vi PLAGUE INDUCED IN OTHER RODENTS 143 



black ship rat take plague readily if inoculated with 

 virulent type of B. pestis, and in this respect they differ 

 from the Norwegian and the sewer rat, since even with 

 virulent B. pestis failures in them are not exceptional. 



Another important fact observed is this : whereas the 

 white tame rat, and to a certain extent also the brown 

 ship rat, breed true the virulent B. pestis, showing no 

 diminution in the virulence of the B. pestis after one or 

 two passages, it is otherwise with the other races of rats. 

 B. pestis, at starting of the virulent type, when passed 

 through the black ship rat for one or two generations is 

 of a distinctly attenuated type ; that is to say, the less 

 susceptible black rat and the less susceptible Norwegian 

 rat infected with type 1 (human) yield a B. pestis of a 

 distinctly less virulent character, i.e. type 2 (rat type). 

 This is to a certain extent also true of the brown ship rat, 

 but to a less extent ; it certainly and distinctly holds 

 good for the sewer rat. 



This attenuation which B. pestis, originally of the 

 human or virulent type, undergoes on its passage through 

 the black ship rat, and may undergo in the brown ship 

 rat, may account for the fact of the notoriously less 

 virulent form of plague in Occidental countries, and the no 

 less notorious fact, as pointed out on a former page (by 

 Dr. Thomson), of the numerous cases where plague in rats 

 on board ship failed to be transmitted to human beings. 



Calmette's statement, that B. pestis bred in one species 

 of rodents and virulent for that species need not be 

 virulent for another species, may therein find its explana- 

 tion. As far as my own experiments are concerned, I 

 have always found that B. pestis of virulent race, when 

 bred in the white rat, loses none of its virulence for 



