152 Arthropods as Simple Carriers of Disease 



the fly season the numbers of bacteria on flies are comparatively 

 small, while later the numbers are comparatively very large. The 

 place where flies live also determines largely the numbers that they 

 carry. The lowest number, 550, was from a fly caught in the 

 bacteriological laboratory, the highest number, 6,600,000 was the 

 average from eighteen swill-barrel flies. Torrey (1912) made exami- 

 nation of "wild" flies from a tenement house district of New York 

 City. He found "that the surface contamination of these 'wild' 

 flies may vary from 570 to 4,400,000 bacteria per insect, and the 

 intestinal bacterial content from 16,000 to 28,000,000." 



Less well known in this country is the work of Cox, Lewis, and 

 Glynn (1912). They examined over four hundred and fifty naturally 

 infected house-flies in Liverpool during September and early October. 

 Instead of washing the flies they were allowed to swim on the surface 

 of sterile water for five, fifteen, or thirty minutes, thus giving natural 

 conditions, where infection occurs from vomit and dejecta of the 

 flies, as well as from their bodies. They found, as might be expected, 

 that flies from either insanitary or congested areas of the city contain 

 far more bacteria than those from the more sanitary, less congested, 

 or suburban areas. The number of aerobic bacteria from the former 

 varied from 800,000 to 500,000,000 per fly and from the latter from 

 21,000 to 100,000. The number of intestinal forms conveyed by 

 flies from insanitary or congested areas was from 10,000 to 333,000,000 

 as compared with from 100 to 10,000 carried by flies from the more 

 sanitary areas. 



Pathogenic bacteria and those allied to the food poisoning group 

 were only obtained from the congested or moderately congested 

 areas and not from the suburban areas, where the chances of infesta- 

 tion were less. 



The interesting fact was brought out that flies caught in milk 

 shops apparently carry and obtain more bacteria than those from 

 other shops with exposed food in a similar neighborhood. The 

 writers explained this as probably due to the fact that milk when 

 accessible, especially during the summer months, is suitable culture 

 medium for bacteria, and the flies first inoculate the milk and later 

 reinoculate themselves, and then more of the milk, so establishing a 

 vicious circle. 



They conclude that in cities where food is plentiful flies rarely 

 migrate from the locality in which they are bred, and consequently 

 the number of bacteria which they carry depends upon the general 



