E. histolytica: distribution in host 



not become a carrier. Dale and Dobell (19 17) state that 

 the incubation period lasts about two weeks if cysts are 

 fed to kittens but averages only about two days if infec- 

 tions are brought about by rectal injections of motile 

 specimens. Wagener (1924) records the incubation 

 period in kittens, following rectal injections, as from 

 2 to 5 days and in adult cats as at least one week. 



3. DISTRIBUTION AND LOCALIZATION WITHIN THE HOST 



Cysts are passively carried to the primary site of in- 

 fection. Natural infections with E. histolytica are brought 

 about by the ingestion of infective cysts, although under 

 extraordinary circumstances (see p. 65) it is possible 

 that trophozoites may be responsible for an infection. 

 Cysts have no powers of locomotion and no hooks or 

 other structures that might anchor them to the wall of 

 the digestive tract, hence they are carried along passively 

 with the food that is swallowed. Since the primary site of 

 infection is the large intestine, they must pass through the 

 stomach and small intestine in a living condition. Food 

 is known to pass through the small intestine of man in 

 about 4 hours ; hence the cysts may reach the large intes- 

 tine in this length of time. If the contents of the large 

 intestine are not well formed, that is, if the bowels are 

 loose, many of the cysts and trophozoites (if excysta- 

 tion has taken place) may be carried directly out of the 

 body. As Dobell and Low (1922) have pointed out, in- 

 fection is most frequent in those parts of the intestine 

 where stasis occurs; and Sellards and Theiler (1924) 

 have emphasized the fact that stasis is an important con- 

 dition in experimental infections in kittens. Trophozoites 



8s 



