146 BACTERIA 



and in the cutaneous pustules, lymphatics and in the lungs in plague 

 pneumonia; more rarely in the blood and other organs. In ani- 

 mals, plague occurs in rats. It is supposed that some tropical soil 

 bacilli infect rats, and becoming accustomed to the rodent's body, 

 are eventually transmitted to man. The bacilli are transmitted 

 from rat to rat in India by the rat fleas which also can bite man. 

 The organisms remain in the flea for some time. Rats are also 

 infected from dead rats. In epidemic times the soil becomes 

 infected and persons going barefoot may be infected. 



Pathogenesis. Highly pathogenic for man. Is the cause of 

 the bubonic or Oriental plague; bacilli gain entrance by way of the 



FIG. 47. Pest bacillus involution forms produced by growing on 3 per cent 

 salt agar. (Kolle and Wassermann.) 



skin, causing localized foci of infection from which buboes develop, 

 followed by pest-sepsis and death. The lungs may be the original 

 site of invasion, and plague pneumonia (worst form of the disease) 

 may result. The typical bacilli can be found in the sputum of the 

 patient thus affected. The mortality from this plague is from 50 

 percent to 80 percent. (Fig. 46.) 



Almost all domestic animals rats, mice, guinea pigs, rabbits and 

 squirrels are affected; horses and swine are very susceptible; cows 



