156 Arthropods as Simple Carriers of Disease 



tions, they are important factors. Howard has emphasized that in 

 such cities there are still many uncared-for box privies and that, in 

 addition, the deposition of feces overnight in uncared-for waste lots 

 and alleys is common. 



Not only unicellular organisms, such as bacteria and protozoa, 

 but also the eggs, embryos and larvae of parasitic worms have been 

 found to be transported by house-flies. Ransom (191 1) has found 

 that Habronema musccB, a nematode worm often found in adult flies, 

 is the immature stage of a parasite occurring in the stomach of the 

 horse. The eggs or embryos passing out with the feces of the horse, 

 are taken up by fly larvffi and carried over to the imago stage. 



Grassi (1883), Stiles (1889), Calandruccio (1906), and especially 

 Nicoll (19 11), have been the chief investigators of the ability of 

 house-flies to carry the ova and embryos of human intestinal parasites. 

 Graham-Smith (19 13) summarizes the work along this line as follows : 



" It is evident from the investigations that have been quoted that 

 house-flies and other species are greatly attracted to the ova of 

 parasitic worms contained in feces and other materials, and make 

 great efforts to ingest them. Unless the ova are too large they often 

 succeed, and the eggs are deposited uninjured in their feces, in some 

 cases up to the third day at least. The eggs may also be carried on 

 their legs or bodies. Under suitable conditions, food and fluids 

 may be contaminated with the eggs of various parasitic worms by flies, 

 and in one case infection of the human subject has been observed. 

 Feces containing tape-worm segments may continue to be a source of 

 infection for as long as a fortnight. Up to the present, however, 

 there is no evidence to show what part flies play in the dissemination 

 of parasitic worms under natural conditions." 



Enough has been said to show that the house-fly must be dealt 

 with as a direct menace to public health. Control measures are 

 not merely matters of convenience but are of vital importance. 



Under present conditions the speedy elimination of the house-fly 

 is impossible and the first thing to be considered is methods of pro- 

 tecting food and drink from contamination. The first of these 

 methods is the thorough screening of doors and windows to prevent 

 the entrance of flies. In the case of kitchen doors, the flies, attracted 

 by odors, are likely to swarm onto the screen and improve the first 

 opportunity for gaining ah entrance. This diffictilty can be largely 

 avoided by screening-in the back porch and placing the screen door 

 at one end rather than directly before the door. 



