HOST-PARASITE RELATIONS! INTESTINAL PROTOZOA 



(1923) record 6.5 per cent of 8,029 persons infected. It 

 is rather strange to find less infection in the tropics, 

 where conditions for the spread of the parasites seem 

 particularly favorable, than in the temperate zone. Thus 

 Jepps (1923) records giardia in only 4.2 per cent of 

 1024 persons in the Federated Malay States and Heg- 

 ner (1925a) in only 2.1 per cent of 286 persons in tropi- 

 cal America. 



Viability of cysts outside the body. The cysts of giardia 

 are resistant to various factors encountered outside of 

 the body of the host. Experiments have been based on 

 the assumption that cysts that become stained in a weak 

 solution of eosin are dead and that those that do not 

 become stained are alive. No one has determined whether 

 these unstained ''living" cysts are capable of infecting 

 a new host. Boeck (1921a) found that giardia cysts are 

 killed at a temperature of 64° C, which is a tempera- 

 ture higher than any they normally are subjected to in 

 nature. In raw feces, the cysts seldom remain alive more 

 than 10 days, but washed cysts kept at room tempera- 

 ture were still alive over two months after they were 

 passed. Thus plenty of time is allowed for distribution be- 

 fore death occurs. Presumably, however, cysts are in- 

 capable of withstanding drying; hence the length of life 

 noted above is dependent upon the presence of moisture. 



Flies as transmitting agents. Giardia cysts resist con- 

 ditions in the digestive tract of flies for considerable 

 periods. Stiles and Keister (191 3) first proved that flies 

 may ingest fecal material containing cysts. Wenyon and 

 O'Connor (1917) recovered living cysts from flies 24 

 hours after such a meal ; also from the droppings of flies 



IS6 



