Experimental infection of Helisoma trivolvis (Colorado strain) snails with cercariae of Echinostoma trivolvis

Abstract

A Colorado (CO) strain of Helisoma trivolvis resistant to miracidial infection, but susceptible to Echinostoma trivolvis cercarial penetration, was used to study encystment of cercariae released from naturally infected H. trivolvis (Pennsylvania strain). All 12 snails, 8–10 mm in shell diameter, each exposed to 25 cercariae and necropsied 1–4 days post-exposure were infected with between 9 and 19 cysts per snail (average 14). In all infectivity experiments, regardless of the size (age) of the Colorado strain snails used, encysted metacercariae were recovered from the kidney-pericardial region of experimentally infected snails within 24 h post-exposure. Snails exposed individually with 500 cercariae averaged 230 cysts per snail at 24 h p.i. Cercariae encysted at 6–8 h p.i., but not at 2 and 4 h p.i. Light microscopical observations of the cysts suggested that greater than 99% were viable. Metacercariae subjected to in vitro excystation in an alkaline trypsin-bile salts medium showed excystation rates of 60–85%. In conclusion, H. trivolvis (CO strain) is an excellent experimental second intermediate host for cercarial encystment of E. trivolvis.