EPIDEMIOLOGICAL AND BIOLOGICAL PROBLEMS 21 



the quiescent cases in Walker and Sellards' experiment (1913) the average 

 incubation period was about 9 days. Even if we take for granted that the 

 incubation period of naturally infected cases is 9 days, it would not be 

 of any help in ascertaining the date of infection unless the population is 

 constantly examined for the presence of the amoebae in the stool. 



In the secondary cases of amoebic dysentery, the incubation period, 

 as stated before, is very obscure, since it is not the period between the 

 ingestion of amoebic cysts and the development of clinical amoebiasis, but 

 the period elapsed after the secondary factors come into operation. 



Hence, from a practical view-point, it is only in the primary cases 

 of amoebic dysentery that the incubation period can be determined with 

 a fair degree of accuracy. In the four cases of dysentery observed in 

 Walker and Sellards' experiment, the incubation period was 20, 57, 87, 

 and 95 days respectively. The incubation period of the cases involved in 

 the Chicago epidemic has been considered by Wenyon (1947) as too short 

 for amoebic dysentery. Wenyon stressed in particular the few cases hav- 

 ing an incubation period of two days and thought that, in order to pro- 

 duce amoebic dysentery with so short an incubation period, the cyst- 

 contaminated water must have been so heavily polluted that it would have 

 shown physical impurities to arouse objections on the part of the guests. 

 In the author's opinion, there must have been misdiagnosed cases in that 

 epidemic, in view of the fact that physicians and technicians did not 

 become aware of the existence of the epidemic until it was over and most 

 of them were probably not too familiar with the diagnosis of amoebiasis. 

 Hence, it is not unlikely that those few cases with an incubation period of 

 less than one week might have been cases of bacterial enteric infection; 

 but there is not much doubt that the epidemic, as will be shown later, 

 was itself amoebic dysentery. Furthermore, the incubation period of the 

 4 cases in Walker and Sellards' experiment cannot be regarded as uni- 

 versally applicable since the virulence of the amoeba, the susceptibility of 

 the host, and the nature of the intestinal bacterial flora may vary from 

 strain to strain, person to person, and place to place. 



In fact, the incubation period of the majority of cases in the Chicago 

 epidemic was not too different from that observed in other studies. Of the 

 216 selected cases who resided in the two hotels involved for a short enough 

 time to allow the determination of incubation period, 25% gave an incu- 

 bation period of less than 11 days, 25% between 12 and 20 days, 25% 

 between 21 and 36 days, and the remaining 25% between 37 and 120 days. 

 It may be interesting to point out that, because of the relatively long 

 incubation period in the majority of cases, the Chicago epidemic was not 

 recognized during the outbreak as there were only 15 cases which were con- 

 nected with the two hotels reported to the Chicago Board of Health, and 

 that the epidemic was only disclosed by the questionnaire survey made later 

 when suspicion was aroused by a report on amoebic dysentery at the 

 American Public Health Association Convention held in Indianapolis (Natl. 

 Inst. Health Bui.). 



