PLAGUE BACILLUS 165 



chronic form of the malady, and by inhabiting ships and ware- 

 houses in foreign countries, spread the disease. Post mortems 

 on infected animals reveal haemorrhagic petechia and serous 

 infiltration into serous cavities. Death is generally due to a 

 profound toxaemia and exhaustion. 



The virulence of the organism can be raised by passing it 

 through a series of animals. 



Serum from infected animals agglutinates plague bacilli. 



FIG. 45. Pest bacillus involution forms produced by growing on 3 percent 

 salt agar. (Kolle and Wassermann.) 



The diagnosis of the plague bacilli is made by rubbing the sus- 

 pected culture upon the freshly shaven skin of a guinea pig; if 

 the animal develops buboes and dies, and polar staining bacilli 

 are found, it is probable that the organism is the plague bacillus. 

 Further, if curious involution forms develop an heavily salted 

 agar (3 percent) the diagnosis is confirmed (Fig. 45). 



Immunity. It is possible to immunize against the disease. 

 Kitasato and Yersin produced an anti-toxic serum, which has, 

 not only a prophylactic, but a curative action. By the use of 

 killed culture Haffkine vaccinated many people against the plague 

 very successfully (see page 83). 



