Cultivation. Pathogenicity 81 



Pathogenicity. In contradistinction to the common char- 

 acteristics described, there are marked differences in the viru- 

 lence of the bacilli which depend on their origin, the nature 

 of the disease and the species of animals affected. However, 

 all the varieties coincide in exerting their strongest pathogenic 

 action when inoculated with blood or exudate of animals which 

 died of the disease (aggressin action, see page 84). 



Another common characteristic of the bacteria belonging 

 to this group is that they develop the highest virulence towards 

 ' that species of animals for which they are naturally pathogenic, 

 while towards other species their action shows a great variance. 

 It is, however, possible to increase the virulence for other 

 species through repeated reinoculations, but such a strain of 

 the bacilli will even then possess the strongest pathogenic action 

 for the species from which it originated. 



The animals which are susceptible to the bipolar bacteria, 

 although not to the same extent, are the guinea pig, rabbit and 

 mouse, while the domestic animals show a great variation in 

 susceptibility. The guinea pig is best adapted for test inocu- 

 lations, and by using this test animal for repeated intraperi- 

 toneal reinoculations the virulence of any of the varieties may 

 be greatly increased. 



The result of artificial infection depends also on the nature 

 and method of the inoculation. The most pronounced action is 

 obtained by direct injection into the circulation, by which means 

 the disease may be transmitted to any species. Somewhat less 

 effective is the intraperitoneal and intratracheal injection of 

 cultures, whereas the subcutaneous and intramuscular inocula- 

 tion produces a severe affection only in very susceptible species 

 of animals. The feeding of virulent material only exceptionally 

 gives positive results, • these being most frequently obtained 

 when the cholera virus of chickens is fed. 



Variability of the Virulence. The bipolar bacilli are very 

 widely spread, and may produce disease in any species of ani- 

 mals, particularly in the domestic animals. As original sapro- 

 phytes they occur in nature in the soil, in slow-flowing or 

 stagnant water, on various plants, and in all kinds of dead 

 organic material. Under normal conditions they do not mani- 

 fest pathogenic characteristics. Hence they occur not infre- 

 quently in the digestive tract and in the air passages. Under 

 certain conditions, which at present are still unknovm, they be- 

 come virulent and then they may attack the tissues of the 

 healthy body. On the other hand, their pathogenic action is 

 favored by all those influences which weaken the resistance of 

 the organism, as colds, exertion, anemia, starvation, mixed in- 

 fection, catarrhs, etc. Parasites'in the air passages and digestive 

 tract also favor the development of infection by loosening the 

 epithelium of the mucous membrane, or by causing penetrating 

 injuries to the mucous membrane, whereby the entrance of the 

 bacteria into the lymph vessels is facilitated. 



