14 MANUAL OF HUMAN PROTOZOA 



sidered abnormal and comparatively small in number. 

 Tetranucleated mature cysts are typical of Entamoeba 

 histolytica. In the faeces, the cysts do not undergo any 

 further changes. The most commonly seen cysts measure 

 about 7-16|j in diameter. 



Viability of cysts 



The dysentery amoeba is transmitted by viable cysts in 

 contaminated food or water. The cyst-passers or carriers 

 do often not show any symptoms of infection. According 

 to Yorke and Adams (1926), cysts kept in faeces at 16-20°C. 

 in the laboratory, begin to die rapidly and all are dead 

 within about 10 days. Keeping the faeces in an ice box at 

 0°C. gives approximately the same result. Washed suspen- 

 sions of the cysts live 17-21 days when kept at 0°C., and 

 10-11 days at 16-20°C. Chang and Fair (1941) state that the 

 cysts are viable 90 days at freezing point, but only 30, 10 

 and 3 days at 50°, 68°, and 86°F. respectively. 



Yorke and Adams state that the cysts survive a tempera- 

 ture of 45° C. for 30 minutes, but are killed within 5 minutes 

 at 50°C. The lethal strengths of various chemicals and drugs 

 when allowed to act upon the cysts for 30 minutes at 20- 

 25°C. and 37°C. are as follows: ' 



20-25 °C 37 °C 



HCl 7.5% 5% 



NaOH 2.5% 2.5% 



CI (sat. aq. solution) 1/64 1/320 



HgCL 1:2500 — 



Potassium permanganate 1% does not kill 1% does not kill 



Formaldehyde 0.5% 0.5% 



Carbolic acid 1% 1% 



Lysol 1% 0.5-1% 



Yatren 5% does not kill 5% does not kill 



Emetin HCl 5% does not kill 5% does not kill 



