EVALUATION OF A SALINE BOILED EXTRACT, CAPSULAR POLYSACCHARIDES AND LONG-CHAIN LIPOPOLYSACCHARIDES OF ACTINOBACILLUS-PLEUROPNEUMONIAE SEROTYPE-1 AS ANTIGENS FOR THE SERODIAGNOSIS OF SWINE PLEUROPNEUMONIA

A saline boiled extract (SBE), capsular polysaccharides (CPS) and long -chain lipopolysaccharides (LC-LPS) of Actinobacillus pleuropneumoniae serotype 1 have been evaluated in ELISA for the serodiagnosis of swin e pleuropneumonia caused by this serotype. Mean optical densities (ODs ) obtained with the three antigens using sera from negative herds as w ell as from animals experimentally and naturally exposed to A. pleurop neumoniae serotypes 1, 9 or 11 were not significantly different. The p ositive ELISA reaction with anti-serotypes 9 and 11 was unexpected wit h the CPS, which are supposed to be serotype-specific; LPS, and to a l esser extent proteins, were present in the CPS and appeared to be resp onsible for this reaction. In addition, sera from animals exposed to a field strain of A. pleuropneumoniae serotype 3 and to Actinobacillus suis presented a significantly lower mean OD (P<0.001) when LC-LPS wer e used. Cross-reacting antigens consisted mainly of LPS core-lipid A p resent in the SBE and CPS. The specificity and the sensitivity of the ELISA were evaluated using three different cut-off values (the OD plus two, three and four times the standard deviation or SD) obtained with 667 negative sera. The diagnostic sensitivity was of 81% with the thr ee antigens and the different thresholds. The diagnostic specificity w as of 84, 86 and 88% for the mean plus two, three and four times the S D respectively using the SBE and the CPS, while that obtained with the LC-LPS was of 96, 98 and 99% using the same thresholds. In conclusion , LC-LPS make an easily obtainable antigen and seem to retain the best specificity while minimizing losses of sensitivity.