FACTORS GOVERNING DISSEMINATION OF DISEASE 219 



etc., though to a less degree, may be responsible for the spread of 

 infectious disease of a bacillary nature, and an account will now 

 be given of the role which this insect plays in the disscini nation 

 of certain diseases ^ 



While flies have been sliown, as tlic evidence advanced latei- 

 will indicate, to cany pathogenic and nun-pathogenic organisms, 

 especially the non-spore-bearing micro-organisms, on the exterior 

 of their bodies, recent investigations, notably the excellent studies 

 of Graham-Smith (1909-13), Torrey (1912) and c.f Cox, Lewis 

 and Glynn (1912), would appear to indicate forcibly that the car- 

 riage of micro-organisms in the digestive tract and infection 

 therefrom is even more important than mechanical transference 

 on the appendages and exterior of the body. The longer duration 

 of the life of the micro-organism carried in the digestive tract of 

 a fly increases the possibility of infection either by " vomit " spots 

 resulting from regurgitation or by faecal spots. This important 

 aspect of the problem will be discussed shortly. 



It should be pointed out that whereas in some of the diseases 

 the epidemiological evidence adduced in support of transference of 

 disease germs by flies is confirmed bacteriologically, in others only 

 the former evidence exists. Should neither form of evidence be 

 available in support of the idea that M. domestica plays a part in 

 the dissemination of the infection of a particular disease, it is 

 essential, nevertheless, that if such a method of transference is con- 

 ceivable the possibility of this insect being able to carry the patho- 

 genic organism should be realised. This possibility is governed 

 by such factors as the presence of M. domestica ; its access to the 

 infected or infective material, this being attractive to the insect 

 either because it is moist or because it will serve as food for itself 

 or its progeny ; and a certain power of resistance for a short time 

 against desiccation on the part of the pathogenic organisms, 

 although, as in the case of the typhoid bacillus, the absence of 

 this factor is not fatal to the idea, as it may be overcome by the 

 fact that the fly is able to carry the organisms in its digestive 

 tract or to take on its appendages an amount sufficient to resist 



^ Though it should be unnecessary, I wish to explain, as I have been occasion- 

 ally misunderstood by medical men and others, that M. domestica is not regarded 

 as being the cause of any disease, but as a carrier of the infection. 



