In vitro excystment of the metacercariae of Cloacitrema michiganensis (Trematoda: Philophthalmidae).

In vitro excystment studies on the metacercariae of Cloacitrema michiganensis indicated that optimal excystment occurred when the cysts were incubated in a complete medium containing trypsin and bile salts at a temperature of 41 to 42 C and a pH of 7.8 in the absence of acid pretreatment. A relatively lower percentage of excystment was obtained following pretreatment with acidified pepsin or Hanks' balanced salt solution (BSS). A low percentage of inactive worms excysted when the cysts were incubated in: trypsin after pretreatment with acidified pepsin; bile salts following pretreatment with acidified pepsin; and the complete medium, after acid pretreatment followed by the reductant (sodium dithionite). Treatment with bile salts after pretreatment with acidified Hanks' BSS, the reductant and the complete medium, and simple media such as Hanks' BSS and warm physiological saline resulted in no excystment. The in vitro excystment process involves two phases, activation and emergence, which are initiated by the metacercaria in response to the appropriate conditions. It is suggested that a synergistic effect between trypsin and bile salts is necessary for excystment whereas acid pretreatment is not.