476 A MANUAL OF BACTERIOLOGY 



end of three months the bactericidal power of the blood 

 will have become very marked, and living cultures are 

 then injected for a further period of about three months 

 until a whole flask -culture is given at a dose. An interval 

 of a fortnight is allowed to elapse between the last dose 

 and the bleeding of the animal. The serum is tested 

 upon mice. 



The anti-plague serum, which is mainly anti-microbic, 

 is not very potent, and to be of service large amounts and 

 early treatment are essential. 1 



Epidemiology. The mode of infection in man has been 

 a matter of controversy. The pneumonic form arises 

 generally from aerial infection by the respiratory tract. 

 It is uniformly fatal and highly infectious, while the 

 bubonic and septicsemic varieties are hardly even con- 

 tagious. Although a gastric and intestinal form of the 

 disease has been described, and there is evidence to show 

 that food or drink may be the vehicle of infection, this 

 must be a rare mode of infection. Yersin claimed to have 

 isolated the bacillus from the dust and earth of a native 

 dwelling, and Hankin from the brackish water in a field. 

 The observations of Hankin and others indicate, however, 

 that contagion is likely to occur only from immediate 

 contact with man or animals, or their excretions, infected 

 with plague, and not from a saprophytic form of the 

 organism. 



Certain animals, especially the rat (Mus rattus and Mm 

 decumanus), are important agents in spreading the disease. 

 The association of sickness and of death among rats 

 with an epidemic of plague has been established by a 

 number of observations, and in some instances the epi- 

 zootic among rats has been definitely shown to precede the 

 epidemic in man. During an epidemic rats may be found in 

 all stages of illness and plague bacilli can be found in large 



1 See Hewlett's Serum Tfarapy, 1910. 



