Lack of Evidence for Encystment in the Parasitic Amoeba Hydramoeba hydroxena (Entz)

SYNOPSIS. It is known that hydras infected with H. hydroxena commonly disintegrate and release scores of amoebae. Spherical resting bodies found among the free amoebae have been described as cysts and it has been suggested that they give rise to amoebulae. Using Hydra pseudoligactis as the host, a study was made of the history of the free amoebae and of the structure, nature, and fate of the so-called cysts. All such amoebae transformed into spherical bodies that resembled cysts, even to the presence of an external membrane. These bodies invariably disintegrated after 2–3 days and they never produced amoebulae. They were called pseudocysts. The external membrane proved to be a layer of gelated cytoplasm. Unlike a true cyst membrane, it dissolved in KOH and it underwent solation when the pseudocyst disintegrated. It is concluded that the pseudocysts are actually weakened, starved, moribund amoebae. At present there is no evidence that H. hydroxena is capable of producing true cysts; its transmission from host to host appears to be solely by means of the amoeboid trophozoite.