Cell-mediated immune response in calves to single-dose, trickle, and challenge infections with Fasciola hepatica

A peripheral blood mononuclear cell (PBMC) proliferation assay was used to study the cell-mediated immune response in eight calves experimentally infe cted with Fasciola hepatica. Hypersensitivity-related eosinophil and mast-c ell responses were also assessed. The primary infection of 500 metacercaria e was administered either as a single-dose or as a trickle infection over a 4-week period. Calves were challenge-infected 4 months later with 100 meta cercariae and slaughtered 24 weeks postprimary infection. Skin eosinophil c ounts (SEC) were determined prior to infection on the basis of the intrader mal reaction (IDR) to phytohaemagglutinin (PHA). These counts correlated ne gatively with the mean fluke length but not with the fluke burden found at necropsy. At the end of the experiment, non-specific (PHA) and specific (ex cretory-secretory parasite products, FhESAg, and whole-worm extract, FhSomA g) immediate type hypersensitivity IDR were elicited in contrast to delayed type hypersensitivity (DTH) responses. The SEC correlated with blood eosin ophilia but not with parasite parameters. These findings suggest that the e osinophil response does not correlate clearly with the development of resis tance to F. hepatica infection in cattle. A specific mononuclear cell respo nse to FhSomAg was detectable as early as 7 days after infection in both in fected groups, being significantly higher during the very early migratory p hase of the juveniles in the single-dose infected calves than in the trickl e infected calves. This response remained significantly higher in infected groups than in the control group throughout the experiment. Challenge elici ted a significant proliferative response, less pronounced than after primar y infection. No production of gamma-interferon (INF-gamma) was recorded 3 w eeks after challenge. At necropsy, the mean number of flukes recovered was similar in both infected groups, suggesting that the rate at which the infe ction is administrated has no effect on protective immunity. Hepatic lesion s, similar in both infected groups, were characterised by marked eosinophil and mast-cell infiltration. Liver biopsies were performed and their diagno stic value is discussed. All results suggest that F. hepatica infection pre dominantly induces a Type-2 response in cattle, and that this response has little protective effect. (C)2000 Elsevier Science B.V. All rights reserved .