102 ORIENTAL PLAGUE chap. 



that after about five or six months, i.e. five or six 

 transferences, not even half to the whole of a surface 

 gelatine culture injected peritoneally was capable of 

 producing a fatal result in the guinea-pig. 



It is interesting in this connection to note that there 

 is presumption that the plague cases that occurred on board 

 the steamer City of Perth (L.P. II.) were caused by 

 rats 1 ; for it may be that this particular (b) variety of B. 

 pestis is in a sense proper to the rat, and that it is moreover 

 normally of comparatively minor virulence. Such assump- 

 tion does not, of course, exclude the possibility 2 that the 

 typical and virulent or (a) race of B. pestis is also 

 capable of transmission through the rat; that animal, 

 indeed, is found to be highly susceptible, experimentally, 

 to both varieties of B. pestis. However this may be, 

 there is strong suggestion that the second type of B. 

 pestis, in so far as it is derived directly from the rat, is 

 less virulent to start with, and may be therefore much 

 less dangerous to man than the first type, for the reason 

 that only highly susceptible persons can take the 

 infection. 3 



1 The City of Perth left Calcutta on May 2, all well. On June 5 the first 

 human case of plague occurred ; later two further cases followed. One of these 

 later cases died at the Denton Hospital, and the lung and swollen inguinal glands 

 yielded the material for investigation. Before the first case occurred on June 5 

 some mortality amongst rats had been observed on board the vessel, and as a matter 

 of fact both the first and second patients had handled and thrown overboard dead 

 rats found on the ship. 



2 As a matter of fact various instances of rat plague on ships are recorded in 

 which several persons who handled sick or dead rats took the disease and died. 

 The Cardiff plague rat, which was described at the commencement of this report, 

 had died of the virulent first (a) type, and both from this rat and from the man 

 presumably infected from similar rats B. pestis of the first or virulent type was 

 obtained. 



3 In this way may be explained the repeatedly observed fact that in some 

 localities mortality and disease of rats had been occurring for a comparatively 

 long time before plague (if any) broke out in man. This is consistent with 

 highly susceptible human individuals, capable of taking and transmitting the 



