34 ORIENTAL PLAGUE chap. 



Statements have been made (Calmette) according to 

 which the virulence of a strain of B. pestis may become 

 enhanced by its passage through a series of animals of the 

 same species, but that while thus increasing in virulence 

 for this species it does not follow that this same strain of 

 B. pestis possesses also increased virulence for another 

 species of animals — in fact the contrary generally is said 

 to take place. Quite a different statement is made by 

 Hankin and Colonel Skinner, viz. that, on the contrary, 

 the virulence of B. pestis decreases on its passage 

 through a series of animals of the same species, including 

 man. 



I have had a considerable experience in testing these 

 propositions and cannot agree with either. I have tested 

 B. pestis derived from acute fatal human cases by injecting 

 the plague material directly into guinea-pigs and rats, in 

 the former subcutaneously, in the latter cutaneously, and 

 have proceeded to do so from guinea-pig to guinea-pig and 

 from rat to rat, both by using the material directly from 

 the spleen of a previous animal, as also by using twenty- 

 four hours' agar culture of the spleen of the previous 

 animal, and as a result I have not noticed anything of an 

 increase in virulence of the B. pestis in its passage 

 through these series of animals. True, B. pestis, like 

 other pathogenic microbes, lose in virulence by trans- 

 mission in the laboratory through series of subcultures on 

 artificial media, and, unless this decrease has become very 

 great, become generally enhanced in virulence by passage 

 through the suitable animal body ; but this has nothing 

 to do with the above statement that a race of B. pestis 

 derived from a particular source (human or animal) 

 undergoes increase in virulence by passage through a 



