20 LIMNOLOGY, WATER SUPPLY AND WASTE DISPOSAL 



viduals who have an intestinal bacterial flora which does not provide 

 conditions anaerobic enough for the perpetuation of the amoebae, may 

 not produce any infection at all. In those persons whose intestinal bacterial 

 flora furnishes a moderately anaerobic condition, the ingestion of cysts 

 may produce an infection without apparent pathology. In those persons 

 whose intestinal bacterial flora creates a profound anaerobic condition 

 which may extend into the mucosa, the introduction of amoebic cysts will 

 establish infection and produce amoebic lesions. 



, Furthermore, it is not entirely unlikely that the amoebae do secrete a 

 cytolysin for their parasitic life in the tissue and that the secretion of this 

 cytolysin may be facilitated by the profound anaerobiasis. 



However, there may be other factors involved in the pathogenicity of 

 E. histolytica, such as the virulence of the amoeba, the nature of the diet, 

 the physiological condition of the intestine, particularly its ability to resist 

 the changes produced by the normal bacterial flora, and the immunity 

 status. All of these may affect in one way or another the development of 

 clinical amoebiasis. 



C. Primary and Secondary Amoebic Dysentery. Whatever the actual 

 mechanism involved in the development of amoebic dysentery, from the 

 epidemiological point of view, the clinical cases may be divided into two 

 groups, namely: the primary and secondary dysentery. The primary cases 

 are those that develop dysentery following the ingestion of infective 

 material. They could be due to either introduction of amoebic cysts of a 

 virulent strain into the intestine where conditions favorable for develop- 

 ment of dysentery exist, or introduction of such amoebic cysts accompanied 

 by a bacterial flora which is capable of altering the normal intestinal flora 

 and favors tissue invasion. This type of case usually occurs during epi- 

 demics or in small outbreaks in poorly sanitated or unsanitated tropical 

 or subtropical regions. Examples are the cases in the Chicago epidemic, and 

 those developed after consuming grossly polluted water in the tropics 

 and subtropics as cited by Strong (1944) and observed in the Burma cam- 

 paign in World War II (personal communication). It is in this kind of 

 cases that the incubation period can be reasonably accurately determined 

 and source of infection established. 



The secondary cases are those that have been carriers for months or 

 years and develop amoebic dysentery upon introduction of certain second- 

 ary factors. The type of case in this group is illustrated by the carriers 

 who developed amoebic dysentery during outbreaks of bacillary dysentery 

 as observed among German soldiers by Hauer (1942) and Horster (1943) 

 and among the Allied troops in North Africa (Coggeshall 1943) and the 

 Near East (Fairley and Boyd 1943). In this type of case, the determina- 

 tion of the course of infection is very difficult, since the disease is not 

 correlated with the primary source but with the occurrence of secondary 

 factor or factors. 



D. Incubation Period of Amoebiasis. Under natural conditions, it is 

 impossible to know the incubation period of asymptomatic amoebiasis. In 



