Protozoa of the Digestive and Urogenital Tracts 563 



to lie upon those who wish to consider this factor unimportant in the 

 epidemiology of amoebiasis. Whether or not it can be assumed that food- 

 handlers are important, the control of amoebiasis at this point would 

 require laboratory examinations at intervals, as well as rigid enforcement 

 of sanitary regulations. The sheer numbers of individuals involved in 

 handling food, the time required for thorough examinations, and the 

 scarcity of experienced laboratory personnel make even a single survey 

 of all food-handlers an utter impossibility. This situation leaves educa- 

 tional measures as the only practical supplement to adequate sanitary 

 codes. 



A source of pure drinking water is another important requirement. 

 That amoebiasis can be spread through polluted water was demonstrated 

 in the Chicago hotel outbreak of 1933 (29), although faulty plumbing 

 rather than inadequate purification of drinking water was involved. In 

 the purification of mimicipal water supplies, rapid sand filtration after 

 preliminary chemical coagulation and sedimentation is reasonably etlec- 

 tive in removing cysts of E. histolytica (12). Although it is not certain 

 that filtration is completely protective, an efficiently operated filtration 

 plant is probably the best safeguard for a large population. The efficiency 

 of chlorination alone vaaies with the concentration of free chlorine, with 

 temperature and pH of the water, and with the amount of organic matter 

 present. Varied results have been obtained on experimental scales. Stone 

 (157) found cysts of E. histolytica no more resistant than Escherichia coli, 

 being killed within 20 minutes by chlorine at 4-10 ppm, whereas Morton 

 (124) believes that even under ideal conditions, chlorine at 30 ppm for 

 30 minutes would be necessary to kill all cysts. Conservative opinion holds 

 that routine chlorination, as currently practiced, cannot prevent the 

 spread of amoebiasis by water supplies. Treatment with high concentra- 

 tions ("superchlorination"), followed by removal of enough chlorine to 

 restore potability, is recommended (128). 



BALANTIDIOSIS 



Balantidium coli 



(Malmsten) Stein 



This ciliate is the only one definitely known to be parasitic in man. 

 The active stage (Fig. 11. 7, A, B), measures 30-200 x 20-70[j,. The cyto- 

 stome is well developed and functional, as are the two contractile vac- 

 uoles. Food vacuoles may contain bacteria and other material from the 

 colon, or sometimes red corpuscles and other tissue elements. A cyto- 

 logical study of B. coli, with special reference to the fibrillar system, has 

 been published by McDonald (113). 



The cysts (Fig. 11. 7, C), which reach a diameter of 60-65[jl, are the 

 largest ones encountered in human stools. Food vacuoles are usually 



