HOST-PARASITE RELATIONS: INTESTINAL PROTOZOA 



that succeed in escaping from cysts and that find suitable 

 conditions in the large intestine may start an infection. 



Where does excystation occur and what factors are 

 responsible? The conditions that bring about excystation 

 are not well known and no one has yet determined 

 exactly where it occurs in the human host. It is generally 

 believed that cysts do not hatch outside of the body and 

 Dobell (Dobell and O'Connor, 1921) goes so far as to 

 state that "cysts never hatch in the colon, where they are 

 formed, or outside the body." Darling (1913), however, 

 as pointed out by Yorke and Adams (1926a), noted the 

 disappearance of cysts and the appearance of tropho- 

 zoites in feces containing histolytica cysts that were kept 

 in a moist chamber. It is not at all certain that the tro- 

 phozoites observed came from the cysts since amoebae 

 of other species often appear in fecal material kept under 

 similar conditions. 



Excystation in laboratory animals. Several investiga- 

 tors have attempted to study excystation by feeding cysts 

 to laboratory animals and then killing the animals after 

 intervals of various lengths and examining the cysts 

 present in various parts of the intestine. Dobell (1919a) 

 had no success with this method, but Chatton (1917b) 

 observed what he considered to be excystation of cysts 

 fed to cats. In the stomach the chromatoid bodies dis- 

 appeared from the cysts but no hatching occurred. 

 Excystation took place, however, in the small intestine. 

 A single quadrinucleate amoeba emerged from each cyst. 

 These amoebae ingested bacteria ; their cytoplasm became 

 vacuolated; and their nuclei clumped together. 



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