IMMUNOLOGICAL APPROACHES FOR THE CONTROL OF FASCIOLIASIS

The immunological relationship between liver flukes and their mammalia n hosts is being unravelled by in vivo and in vitro studies, Vaccine s tudies in cattle and sheep with purified antigens (fatty acid binding protein, FABP; glutathione S-transferase, GST; cathepsin L, CatL; hemo globin) have shown that high reductions in worm burdens (31-72%) and e gg production (69-98%) can be achieved, raising the realistic possibil ity that immunological control of Fasciola infection is a commercially achievable goal, Combination vaccines may also be feasible since a co cktail of CatL and hemoglobin elicits a significant 72% protection in cattle. Analysis of immune responses to Fasciola during infection in r uminants suggests that chronic infection correlates with a type 2 help er T cell response, implying that type 1 helper T cell responses are d own-regulated in fasciolosis. Recent results studying the resistance o f Indonesian Thin Tail (ITT) sheep to F. gigantica have shown that thi s breed exhibits high innate (or rapidly acquired) resistance to infec tion and acquires a higher level of resistance after a primary challen ge. Initial studies suggest that the resistance of ITT sheep to F. gig antica may be determined by a major gene. Merino sheep also acquire re sistance to F. gigantica. In contrast, ITT and Merino sheep do not exh ibit resistance to F. hepatica. These results suggest that there are f undamental differences between these two species of Fasciola in the bi ology of their interaction with the sheep immune system, Irt vitro stu dies on immune mechanisms of killing of juvenile fluke have shown that juvenile larvae of F. hepatica are susceptible to antibody-dependent killing by activated rat macrophages in vitro which is mediated by nit ric oxide. Future studies on the immune effector mechanisms expressed by resistant sheep which control infection by F. gigantica will lead t o new knowledge which may allow the design of more effective vaccines for fasciolosis. (C) 1997 Australian Society for Parasitology. Publish ed by Elsevier Science Ltd.