E. HISTOLYTICA CYSTS I FLIES 



Other protozoan cysts has been recognized for many 

 years. As long ago as 19 13 Stiles and Keister recovered 

 giardia cysts from flies that had been fed on fecal matter 

 containing cysts of this flagellate. Kuenen and Swellen- 

 grebel (1913) did not find cysts in or upon flies that 

 were allowed access for 48 hours to fecal matter contain- 

 ing histolytica cysts ; .and cysts that were present on the 

 outside of flies that they soiled with infected material 

 soon died because of dessication; they therefore con- 

 cluded that flies are unimportant carriers. Several years 

 later, however, Thomson and Thomson (1916b) noted 

 that flies may ingest cysts and deposit them in their 

 feces. A number of interesting and important experi- 

 ments were conducted by Wenyon and O'Connor (19 17) 

 with flies belonging to several genera. They found that 

 a fly that had not fed for 2 or 3 hours could ingest one 

 milligram of feces in half an hour; Root (1921) records 

 a larger capacity, a single house fly {Musca domestica) 

 ingesting 0.0068 cc. of fluid and a single blow fly (Calli- 

 phora erythrocephala) 0.022 cc. after being without food 

 for from 17 to 21 hours. Flies are thus able to ingest a 

 large number of cysts at a single meal. With the use of 

 the eosin test, Wenyon and O'Connor found that cysts 

 are not killed by conditions in the fly's digestive tract, 

 but may remain alive there for as long as 24 hours, and 

 that the fly may deposit living cysts in its feces from 5 

 minutes to at least 16 hours after feeding. Of particular 

 interest is their discovery that 18 of 229 wild flies cap- 

 tured in a hospital compound passed cysts or eggs of 

 parasites in their feces; of these, 6 deposited coli cysts 

 and 5 histolytica cysts. Cysts were not found in material 



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