Evaluation of enzyme-linked immunosorbent assay in comparison with complement fixation test for the diagnosis of subclinical paratuberculosis in cattle.

An enzyme-linked immunosorbent assay (ELISA) was evaluated and compared in parallel with the standard complement fixation test (CFT) for the diagnosis of bovine subclinical paratuberculosis. Bovine sera preabsorbed with the mixture of Mycobacterium phlei and kaolin suspension were assayed for antibody activities to the crude protoplasmic antigen of Mycobacterium paratuberculosis in the ELISA. ELISA antibody titer was expressed as ELISA antibody index (EAI) value: EAI = (At-An)/(Ap-An), where At, Ap and An are the absorbance values of a 1:200 dilution of unknown test sera, a 1:400 dilution of positive control serum, and a 1:200 dilution of negative control serum. An EAI of 0.6 or greater was established as a reasonable cutoff point for a positive antibody titer by ELISA. Of the 156 sera from cattle with subclinical M. paratuberculosis-infection, 106 (67.9%) were positive by ELISA and 41 (26.3%) by CFT. Of the 3,880 sera from cattle in the herds which had no history or evidence of paratuberculosis, 3,875 (99.9%) were negative by ELISA, and 3,787 (97.6%) by CFT. Positive ELISA titers were detectable 1 to 5 months earlier than positive CFT titers in experimentally infected cattle, and 7 to 10 months earlier in naturally infected cattle. These results indicate that the ELISA should replace the CFT as the routine test of choice for the diagnosis of bovine paratuberculosis.