47 



establishing an acquired immunity. We may mention here the 

 fact that has been observed by many workers, and which we have 

 amply confirmed, that rats are readily immunized by antiplague 

 serum. 



The subject of the transfer of infection directly from rat to rat by 

 cutaneous or subcutaneous inoculation through a series of the animals 

 is one that is evidently intimately associated with the preceding 

 subject, as it is quite evident that an immune rat or several of them 

 might terminate a series without any actual diminution in the viru- 

 lence of the organism transferred. It is quite plain that the success 

 of such an experiment would depend largely upon the number of 

 rats used in each transfer. The Indian Plague Commission (19) 

 had no difficulty in carrying infection through twenty-six transfers, 

 using from six to fifty rats in each transfer. 



Pound (7) in a series of eight experiments, was never able to convey 

 the infection successfully beyond the sixth rat, using but one rat for 

 each transfer. There was no apparent lessening of the virulence 

 of the organism and each series appears to have been terminated 

 abruptly by encountering an immune rat. 



Baxter-Tyrie (15) says: 



It is probable that under certain natural circumstances a reduction in the viru- 

 lence of the organism is effected and a comparative immunity is conferred on the rats. 

 The infection of immigrant rats is, however, severe, and their arrival is heralded 

 by a heavy mortality. In the same manner an infected rat imported into a fresh 

 locality produces a similar result. This attenuation of virulence is responsible for the 

 condition known as chronic rat plague. 



Several experiments conducted in San Francisco to determine 

 this point have given results that I regard as showing merely the 

 presence of a considerable percentage of immunity among the rats. 

 It was observed that in each case certain of the rats died of acute 

 plague even in the last transfer. It was very evident that had cer- 

 tain combinations of immune rats been encountered the experiment 

 might have terminated at any point. On the other hand, by being 

 especially fortunate in using nonimmune rats, the experiments might 

 have given a much higher percentage of cases of acute plague. Unfor- 

 tunately it was necessary to terminate these experiments in each 

 instance before they could be regarded as completed. 



The reason for the natural subsidence of plague among rats in any 

 community is a point about which much more evidence must be 

 obtained before we can speak with any degree of certainty. It may 

 be due to the lack of susceptible material, possibly to a loss of viru- 

 lence of the organism; but it seems more probable that it is due to 

 a change in the number or relations of the ecto-parasites of the rat. 



Adequate measures of rat extermination, while they may never 

 bring about the ideal condition of a community that is free from 

 rats, are, as is shown by the recent experience in San Francisco, of the 

 utmost value in shortening the epizootic. 



