Serodiagnosis of leptospirosis in pigs using an axial filament enzyme-linked immunosorbent assay.

The axial filament (AF) from Leptospira interrogans serovar canicola was isolated by cesium chloride density gradient centrifugation of 2% sarcosyl treated whole cells. Isolation of AF was confirmed by electron microscopic examination, by protein-A immunogold labelling, sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE), and immunoblotting. Analysis by SDS-PAGE of the purified preparation showed relatively weak bands of molecular size 41 kDa and 21 kDa, and strong bands of 35 kDa and 34.5 kDa. Immunoblot analysis using antiserum to the AF against sonicated leptospires of a variety of serovars showed prominent reaction against the 41, 35, and 34.5 kDa protein bands, as well as against minor bands of molecular weight 43, 39, and 37 kDa. Antisera prepared against leptospiral serovars also identified minor bands at 33 and 32 kDa. Immunoblots with antiserum to whole cells of serovar bratislava detected the 35 and 34.5 kDa AF bands of Borrelia burgdorferi moderately and of Treponema hyodysenteriae only slightly in comparison to leptospiral AF. Antibody to B. burgdorferi did not detect the leptospiral AF antigen. Immunoblots with antiserum to T. hyodysenteriae showed a marked reaction with a 41 kDa band of B. burgdorferi but only a very minor reaction with leptospiral AF. The AF was tested in an AF-ELISA against sera from 260 pigs, many of which reacted in the microscopic agglutination test (MAT) against one or more leptospiral serovars. A sensitivity of 97.1% and a specificity of 93.1% was determined in comparison to the MAT. Only moderate correlation was observed between titres detected in the AF-ELISA and the MAT (r = 0.4). When sonicated whole cells (WC) of serovar canicola were used in an ELISA (WC-ELISA), high correlation was observed between AF-ELISA and WC-ELISA (r = 0.97). These findings show that the AF-ELISA can be used effectively as a species-specific antigen for the serological diagnosis of leptospirosis in swine and that sonicated whole cells can substitute excellently for purified AF as the antigen source. These findings may be extrapolated to the use of AF in immunodiagnosis of leptospirosis in other species.