PLAGUE IN THE RAT 



87 



way related to that disease. And we further know that one of 

 these maladies is characterised by the presence in the rat of a micro- 

 organism that cannot be clearly differentiated from that of plague 

 by microscopic examination alone ; and that, indeed, definite 

 differentiation cannot be satisfactorily established without physio- 

 logical experiment in addition to microscopical and cultural exami- 

 nation. So that it is open to serious doubt whether the numbers 

 I have given of ships that have had plague among rats on board 

 them should not be materially reduced. 



But, on the other hand, it is claimed that plague may affect rats 

 on board and yet remain unrecognised, since only a notable mortality 

 among them is likely to attract attention, while the presence of 

 plague among rats may not necessarily set up such a mortality. 

 Certain observations that point in this direction will be cited at a 

 later stage of this memorandum." 



(2) The danger of transmission of infection from plague-infected 

 rats to man on board ship. 



" The number of ships on which plague is known to have occurred, 

 among man or rats or both man and rats, during the period 1898- 

 1901, according to the records I have consulted, is, as already stated, 

 95. The yearly distribution is as follows : — 



These figures are subject to two criticisms of opposite intention. 

 On the one hand, it may be contended that some of the instances 

 classed as ■ man alone ' were in reality instances in which there was 

 unrecognised plague among rats on board. On the other, it may be 

 claimed that in some instances the disease observed among rats was 

 not plague. 



