18 LIMNOLOGY, WATER SUPPLY AND WASTE DISPOSAL 



ized, only 4 developed dysentery, respectively 20, 57, 87, and 95 days after 

 the feeding. Some of the quiescent cases were followed for over 2 years and 

 never showed symptoms related to the infection. Furthermore, Wenyon and 

 O'Conner (1917) reported that of the 106 carriers of E. histolytica found 

 among 1979 healthy persons in Egypt, only 16 gave any history of 

 dysentery. 



Hence, it seems that even in the tropics and subtropics, the majority 

 of cases of amoebiasis exist in the native population in a quiescent state. 

 For these asymptomatic cases, the determination of the source of infection 

 would be extremely difficult, if not impossible. 



B. Secondary Factors in the Etiology of Amoebic Dysentery. Although 

 amoebic dysentery has been known to us since the end of the last century, 

 we must admit that we are still uncertain about the factors concerned in 

 the etiology of the clinical disease. As stated before, introduction of E. 

 histolytica into the human intestine oftentimes results in a carrier state and 

 dysentery may not develop until certain secondary factors are present. 

 Evidently the factors involved in the development of dysentery are complex. 

 Some laboratory investigators and clinicians (Dobell and O'Conner 1921; 

 Wenyon 1926; James 1927; Westphal 1937; Deschiens 1938; Nauss and 

 Rappaport 1940; Manson-Bahr 1947) have reached the conclusion that 

 under normal conditions, E. histolytica is a harmless parasite in the human 

 intestine and will not invade tissue and provoke dysentery or liver abscess 

 until other factors exist, such as injury of the tissue by pathogenic bacteria 

 or chemical agents. The fact that many of the German soldiers who had 

 been carriers for years in Germany developed amoebic dysentery during 

 outbreaks of bacillary dysentery in North Africa made Horster (1943) 

 believe that it was the presence of pathogenic bacteria such as the shigellae 

 that damaged the tissue and provoked clinical amoebiasis in those carriers. 

 Very recently, Wenyon (1947) has suspected that the Chicago epidemic 

 might have been an outbreak of bacillary dysentery among a population 

 with a high carrier rate of E. histolytica. 



The other group of investigators led by Craig (1944), Faust (1941) 

 and Johnson (1941) believe that E. histolytica is pathogenic under all cir- 

 cumstances. While there are some reports in the literature of the occur- 

 rence of typical amoebic ulcers in the intestine of apparently symptomless 

 persons, these observations have usually been made in the tropics and sub- 

 tropics. Those who favor this belief frequently quote the careful observation 

 of Faust (1941) made on 202 autopsies of accidental death in New Orleans 

 in which E. histolytica was found in 13, with 8 showing bowel lesions but 

 only 5 with amoebae in the lesion. Commenting on Faust's observation, 

 Napier (1947) has very recently pointed out that in none of these autopsies 

 was a typical amoebic lesion demonstrated, and the description of the 

 lesions, particularly of the superficial erosions, do not distinguish them 

 from post-mortem changes. 



In a previous communication, the author (1946) showed that E. 

 histolytica is an obligate anaerobe flourishing in natural or artificial media 



