Use of an enzyme-linked immunosorbent assay for serodiagnosis of clinical paratuberculosis in goats. Study by western blotting of false-positive reactions.

An enzyme-linked immunosorbent assay (ELISA) was performed for diagnosis of paratuberculosis in goats, using as antigen a protoplasmic extract (PPA-3). The test was developed on the basis of the results obtained with two serum reference pools, positive and negative respectively. To avoid day-to-day variations, dilutions of the positive serum pool were included in each plate to obtain an arbitrary system, transforming absorbance into immunoglobulin (Ig) G anti-Mycobacterium paratuberculosis units. The ELISA was used on sera of two reference groups of animals. One group consisted of 35 goats suspected of being infected with paratuberculosis, which was confirmed by histological findings and isolation of M. paratuberculosis. The negative group consisted of 61 healthy goats from a farm free of paratuberculosis. The test showed a sensitivity of 100% and a specificity of 91.8%. Absorption of sera with a Mycobacterium phlei suspension did not modify either the sensitivity or the specificity of the test. Sera from the negative group were analysed by Western blotting, and four of them recognized two fractions with a molecular weight of 17.3 and 28.1 kDa.