898 



THE ANIMAL CARRIERS OF DISEASES 



human food, cooked and raw, such as milk, meat, butter, sweets, 

 etc. ; and the distances within reason to which flies travel is limited 

 by the necessities of food and shelter, but they go to the nearest 

 place, and if both exist at hand they do not travel. 



Nicholls, working in St. Lucia, has shown that the fly Limosina 

 punctipennis Wiedemann lives and breeds almost exclusively upon 

 human excrement, and that it is a carrier ot Bacillus coli communis. 

 In Africa flies belonging to the gemx^Pycnosoma — e.g., P. marginale 

 — do the same as do other species in India and China. 



These flies one and all are great feeders, and are accustomed to 

 vomit frequently, while they pass a considerable amount of excre- 

 ment. Graham Smith records 1,102 vomit marks and nine fsecal 

 deposits on an area of a cupboard window 6 inches square. 

 Bearing this in mind, it can readily be appreciated how well they 

 contaminate food and what efiicient disseminators of germs they 

 maybe. 



Graham Smith describes fsecal deposits as round, opaque, often 

 raised, spots of a yellowish, brownish, or whitish colour, while vomit 

 spots have an opaque centre and a clear periphery bounded by a 

 darker zone. 



But the body of the fly is thickly clothed with hairs or setae, and 

 as it walks over filth, particles containing bacteria are apt to cling 

 to these hairs. 



We therefore have to consider : — ■ 



A. The external carriage of germs. 



B. The internal carriage of germs. 



With regard to the former, Graham Smith's experiments with 

 B. prodigiosus show that this bacillus can be cultivated from the 

 legs and wings of infected flies for eighteen hours after infection. 

 It must be remembered that flies are everlastingly cleaning them- 

 selves, and it is a matter of common knowledge how the proboscis 

 is rubbed by the anterior pair of legs, which become contaminated 

 therefrom. 



Therefore the external carriage of germs from filth to food is 

 possible, provided that it takes place within a relatively short time. 



With regard to the internal carriage, the same observer has shown 

 that, though there is no evidence that B. prodigiosus multiplies 

 therein, it can live in the alimentary canal of flies for four to five 

 days. 



The investigation of the presence of B. typhosus in a fly is most 

 difficult, because there are non-lactose fermenters present as normal 

 denizens of the fly. Faichnie has shown that it tends to be present 

 in the intestine, and not on the legs, but Cochrane's experiments 

 show that it may be recovered from the external washings of flies. 

 The bacilli so obtained were tested with typhoid serum, and gave 

 positive reactions, and even by immunizing animals therewith 

 and testing the serum so obtained against stock B. typhosus, 

 positive results were obtained. 



