An enzyme-linked immunosorbent assay using immonoaffinity-purified antigen in the diagnosis of caprine paratuberculosis and its comparison with conventional ELISAs.

An enzyme-linked immunosorbent assay (APA-ELISA) using an immunoaffinity-purified antigen was developed and compared with the unabsorbed and absorbed ELISA procedures, using a crude antigenic preparation, for its efficacy in detecting antibodies in goat sera against Mycobacterium ovium paratuberculosis. Serum samples from 89 goats belonging to three different flocks, two with a history and evidence of paratuberculosis and one without it, were subjected to each ELISA, which had been standardized on known positive sera from goats experimentally infected with paratuberculosis. Faecal culture, faecal examination and histopathology were used as indicators of infection. The diagnostic sensitivities of the unabsorbed, absorbed and APA-ELISA were 81.8%, 77.3% and 77.3% and the specificities were 90.6%, 93.7% and 96.8%, respectively. The positive predictive values of APA-ELISA (94.4%) was the highest, followed by absorbed ELISA (80.9%) and unabsorbed ELISA (72.0%). The negative predictive values for APA-ELISA, absorbed ELISA and unabsorbed ELISA were 93.0%, 92.7% and 93.8%, respectively. The results indicated the value of APA-ELISA in avoiding the need to absorb individual test sera with Mycobacterium phlei and giving more consistent results than the absorbed ELISA. The APA-ELISA was also better than the other two procedures in terms of specificity and positive predictive values.