E. histolytica: prepatent period 



reports data obtained by him in tropical Batavia in 191 7 

 to 1924; these indicate August, October and November 

 as the months of greatest incidence. The conclusion 

 reached is that climate is responsible for the change from 

 the carrier condition to that of acute dysentery and that 

 the greater incidence of clinical amoebiasis in the tropics 

 is explained by the fact that hot weather is favorable 

 and colder weather unfavorable for the production of 

 symptoms. Just how differences in temperature operate 

 to bring about these results is not indicated. 



2. PARASITOLOGICAL AND CLINICAL PERIODS 



The prepatent period. Usually the exact time when in- 

 fective cysts were ingested by persons infected with E. 

 histolytica is not known. On this account we must depend 

 on the very few human experiments that have been re- 

 ported and on animal experiments for our data. The best 

 work on human beings is that of Walker (Walker and 

 Sellards, 1913). Specimens, mostly cysts, in gelatine cap- 

 sules, were fed to 20 different persons whose stools 

 had previously been found to be free from this species. 

 These men were then kept under observation for periods 

 of time ranging from 2 months to one year — 14 for the 

 latter period. Eighteen of the 20 became infected; no 

 specimens were recovered from the other two during 

 the year following the initiation of the experiment. The 

 prepatent periods of those who became infected were as 

 follows : in 2 cases, i day ; in 2 cases, 2 days ; in i case, 

 3 days ; in 4 cases, 4 days ; in 3 cases, 5 days ; in i case, 

 8 days; in 2 cases, 11 days; in i case, 21 days; in i case, 

 33 days; and in i case, 44 days. The shortness of the 



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