Acquired Immunity and Age Resistance in Rats with Chronic Fascioliasis

Abstract

Rats with chronic fascioliasis (7 or 12 months' duration) exhibited a significant degree of acquired immunity to superinfection with Fasciola hepatica. Age resistance to primary exposure given at the time of challenge was also demonstrated. Thus, rats challenged with 20 metacercariae, 28 or 48 weeks after a single previous exposure to 5 metacercariae, yielded 66 or 50% less flukes, respectively, from challenge than previously uninfected controls. No significant effect of the challenge on the first (existing) infection, however, was detected. Moreover, in contrast to rats 3 or 4 weeks of age at the time of challenge, old rats (8 or 13 months) given the challenge exposure as a primary infection yielded 58 or 56% less flukes, respectively, than singly infected young rats. Hence, fluke burdens in old rats can be influenced both by previous exposure and host age. These results are presented as further evidence of immunity to superinfection in the F. hepatica-infected rat, and show the continued presence of immunity in long-standing infections. Although previous investigators have reported that the rat can resist challenge with Fasciola hepatica following exposure to irradiated metacercariae, multiple stimulating infections, or lymphoid cells from infected donors (Thorpe and Broome, 1962; Ruter cited in Geyer, 1971; Corba et al., 1971), it has only recently been reported that rats exposed to a single infection exhibited a marked resistance to subsequent challenge exposure (Hayes et al., 1972, 1973; Goose and Macgregor, 1973). Superinfection in the last-mentioned studies was given when the rats were in the subacute phase of the first infection (e.g., 7 weeks); the study reported herein was conducted to determine the response of rats with chronic fascioliasis (7 or 12 months) to a superinfec-