Possible role of excretory/secretory products in evasion of host defences by Fasciola hepatica

Abstract

RATS develop a marked resistance to reinfection with Fasciola hepatica after a single immunising infection^1,2. This resistance is apparent within 2 weeks of initial infection^3,4 but diminishes in long-standing infections⁴. Resistance does not depend on the presence of the initial infection at the time of challenge², and operates against intraperitoneally implanted metacercariae, juvenile or adult flukes³ as well as against orally administered metacercariae. Implantation (subcutaneous or intraperitoneal) with adult flukes does not stimulate marked resistance to challenge^3,5, and these implanted flukes may persist for many months in the cysts which form around them in the rat. Resistance may be transferred by cells⁶ or, provided that it is given at the time of challenge, by immune serum⁷; serum administered 6–8 d after infection is not effective⁷. However, the flukes of the immunising infection persist in the rat in an apparently normal healthy condition^1,2 and clearly evade the mechanism which controls challenge infections. We report here experiments which show that F. hepatica produces substances which are toxic to its host's lymphocytes and that these substances may protect the parasite from its host's immune defences.