On the Viability of Cysts of Endamoeba Histolytica under Variable Conditions

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In an article by Yorke and Adams (1), it is claimed that the cysts of Endamoeba histolytica could not exist for more than ten days in a watery medium at room temperature. On the other hand Boeck (2), basing his estimate of viability of cysts on the penetrability of eosin through the cyst wall, gave the extraordinary figures of from one hundred -fifty to two hundred eleven days.

Dobell (3), in 1919, in his book “Amoebae Living in Man,” p. 47, makes the observation that: “The longest time of survival which I have observed is five weeks (cysts kept in water), but as a rule they will not live so long.” Since that time he has offered cultural proof of possibility of reproductive survival up to thirty-seven days (4) and in the same monograph states that possibly this is not the limit of viability. We have the following proof of the truth of this conjecture.