THE BACILLUS OF PLAGUE. 115 



been studied by Nuttall (20). It had been already noticed 

 by Yersin (4) that flies died of plague, and that an infected 

 insect could establish the disease in a guinea-pig. Many 

 flies, however, die of water, and the laboratory experi- 

 ments by Yersin have been greatly extended by Nuttall's 

 researches, who has worked out this subject with the 

 greatest care, making use of cultures from Kolle and 

 A. Macfadyean of London, which were sufficiently virulent 

 to kill mice in. thirty-six to forty-eight hours. To any one 

 familiar with the swarms of flies in the East, it would 

 appear very probable that if these insects can be infected, 

 which undoubtedly is the case, they might be an efficient 

 means in spreading the disease. In all experiments to 

 decide this question control experiments were devised. 

 Flies fed with infected bouillon died much earlier than those 

 in the control apparatus, and their bodies contained pest- 

 bacilli which were pathogenic for mice. Infected flies 

 which had not died contained virulent bacilli for forty- 

 eight hours and longer, after they were removed from the 

 apparatus. Similar experiments were made with bugs, but 

 in these insects the bacilli gradually die. In the case of 

 mosquitoes and fleas no experimental evidence exists, 

 though it is a well-established fact that the former can in- 

 fect man with filarice, and virulent pest-bacilli have been 

 found in fleas taken from plague-stricken rats (25). Han- 

 kin has observed that ants rapidly eat up rats dead of 

 plague, and their excreta contains virulent bacilli. Ants, 

 however, do not die of plague, nor do they retain the in- 

 fection for any length of time (27). From a practical point of 

 view, though the actual proof in any epidemic has not yet 

 been given, it cannot be denied that flies at any rate may 

 play a part in spreading the plague, and at any rate may 

 contaminate food or water by their excreta. 



Like other pathogenic bacteria the bacillus of plague 

 produces toxins, which accumulate in the micro-organisms 

 and are discharged by these into the surrounding medium. 

 The fever of plague is directly due to these products. 

 Lustig and Galeotti (29) have obtained various products by 

 extracting cultures of the microbe for twelve to twenty-four 



