Scanning electron microscopy and chemical excystation of Fascioloides magna (Trematoda) metacercariae.

Scanning electron microscopy was used to study encysted metacercariae and newly excysted juveniles of Fascioloides magna. The outer cyst was rough, coarse and discontinuous in the ventral aspect; the inner cyst was smooth. The newly excysted metacercaria was plump and contained numerous tegumentary spines; large dome-shaped papillae were prominent around the oral sucker and on the rim of the acetabulum. Encysted metacercariae with outer cysts were excysted in an alkaline bile salts-trypsin medium at an elevated temperature in the absence of acid saline or acid pepsin pretreatment. Pretreatment in acid saline slightly decreased subsequent excystation, while pretreatment in acid pepsin slightly enhanced subsequent excystation in the alkaline bile salts-trypsin medium.