EPIDEMIOLOGICAL AND BIOLOGICAL PROBLEMS 17 



ever, as to the spread of the infection in a large community, two schools 

 of thought seem to have prevailed, one of which emphasized the importance 

 of contaminated water supply while the other stressed the presence of car- 

 riers among food handlers. 



Many epidemiologists have not yet fully accepted the view that 

 amoebiasis is water-borne. The skepticism seems to be based on the fact 

 that amoebiasis in general does not follow the epidemiological pattern of 

 classic water-borne bacterial diseases. For instance, amoebic infection is 

 usually not directly traceable to water supply ; it is not explosive in nature ; 

 and the cysts of E. histolytica have not been demonstrated in water. The 

 Chicago epidemic, in the opinion of these epidemiologists, is so unusual that 

 it should be considered as an exception rather than the rule. 



In an attempt to ascertain the role played by water in the spread of 

 amoebiasis, the author has endeavored in the first two sections of this paper 

 to bring together and analyse the important reports pertaining to this sub- 

 ject and to discuss certain problems encountered in determining the mode of 

 transmission of this protozoan infection. 



Epidemiologically speaking, the mode of transmission of any infectious 

 disease is generally detected by a close correlation of the disease in one way 

 or another with its source of infection. In establishing this relationship, the 

 following conditions are desirable: 



(1) The infection should have a relatively short incubation period. 



(2) The infection should give rise to certain distinct clinical symp- 

 toms that call for medical attention, thus establishing the diag- 

 nosis. 



(3) The causative organism of the disease should be demonstrable 

 in the transmitting agent, or agents. 



Unfortunately, not all infectious diseases satisfy these above-mentioned 

 conditions. This is particularly true in the case of amoebiasis which has 

 certain features that make the establishment of the relationship of the 

 infection to its source extremely difficult under ordinary circumstances. 

 These features are discussed in the subsequent section of the paper. 



I. Certain Peculiar Clinical Features of Amoebl\sis 



A. Symptomless Infection. One of the characteristics of the behavior 

 of E. histolytica is its great tendency to produce asymptomatic infection, 

 particularly in the temperate zone. Ingestion of the cysts of E. histolytica 

 with subsequent appearance of the organism in the stool does not necessarily 

 mean that the infected person is going to develop chnical symptoms. In 

 fact, most of the infected cases develop into a quiescent state. This phe- 

 nomenon is not only confirmed by the existence of carriers in various coun- 

 tries (Craig and Faust 1943) but also by the outstanding work of Walker 

 and Sellards (1913) in the Philippines. These two authors fed a faecal 

 suspension containing cysts of E. histolytica from a carrier to 20 volunteers 

 and found that 18 became infected and 2 did not. Among the 18 parasit- 



