Use of a pre-selected epitope of cathepsin-L-1 in a highly specific peptide-based immunoassay for the diagnosis of Fasciola hepatica infections in cattle

A peptide-based indirect ELISA to detect cattle antibodies against Fasciola hepatica was developed and evaluated for its sensitivity and specificity. An immunogenic antigen released in vitro by F. hepatica was purified. After purification the sequence of the first 20 N-terminal aa of this protein sh owed considerable homology with cathepsin L-like proteinase. Based on its h omology with cathepsin-L-1, we further focused on this protein for diagnost ic purpose. Predicted B-cell epitopes of cathepsin-ll were synthesised as s ingle synthetic peptides and tested with respect to their diagnostic potent ial. An indirect ELISA based on one of these peptides was (i) evaluated fur ther and (ii) compared to the potential of an indirect ELISA with excretion /secretion antigens from adult F. hepatica, or (iii) purified cathepsin-L-1 . Specificity and sensitivity of the three ELISAs were assessed using sera from calves experimentally infected with pure isolates of Dictyocaulus vivi parus, Ostertagia ostertagi, Cooperia oncophora, Nematodirus helvetianus, S chistosoma mattheei, Ascaris suum, Taenia saginata or F. hepatica, respecti vely, and sera from parasite-naive calves. In addition, sera were analysed from calves naturally infected with F. hepatica. The sensitivities of all three ELISAs were also very high: 98.9% (i), 100% (ii) and 100% (iii). The specificity of the peptide ELISA was very high, 99 .8%, whereas specificities of the ES antigens and cathepsin-L-1 ELISAs were only 82.8% and 94.6%. In experimentally infected cattle, F. hepatica-speci fic antibodies were first detected between days 21 and 28 p.i. with all thr ee ELISAs, and the antibody levels persisted in the peptide ELISA until day 183 p.i. All sera from naturally infected calves were positive in the pept ide ELISA. These results demonstrate that the peptide-based F. hepatica ELISA is a use ful method for detecting antibodies in the sera from cattle infected with F . hepatica. This type of immunodiagnostic will therefore contribute to more accurate diagnosis and to timely curative treatment of animals. (C) 1999 A ustralian Society for Parasitology. Published by Elsevier Science Ltd. All rights reserved.