MEDICAL ENTOMOLOGY HOWARD. 579 



means of rat fleas was confirmed by Verjbitski in 1903 and by Liston 

 in 1904. The British plague commission in India proved most con- 

 clusively that the rat flea is the principal means of transmission of 

 the bubonic form of the plague. No less than eight species of fleas 

 have been shown by various workers to be carriers of the plague 

 bacillus, but the rat flea, known as Xenopsylla cheopsis, is the princi- 

 pal carrier in great epidemics. The Indian plague commission, find- 

 ing the bacilli only in the alimentary tract of the fleas, and not in the 

 salivary glands, concluded that the bacilli are present in the excreta 

 of the flea and are inoculated into the human host by scratching. 

 Experiments by Bacot and Martin, however, indicate that the plague 

 can be transmitted by the bite of fleas when a temporary obstruction 

 of the proventriculus of the insect causes the blood laden with bacilli 

 to be regurgitated into the bite puncture. 



This is the first of the diseases which we have mentioned in which 

 the insect seems to be simply the mechanical carrier of the disease 

 organism. In this case, as in many others, some of which we shall 

 briefly mention, there is obviously no development of the disease 

 germ in the body of the insect which is therefore not a necessary sec- 

 ondary host, but simply a carrier which, through its presence in great 

 numbers, causes the inordinate spread of disease. There are a num- 

 ber of insect-borne diseases in which the causative organism has not 

 yet been discovered, in which this point has not clearly been ascer- 

 tained. When Reed, Carroll, and Lazear failed to find the cause of 

 yellow fever, and yet were able to produce the disease from the 

 blood serum which had been filtered, they were, nevertheless, of the 

 opinion that there must be a causative organism too small to be seen 

 by the highest power of the microscope, and which undergoes a part 

 of its development in the body of the yellow-fever mosquito, since a 

 certain definite period must elapse between the infection of the mos- 

 quito and the time when its bite gave the infection to a nonimmune, 

 just as is the case with malaria where the organism and its full life 

 history is so well known and so easily demonstrable. 



But we must return to Ricketts and his second great discovery. 

 This excellent investigator was connected with the medical depart- 

 ment of the University of Chicago. Upon the conclusion of his 

 work in Montana, his attention was attracted to typhus fever. 



Typhus fever, a serious disease formerly prevalent under unsani- 

 tary conditions in the most civilized countries when people were 

 crowded together in camps, hospitals, or jails and which, in fact, a 

 hundred years and more ago was known as "jail fever" in England — 

 the disease which nearly killed the "Vicar of Wakefield" in his 

 life of many tribulations — while practically nonexistent in the United 

 States for many years, was still rife in the prisons of the principal 



