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HOW THE FLEA INFECTS ITS HOST. 



The exact method by which the flea can transmit plague from 

 animal to animal has, in our opinion, never been satisfactorily 

 explained. There have been several explanations offered: First, 

 that the rat may eat the flea. Miller (7) has found that the 

 Hepatazoon perniciosum is transmitted from rat to rat through the 

 rat eating the mite, Lelaps echidninus, which acts as the intermediate 

 host. We know, however, that when a rat is fed on plague material 

 a mesenteric bubo is the rule, while in naturally infected rats a 

 mesenteric bubo is a rare condition. This, then, negatives the possi- 

 bility of plague being contracted through eating the flea. 



Another explanation is that the infection comes from the saliva 

 injected at the time of biting. We have already stated that after 

 repeated examinations, both by dissecting out the salivary glands 

 and by serial sections of the entire flea, plague bacilli have never 

 been demonstrated in these glands or anywhere outside of the 

 alimentary tract. 



Another explanation has been advanced, that the bacillus is intro- 

 duced by the contaminated mandibles. It is not possible to exclude 

 this as a means of infection, although the Indian Plague Commission 

 made numerous investigations and was unable to demonstrate the 

 bacillus on the mandibles. 



The possibility of infection taking place by regurgitation from the 

 stomach has also been considered. As the stomach is guarded by 

 the finger-like processes in the gizzard which seem to act as compe- 

 tent valves, and as the movement of the blood aspirated by reason of 

 the mechanism already explained is in a backward direction, it 

 would seem improbable that there is any regurgitation from the 

 stomach. 



The most plausible explanation that has been advanced has been 

 based on an observation that blood-sucking insects at the time of 

 biting frequently eject a drop of blood from the rectum. We know 

 that the rectum may contain numerous plague bacilli, and it is sup- 

 posed that this blood ejected in the vicinity of the bite is either 

 brought in contact with the slight wound by the feet or mandibles 

 of the flea itself or is rubbed in as a result of scratching. Verjbitski 

 has shown that an emulsion of the feces of fleas or any plague material 

 when placed upon the bitten part before the expiration of twenty-- 

 four hours is sufficient to give the animal plague. After twenty- 

 four hours the animals did not develop plague, it being supposed that 

 the slight wound in the skin made by the biting apparatus had healed. 

 It is probable that this ejection of blood is purely accidental and does 

 not necessarily occur at the time of biting, but it is likely that the 

 insect had just previously had a full meal, which had been digested 

 and passed into the rectum. In the many biting experiments done 



