SOURCES OF INSECT INFECTION. 71 



derive the micro-organisms. Commonly, the trans- 

 mission occurs only between different members of 

 the same species : the insect obtains the virus from 

 one individual and inoculates it into another. In 

 so far as is known, man alone suffers from yellow 

 fever, and the human type of malaria, and he 

 constitutes the only source of infection for the 

 mosquitoes which are concerned in the mainte- 

 nance of these diseases. The same conditions ap- 

 pear to prevail regarding piroplasmosis in cattle 

 (Texas fever) and in other animals, and also in 

 the South African tick fever of man (a spirillosis), 

 diseases in which ticks are transmitters. Pre- 

 sumably this is also true of some other insect- 

 borne diseases among animals, as the spirillosis of 

 fowls and geese. 



In some other instances the existence of a third 

 host has been demonstrated. Man is an important 

 source of infection for the flies which carry sleep- 

 ing-sickness, but the evidence is strong that some 

 of the native animals of Africa also harbor the 

 trypanosome concerned and that tsetse flies be- 

 come infected from them as well as from man. In 

 Eocky Mountain spotted fever man is virtually a 

 negligible factor for the infection of the ticks. 

 The circumstances indicate that one or more spe- 

 cies of small, wild animals, of demonstrated sus- 

 ceptibility, are the means of keeping the diseases 

 alive in the ticks. It appears to play back and 

 forth from tick to animal, and it is only occa- 

 sionally that an infected tick becomes attached to 

 man. Fleas probably derive the micro-organisms 

 of plague from rats, in large measure, but experi- 

 ments also show that they may become infected 

 from man. 



