v PLAGUE IN THE RAT 127 



would be those infected with the chronic form of plague, 

 i.e. with B. pestis which had passed through a series of 

 rats less susceptible to the disease. As a result the 

 B. pestis passed on from the remaining rats, after arrival 

 of the vessel at her destination, to rats on shore would 

 tend to be of no great consequence, for the reason that 

 by the passage of a weakened strain of plague through a 

 succession of rats the acquired attenuation of virulence 

 would be likely to have become permanent. Further, 

 plague of this sort communicated by the shore rats to the 

 human community ashore would in all probability be little 

 diffused and of a non-virulent type ; so that only amongst 

 the more highly susceptible inhabitants would there occur 

 recognisable plague cases and deaths. It is probable that 

 for some such reason many outbreaks of plague presumably 

 originating in plague of rats in Occidental countries have 

 proved, both as to incidence and mortality, of a less severe 

 type than in Oriental countries where the virulent or 

 human type is the predominating character of the 

 B. pestis, and where also rats are constantly being 

 infected directly from the human subject. 



While this view of an attenuated type of plague bred 

 in the rat appears indicated by these experiments, it does 

 not necessarily follow that, when after its passage through 

 a succession of human beings the B. pestis again enters 

 the rat, virulence in the sense of type No. 1 will not again 

 be restored to it, and with the result that, for a time at 

 any rate, the virulent or human type of B. pestis might 

 prevail among the local rats, until, indeed, by a succession 

 of passages through the rat, attenuation of type was again 

 brought about. Further observations on this point will 

 need to be made. 



