EPIDEMIOLOGICAL AND BIOLOGICAL PROBLEMS 25 



Disinfectants. In the last 5 years, numerous experiments on the destruc- 

 tion of amoebic cysts in water by various chemicals have been conducted 

 in our laboratory. A few reports by other investigators have also appeared 

 in the literature. It is beyond the scope of the present paper to deal in 

 detail with the data on this subject, but it is felt desirable to present here 

 a brief account of the conclusions reached in our studies and by other 

 investigators. The early studies in which the eosin staining method was 

 used (Wenyon and O'Conner 1917; Mills, Bartlett and Kessel 1925; Baylis, 

 Gullans and Spector 1936) for determining the viability of treated cysts 

 and the more recent study made by Stone (1937), in which the viability 

 of the treated cysts was tested by the culture method without restoring 

 the bacterial flora are not included in this presentation because these 

 methods of testing the viability of amoebic cysts are unreliable. 



1. Acids and Bases. Cysts of E. histolytica are very resistant to acids 

 and bases. The cysticidal effect seems to be chiefly a function of the hydro- 

 gen or hydroxide ion concentration. In our studies on water disinfection 

 (Fair, Chang and Morris 1945), it was found that the culture-induced 

 cysts were not destroyed by acids at a pH value of 0.5 with a contact time 

 of 2 hours. With bases reaching a pH value of 14.0, the cysts were 

 destroyed in 10 minutes; but a pH value of 13.0 showed no cysticidal effect 

 in 2 hours. Hence, it seems that the OH-ions are more cysticidal than 

 the H-ions. Unfortunately, this cysticidal pH value is too high to be of 

 practical use in ordinary water treatment. 



2. Halogens and Halogen Compounds. In an earlier report by Chang 

 and Fair (1941), it was shown that amoebic cysts were definitely more 

 resistant to active chlorine than vegetative bacteria, such as Escherichia 

 coli and that the cysticidal efficiency of active chlorine increased with 

 lowering of pH value, rise of water temperature, and increase in contact 

 time, and, to a less extent, with decrease in cyst concentration. It was 

 concluded that for complete destruction of amoebic cysts that may be 

 present in water, doses of active chlorine in the lower range of super- 

 chlorination vrould be required provided that the pH of the water could 

 be held below 7.5 and the chlorine demand was not too high. Brady and 

 his associates (1943) also reported that superchlorination is necessary to 

 destroy the cysts; but they also noticed that the contact time is more 

 important than the chlorine dosage, since increase in chlorine dosage some- 

 times resulted in a lowering of the cysticidal efficiency. In view of the 

 fact that these investigators used calcium hypochlorite in their tests and 

 that the increase in chlorine dosage by the use of calcium hypochlorite 

 automatically raises the pH of the chlorinated water (Fair, Chang and 

 Morris 1945), thus decreasing the amount of more cysticidal hypochlorous 

 acid (HOCOl) and increasing the amount of less cysticidal hypochlorite 

 ions (0C1~), this phenomenon observed by Brady et al. (1943) was 

 to be expected. It is regretted that these authors did not determine the 

 pH value of their treated water. 



Becker, Burks and Kalieta (1946), working with amoebic cysts 



