The different stages of Fasciola hepatica capable of inducing immunity and the susceptibility of various stages to immunological attack in the sensitised rat.

Intraperitoneal sensitisation of female Piebald Virol Glaxo rats by the migratory immature stages (two-hours-, six-days- and 14-days-old) of the liver fluke Fasciola hepatica protected them from intraperitoneal challenge with living adult flukes three weeks later. Sensitisation with adult fluke antigen and Freund's complete adjuvant had no effect. Flukes attenuated by gamma irradiation (3-8 kiloroentgen), and so prevented from developing beyond the eight to 10 day stage, did not protect against adult challenge. When rats were sensitised orally with metacercariae and challenged with 14-day-old flukes intraperitoneally the challenge flukes were not adversely affected. It is suggested that although 14-day-old flukes can immunise rats adequately, the fact that they are not killed when placed in sensitised rats may be due to the protective effect of tegumental replacement in these flukes before gaining access to, and safety of, the liver site.