Echinostome cercarial penetration and metacercarial encystment as mortality factors for a second intermediate host,...

Cercarial penetration and metacercarial encystment of Echinostoma liei in Biomphalaria glabrata caused high mortality of juvenile snails (3-8 mm in diameter) after 4 to 6 days of continuous exposure to about 150 cercariae per snail per day. Larger snails (10-13 mm) withstood cercarial penetration longer, significant mortality appearing 16 days after exposure. Life-table analysis showed that the snail mortality rate per age interval increased with increasing exposure to echinostome penetration. Single exposures of 3- and 6-mm snails to 500 cercariae caused 97% mortality for the former and 84% for the latter within 2 days. Snails surviving the first 2 days did not suffer further mortality when infected with as many as 488 metacercariae. Thus, mortality seems to result from penetration and early stages of encystment, rather than the presence of large numbers of metacercariae in the pericardial region. Growth of 3-mm snails exposed to 10 or 100 cercariae and of 6-mm snails to 500 cercariae was significantly less than the growth of unexposed controls after 20 days. Cercariae rapidly located the snails, 90% penetrating within 1 hr when individual, 6-mm snails were exposed to 100 cercariae in 5 ml of water. The high rate of cercarial penetration needed to cause snail mortality suggests that echinostome penetration and encystment will not cause much mortality of snail intermediate hosts in nature unless the density of echinostome cercariae is high.