Acquired immunity to the trematode Fasciola hepatica in rats

Following a primary infection of 5 metacercariae (mc) of Fasciola hepatica, rats became immune to challenge with 30 mc. Immunity was less consistently demonstrated following challenge infection with 5 mc. The levels of precipitating and miracidial immobilizing antibodies, and antibodies detected by an indirect fluorescent antibody test, were monitored during the course of a primary infection of 5 mc and challenge infection of 30 mc. Following primary infection, peaks in the antibody response were associated with the invasion and migration of juvenile flukes in the liver parenchyma. Antibody titres fell when flukes entered the bile duct. There was a slight elevation in the titre of all antibodies following challenge, but, apart from those detected by the fluorescent antibody test, this elevation was not as marked as in control animals. It is suggested that flukes become isolated from, or have diminished contact with, the immune system once they enter the bile duct. This might explain why adult flukes survive in immune hosts whereas challenge metacercariae are destroyed.