In Vitro Excystment of the Metacercaria of Acanthoparyphium spinulosum (Trematoda: Echinostomatidae)

In vitro excystment studies on the metacercaria of Acanthoparyphium spinulosum indicated that optimal excystment occurred when the cysts were incubated in either a complete medium containing trypsin and bile salts, or bile salts alone following acid pretreatment, and the reductant (sodium dithionite) at a temperature of 42°C and a pH of 7.8. A relatively lower percentage of excystment was obtained following pretreatment with acidified pepsin or Hanks' BSS, the reductant, and incubation in trypsin at a pH of 7.8. Worms that excysted in any medium containing trypsin became sluggish after 1 hr in comparison to worms that excysted in bile salts, indicating that trypsin may have an inhibitory effect. Worms excysted in bile salts medium after pretreatment in acid and the reductant were more active, suggesting that bile salts may act as a muscular stimulant. Pretreatment with acidified pepsin followed by incubation in the complete medium, in the absence of the reductant, resulted in a low percentage of excystment; whereas, pretreatment with acidified Hanks' BSS followed by incubation in the complete medium in the absence of the reductant resulted in no excystment. The in vitro excystment process was initiated by acid pretreatment that resulted in vigorous muscular activity of the enclosed larva that eventually produced the escape aperture for emergence. A syncrgistic effect between trypsin and bile salts does not appear to occur in the excystment of this parasite. Several reports have been concerned with the in vitro excystment of metacercarial cysts of echinostomes. Howell (1970) studied the conditions necessary for the excystment of Echinoparyphium serratum; Fried and Grigo (1975) reported on E. flexum; Fried and Butler (1978) observed the excystment of Echinostoma revolutum; Kirschner and Bacha (1980) described factors involved in the excystment offfimasthla quissetensis; and LeFlore and Bass (1982) investigated the conditions responsible for the excystment of H. rhigedana. There have been no similar studies on the metacercaria of Acanthoparyphium spinulosum. The first and second intermediate host of A. spinulosum is the brackishwater snail Cerithidea hegewischi californica. Cercariae that develop in daughter rediae excyst in the radular mass of the snail. The natural adult hosts are the black-bellied plover, Pluvialis squaturola, and the American avocet, Recwvirostra americana, which acquire the parasite by eating infected snails. Adults have been grown experimentally in the domestic chick by Martin and Adams (1961). The present study was undertaken to examine the factors that bring about excystment of A. spinulosum in vitro and to describe the behavior of the metacercaria during