Serological characterisation of Actinobacillus pleuropneumoniae isolated from pigs in 1993 to 1996

OBJECTIVES: To clarify the serological identity of the prototype strain of a group of Actinobacillus pleuropneumoniae isolates that could not be serotyped in previous studies and to establish the serovar of 378 isolates of A pleuropneumoniae obtained from pigs in Australia over the period 1993 to 1996. DESIGN: After initial validation, QGD and IHA tests were used to characterise the prototype isolate (HS143) selected to represent the cross-reacting isolates that were found in a previous study. Next, 378 recent field isolates of A pleuropneumoniae were characterised using the existing gel diffusion serotyping technique and/or the IHA or QGD tests. RESULTS: The indirect haemagglutination test was shown to be capable of correctly recognising the reference strain for all serovars except serovar 11. While the quantitative gel diffusion test was not as effective as indirect haemagglutination, it could recognise serovar 11. When the two tests were used to examine the prototype strain (HS143) of the cross-reactive isolates, the results indicated that HS143 is serologically distinct from all 12 of the recognised serovars of A pleuropneumoniae. However, as HS143 was subsequently identified as serovar 12 by one of the leading international reference laboratories, the antiserum to isolate HS143 was used as the serovar 12 antiserum. A total of 346 of the 378 A pleuropneumoniae field isolates examined could be confidently serotyped with almost 90% of the isolates being either serovar 1 (104 isolates); serovar 7 (83 isolates) or serovar 12 (142 isolates). A range of other serovars and some cross-reactive isolates made up the remainder of the isolates. CONCLUSION: The serovar 12 antiserum produced against the international reference strain (1096) does not recognise Australian serovar 12 isolates. The antiserum raised against isolate HS143 does recognise the Australian serovar 12 isolates. The dominant serovars of A pleuropneumoniae infecting Australian pigs are (in decreasing order) serovars 12, 1 and 7.