102 Croui)ous Pneumonia. 



(swine plague, horse influenza, contagious pleuropneumonia of 

 cattle) and it appears more or less widespread according to the 

 pathogenic characters of the particular causative microorgan- 

 isms. Microorganisms which are morphologically and cultur- 

 ally identical with such disease producers are found not uncom- 

 monly as saprophytes in the air passages or in the environments 

 of domestic animals without interfering with their health. If, 

 however, the power of resistance of the organism as a whole or 

 of the pulmonary tissue alone has been diminished by definite 

 external influences or by other diseases, such microorganisms 

 are enabled to penetrate into the organs of the body, to multiply 

 in the lungs and to produce an inflammatory process in them. 

 Their virulency may so increase that they are able to invade 

 healthy animals without the auxiliary effect of external influ- 

 ences. It cannot be denied, on the other hand, that microorgan- 

 isms living as saprophytes outside or inside of the animal or- 

 ganism may become pathogenic under special circumstances, 

 and may be able to attack an animal the power of resistance of 

 which has not been previously lessened. Such bacteria which 

 have become pathogenic may lose their virulency in the dis- 

 eased animal, or they may, on the contrary, retain it and 

 spread the disease. 



It has been customary for a long time to consider as a 

 disease sui generis, and to call genuine pneumonia, that form 

 of croupous inflannnation of the lungs which appears, after 

 certain external stimuli or without them, as a primary disease 

 which does not, hoAvever, manifest a distinctly contagious type. 

 This form has been separated from other types of pneumonia. 

 Such a separation of diseases, which differ essentially only in the 

 virulency of the infective agents concerned, does not, however, 

 appear justified either from a scientific or from a practical 

 standpoint, particularly since we are unable to determine at 

 the onset the further behavior of the causative microorganism 

 as to its virulency. Croupous pneumonia of man furnishes an 

 analogous example ; it appears sporadically at one time, 

 endemically at other times, though the causative microorganism 

 is the same under both conditions. We have no proof based 

 upon sufficient bacteriologic investigations that there exists a 

 separate clinical picture of croupous pneumonia occurring in 

 any other way except by localization of well known infectious 

 diseases. 



Confusion as to the nature of croupous pneumonia has 

 also been caused since the pathologic anatomical picture has 

 been made the main basis of consideration, and therefore those 

 cases have been designated as genuine croupous pneumonia 

 in which a fibrinous exudate is found in the lungs, and where 

 the specific infectious character of the disease cannot be 

 recognized from the external clinical picture. It appears that 

 the predisposition of the animal organism, to form a fibrinous 

 exudate in the lungs after certain noxious stimuli, has not been 



