. In 

 bubonic plague, the lymphatic glands may be said to act as filters 

 against the general invasion of the organism by the plague 

 bacillus, while in primary pneumonic plague there is no such 

 mechanism for the defense of the host, the bacilli spreading 

 rapidly throughout the lung and invading the circulation in 

 every instance in a comparatively short time and apparently 

 before the organism has had time to produce any appreciable 

 quantity of immune substances. The bronchial lymphatic glands 

 in primary pneumonic plague offer resistance to the invasion of 

 the plague bacillus, and in every case of this disease these glands 

 are very acutely inflamed and frequently almost of a black color 

 from the resulting toxic haemorrhages in the glandular substance. 

 However, by the time the bronchial glands have become involved, 

 the bacteria have already spread so extensively throughout the 

 lung substance that a bactersemia has usually occurred. Micro- 

 scopical preparations made at necropsy from the lungs of these 

 pneumonic cases invariably contain enormous numbers of plague 

 bacilli. In no other disease are the organisms found in such 

 great abundance. In primary pneumonic pl^ue, the bacilli are 

 found in very much greater number in the lung than in the spleen, 



