224 THE DISSEMINATION OF PATHOGENIC ORGANISMS BY FLIES 



suck at dried milk spots, and it was shown that clean, uninfected 

 flies will infect themselves from the deposits, both faecal and vomit, 

 of infected flies. The clean flies will sometimes infect themselves 

 from the vomit or faeces deposited by infected flies several days 

 after infection. In no case were the limbs of the flies infected. 



The fly in the milk is unfortunately an occurrence altogether 

 too common. The infection of milk by the immersion of infected 

 flies was shown by Graham-Smith in his experiments, in which 

 B. prodigiostis, B. pyocyaneus and a pink Coccus were used, to be 

 possible for as long a period as 74 hours after the flies had fed 

 upon the infected matter. The micro-organisms were usually to 

 be found in the crops or intestinal contents of the flies during 

 that period. 



Experiments along similar lines were carried out with blow- 

 flies (Calliphora erythrocephala) which were able to produce gross 

 infection with non-spore-bearing bacteria {B. prodigiosus and 

 B. pyocyaneus) for six to nine days. 



The foregoing experiments clearly indicate the manner in 

 which and extent to which infected flies may infect milk and sugar, 

 and no doubt other foods containing these ingredients, by feeding 

 upon them. They also demonstrate the more serious nature of 

 such infection compared with infection by means of the limbs or 

 body owing to the infective organisms persisting for a much 

 greater length of time in the alimentary tract of the fly. 



Infection from Flies bred in infected material. 



In addition to the direct mode of transference of the typhoid 

 bacillus, by mechanical means or by feeding on infected material, 

 some experiments of Faichnie (1909) indicated what may be a 

 still more important means of dissemination. He believed that 

 one of the most important sources of fly infection was from flies 

 bred in enteric excreta ; such insects thereby became carriers of 

 bacilli for the remainder of their lives, spreading the infection 

 chiefly by means of their excreta. They were, in fact, typhoid 

 " carriers." 



On August 12th, 1909, three ounces of faeces infected with 

 B. typhosus were placed in a box of earth and covered with a wire 



