Arthropods and Disease Transmission 351 



controlled fundamental experimental work has been conducted 

 with these insects to prove that they are active transmitters of 

 intestinal disease. Hawley, et ah (1951), by mounting individual 

 Musco domestica and feeding them a controlled diet and a known 

 number of Salmonella schottmidleri, Escherichia coli, or Shigella 

 dysenteriae, found that unless at least one thousand viable organ- 

 isms were fed to the flies, recovery in the feces of the fed bacteria 

 was not possible. But when the number of ingested organisms 

 increased above one thousand per feeding, decided multiplication 

 occurred within the insects as evidenced by the excretion of 

 significantly greater numbers of the bacteria than were fed. More 

 such controlled investigations need to be conducted to indicate 

 more clearly what the true nature of the problem is with respect 

 to insect transmission of disease. 



While it is conceivable that a fly might ingest large enough 

 numbers of pathogenic bacteria to allow active multiplication to 

 occur within the digestive tract of the insect with a subsequent 

 spill-over into the excreta, it would appear that mechanical transfer 

 of organisms on the outside of the insect is probably more often 

 the case. Survival of bacteria on the hairy surface of the insect's 

 body will depend upon a number of factors, but studies indicate 

 that survival may extend for as long as several days, and a fly 

 can contaminate many objects during this period of time. 



The number of bacteria it is possible to wash from the outer 

 surface of a fly will naturally vary with the living habits of the 

 specific insect, but studies have shown that millions of organisms 

 per fly is a common finding. In general the number of bacteria 

 found within the digestive tract of flies is many times that found 

 on the outer surface of the insect. The honeybee, on the other 

 hand, harbors few or no bacteria within the body. Still other in- 

 sects may have certain segments of the digestive tract that are free 

 of viable microorganisms, while other segments are heavilv loaded 

 with bacteria. Over 250 species of microbes have been found 

 associated with insects. 



As early as the sixteenth century Pare reported that severe 

 wounds in which blowflies had deposited their eggs healed more 



