BUBONIC PLAGUE. 559 



< tgata found that the plague bacillus existed in the 

 bodies of fleas found upon diseased rats. One of these 

 he crushed between sterile object-glasses and introduced 

 into the subcutaneous tissues of a mouse, which died 

 in three days with typical lesions of the plague, a con- 

 trol-animal remaining well. Some guinea-pigs taken 

 for experimental purposes into a plague district, and 

 kept carefully isolated, died spontaneously of the disease, 

 presumably because of insect infection. 



The animal most prone to spontaneous infection seems 

 to be the rat, and there is much evidence in support of 

 the view that it aids in the spread of epidemics. At 

 several of the Asiatic plague districts and at Santos the ap- 

 pearance of plague among the inhabitants was preceded by 

 a large mortality among the rats, some of which when ex- 

 amined showed buboes and had died of plague-septicemia. 



It is rather improbable that men become infected with 

 plague through the bites of the fleas leaving the bodies 

 of plague-destroyed rats, as was once supposed. Galli- 

 Valerio 1 thinks the fleas of the mouse and rat are incapa- 

 ble of living upon man and do not bite him, and that it 

 is only the Pulex irritans, or human flea, that is capable of 

 transmitting the disease from man to man. 



Yersin found that when cultivated for any length of 

 time upon culture-media, especially agar-agar, the viru- 

 lence was rapidly lost and the bacillus eventually died. 

 On the other hand, when constantly inoculated from 

 animal to animal the virulence of the bacillus is much 

 increased. 



The bacillus probably attenuates readily. Kitasato 

 found that it did not seem able to withstand desicca- 

 tion longer than four days ; but Rappaport (quoted by 

 Wyman) found that they remained alive when kept dry 

 upon woollen threads at 20 C. for twenty-three days, and 

 Yersin found that although it could be secured from the 

 soil beneath an infected house at a depth of 4-5 cm., 

 the virulence of such bacilli was lost. 



1 Centralbl. f. Bakt. u. Parasitenk., Jan. 6, 1900, xxvii., No. I, p. 1. 



