Immune responses of chronically infected adult cattle to Fasciola hepatica.

Eight adult cows, with an existing chronic Fasciola hepatica infection, were experimentally infected with 1300 metacercariae of F. hepatica, given as trickle infections, over two separate 10-day periods. Two fluke-naive heifers were similarly treated. Analysis of parasite-specific immunoglobulin isotypes IgM, IgG1, IgG2 and IgA showed IgG1 to be the dominant isotype in both chronically infected and previously naive animals. Lymphocyte proliferation assays demonstrated (a) an association between lymphocyte response and mature fluke burden in the chronically infected cattle and (b) no association between lymphocyte response and mature or immature fluke burden in naive heifers. There was no production of gamma-interferon (IFN gamma) by lymphocytes responding to adult fluke antigen. At post-mortem examination the burden of immature flukes in chronically infected and previously naive heifers was similar. This suggests that chronically infected animals may be tolerant to a moderate superinfection and that the prevailing immune mechanism in operation may be a non-protective response generated by the Th2 lymphocyte subset.