170 Arthropods as Direct Inoculators of Disease Germs 



After the discovery of the causative organism, one of the first 

 advances was the establishment of the identity of human plague 

 and that of rodents. It had often been noted that epidemics of the 

 human disease were preceded by great epizootics among rats and 

 mice. So well established was this fact that with the Chinese, 

 unusual mortality among these rodents was regarded as foretelling 

 a visitation of the human disease. That there was more than an 

 accidental connection between the two was obvious when Yersin, 

 the discoverer of Bacillus pestis, announced that during an epidemic 

 the rats found dead in the houses and in the streets almost always 

 contain the bacillus in great abundance in their organs, and that many 

 of them exhibit veritable buboes. 



Once it was established that the diseases were identical, the atten- 

 tion of the investigators was directed to a study of the relations 

 between that of rats and of humans, and evidence accumulated to- 

 show that the bubonic plague was primarily a disease of rodents 

 and that in some manner it was conveyed from them to man. 



There yet remained unexplained the method of transfer from rat 

 to man. As long ago as the i6th century, Mercuralis suggested 

 that house-flies were guilty of disseminating the plague but modern 

 investigation, while blaming the fly for much in the way of spreading 

 disease, show that it is an insignificant factor in this case. 



Search for blood-sucking insects which would feed on both rodents 

 and man, and which might therefore be implicated, indicated that 

 the fleas most nearly met the conditions. At first it was urged that 

 rat fleas would not feed upon man and that the fleas ordinarily attack- 

 ing man would not feed upon rats. More critical study of the habits 

 of fleas soon showed that these objections were not well-founded. 

 Especially important was the evidence that soon after the death of 

 their host, rat fleas deserted its body and might then become a pest 

 in houses where they had not been noticed before. 



Attention was directed to the fact that while feeding, fleas are in 

 the habit of squirting blood from the anus and that in the case of those 

 which had fed upon rats and mice dying of the plague, virulent plague 

 bacilli were to be found in such blood. Liston (1905) even found,, 

 and subsequent investigations confirmed, that the plague bacilli 

 multiply in the stomach of the insect and that thus the blood ejected 

 was richer in the organisms than was that of the diseased animal. 

 It was found that a film of this infected blood spread out under the 

 body of the flea and that thus the bacilli might be inoculated by the 

 bite of the insect and by scratching. 



