The effects of various chemical and physical factors on encystment and excystment of Zygocotyle lunata

Abstract

The effects of various chemical and physical treatments on the cercariae and metacercariae of Zygocotyle lunata were studied. Cercariae that actively emerged from Helisoma trivolvis snails were examined in 4 ml of test solution in a 3-cm diameter Stender dish. Observations were usually made on encystment and excystment from 2 to 24 h after cercariae were placed in the Stender dishes. Preferences for certain substrates for encystment were noted, such as smooth glass rather than rough glass, the macrophyte Eleocharis acicularis, and plastic Petri dishes. Many of the solutions tested allowed for encystment, i.e., artificial spring water (ASW), 10% artificial ocean water (AOW), and saline (0.85% NaCl). Certain hypertonic solutions allowed for both in vitro encystment and subsequent excystment, i.e., Locke's solution, 25% AOW, and Locke's 1:1. Serial dilutions of ammonium hydroxide, neutral buffered formalin, potassium permanganate, hydrochloric acid, and acetic acid killed cercariae at all concentrations. Ethanol was effective in killing cercariae at all concentrations except at 3.1%, which allowed for encystment. Cercariae were able to encyst at a low temperature of 16 degrees C and a high temperature of 41 degrees C. Morphometric analysis was performed on both live and fixed cercariae of Z. lunata as well as metacercarial cysts formed in Locke's, Locke's 1:1, and ASW. Live cercariae had larger body dimensions than cercariae fixed in hot 5% neutral buffered formalin and mounted in glycerin jelly. Cysts formed in ASW had very similar length and width measurements. Cyst dimensions became less similar to one another as the tonicity of the medium increased. The phenomenon of spontaneous excystation in Z. lunata after in vitro encystment in a hypertonic medium appears to be reported for the first time in digeneans.