Liver flukes produce cathepsin B and cathepsin L in their excretory–secretory material. These proteases are proposed to be key virulence factors for parasite infection, and are therefore targets for vaccination. Cathepsin B is predominately released in the juvenile stage of the life cycle, while different cathepsin L's are released throughout the cycle. Three proteases (cathepsin L5, cathepsin L1g and cathepsin B) were expressed in yeast from cDNA clones isolated from adult, metacercariae and newly excysted juvenile flukes respectively. Each was used singly or in combination to vaccinate rats that were subsequently challenged with Fasciola hepatica metercercariae. Each protein induced an immune response, and all groups vaccinated with recombinant protein yielded significantly fewer and smaller flukes than the control group. Maximal protection of 83% was seen in the group vaccinated with cathepsin B and cathepsin L5 in combination.
Jayaraj, R., Piedrafita, D., Dynon, K., Grams, R., Spithill, T. W., & Smooker, P. M. (2009). Vaccination against fasciolosis by a multivalent vaccine of stage-specific antigens. Veterinary Parasitology, 160(3-4), 230-236. https://doi.org/10.1016/j.vetpar.2008.10.099