SPECIFIC DISEASES 1 25 



(i) Experiments showing to what extent and in what 

 manner flies can carry and distribute the causative organisms. 



(2) The discovery of the specific organism or related 

 types in ' wild ' flies, 



(3) General observations on the relationship of flies to 

 outbreaks of the disease. 



In considering the bearing of experimental work on natural 

 transmission of the disease organisms, it must be borne in mind 

 that most experiments have been conducted with cultures. 

 Much grosser infection of the fly is, therefore, produced than 

 can usually occur under natural conditions, and moreover 

 ' cultivated ' as opposed to ' uncultivated ' bacteria are used. 

 The latter fact introduces a factor about which we have at 

 present little accurate knowledge. We know that ' cultivated ' 

 bacteria usually grow better under artificial laboratory conditions, 

 but we have little information as to how they behave, as 

 compared with ' uncultivated ' bacteria direct from the body, in 

 competition with the different organisms present under varying 

 conditions in the alimentary canal of the fly. While it is 

 probable that the ' uncultivated ' strains persist longer in the fly, 

 it is certain that their powers of producing infection in the 

 human subject or in experimental animals are greater. 



In judging of the value of the records relating to the finding 

 of 'specific' pathogenic bacteria in 'wild' flies, it must be 

 remembered that many of the records were published several 

 years ago, when the means of diff"erentiating allied organisms 

 were not so complete as at present, and further, that com- 

 paratively little was then known of the numerous species, which 

 very closely resemble the specific pathogenic forms, both in 

 appearance and in culture. The greatest caution must therefore 

 be observed in accepting statements unaccompanied by full 

 descriptions of the methods employed. 



The general observations which have been published on the 

 relationship of flies to outbreaks of disease are obviously of very 

 unequal value. Many are mere surmises unaccompanied by 

 evidence; in many cases other possible sources of infection have 

 not been excluded and the evidence brought forward is of very 

 doubtful value; in some statistical evidence requiring very careful 



