198 ORIENTAL PLAGUE chap. 



crowded with B. pestis ; that is to say, with the appear- 

 ances described in detail on a former page. 



This failure of transmission of plague from a plague 

 rat to a healthy rat living amicably in the same or in an 

 adjoining open wire cage is not an isolated instance. 1 

 In my reports to the Medical Officer of the Local 

 Government Board for 1902-1903 and 1903-1904 I have 

 mentioned a good many such instances ; and in the 

 present chapter I have recorded several in addition where 

 only one of two animals (rats, mice) contracted and died of 

 plague, be it by inoculation or by feeding, the companion 

 remaining unaffected, though highly susceptible, as proved 

 afterwards by inoculation with active plague material. 



While, then, the transmission of plague from animal 

 to animal is experimentally established both as regards 

 cutaneous inoculation and feeding with semi-dry infective 

 material, there is a distinct failure of evidence that trans- 

 mission of the disease is effected by fleas or lice from an in- 

 fected animal to a healthy one. It is not, therefore, in my 

 view, justifiable to regard this mode of transmission, if, in- 

 deed, it happens at all under natural conditions, as anything 

 but exceptional, at any rate as far as the sewer rat and the 

 tame white rat are concerned. Theoretically, such a trans- 

 mission is possible and easily imaginable, as I have discussed 

 on a former page ; it is possible, I mean, that a flea or louse 

 which has just sucked from a rat blood well charged with 

 B. pestis may, by biting a neighbouring rat, directly inocu- 



1 Hitherto I have searched in vain during autumn, winter, and spring for fleas 

 on sewer rats and on the tame or white rats. I have been able to discover only- 

 lice. I would further point out that, as mentioned on a former page, the above 

 experiment is not altogether satisfactory, since the sewer rat is not an animal with 

 high susceptibility for plague. It is probable that in these respects an important 

 distinction may have to be drawn between this species and the Oriental rat or 

 Mus rattus. 



