CHAPTEE VI. 



SOURCES OF PATHOGENIC MICRO-ORGANISMS 



(Concluded.) 



(6) Dissemination and Transmission by Insects. 



A. Dissemination. 



Role of The demonstration that insects may play a role 

 in the transmission and maintenance of infections 

 dates from the work of Smith and Kilbourne, 

 which disclosed the relation of the tick (Margaro- 

 pus annulatus) to Texas fever in cattle. 



Insects may act simply as disseminators of 

 virus, or as the agents of actual inoculation 

 through their bites i. e., as transmitters. It is 

 important to keep this distinction in-mind. 

 B Meciumicai -^ n their ro ^ e as P ure disseminators they may 

 Transmission, carry micro-organisms from one point to another 

 on their feet, mouth parts, or in their intestinal 

 contents ; or, they may act as temporary hosts, the 

 microbes proliferating in their intestinal tract and 

 subsequently being deposited with the feces, in in- 

 creased numbers, at some new point. The first is 

 pure mechanical dissemination, whereas in the sec- 

 ond a biologic factor enters, that of proliferation, 

 and it may be spoken of as biological dissemina- 

 tion. For example, when flies carry typhoid bacilli 

 or cholera vibrios from feces to food, on their feet 

 or mouth parts, or tubercle bacilli from sputum to 

 food or other objects, this is, of course, a purely 

 mechanical process. It would be complicated by 

 the biological feature, however, in case the or- 



