Use of recombinant ApxIV in serodiagnosis of Actinobacillus pleuropneumoniae infections, development and prevalidation of the ApxIV ELISA.

Actinobacillus pleuropneumoniae is the etiological agent of porcine pleuropneumonia, which causes worldwide severe losses in pig farming. The virulence of the 15 serotypes of A. pleuropneumoniae is mainly determined by the three major RTX toxins ApxI, ApxII and ApxIII, which are secreted by the different serotypes in various combinations. A fourth RTX toxin, ApxIV, is produced by all 15 serotypes only during infection of pigs, but not under in vitro conditions. Pigs infected with A. pleuropneumoniae show specific antibodies directed against ApxIV. In contrast, antibodies against the other three toxins ApxI, ApxII and ApxIII are also found in pigs free of A. pleuropneumoniae. The antibodies to the three latter might result from other, less pathogenic Actinobacillus species such as A. rossii and A. suis. We used a recombinant protein based on the N'-terminal part of ApxIV to serologically detect A. pleuropneumoniae infections in pigs by immunoblot analysis. The analysis of sera of experimentally infected pigs revealed that ApxIV-immunoblots detected A. pleuropneumoniae infections in the second to third week post infection. We developed an indirect ELISA based on the purified recombinant N'-terminal moiety of ApxIV. The analysis of sera from pigs that were experimentally or naturally infected by A. pleuropneumoniae, and of sera of pigs that were free of A. pleuropneumoniae, revealed that the ELISA had a specificity of 100% and a sensitivity of 93.8%. The pre-validation study of the ApxIV-ELISA revealed that the latter was able to detect A. pleuropneumoniae-positive herds, even when clinical and pathological signs of porcine pleuropneumonia were not evident. Pigs vaccinated with a subunit vaccine Porcilis App were serologically negative in the ApxIV-ELISA.

Salmonella level of Danish swine herds based on serological examination of meat-juice samples and Salmonella occurrence measured by bacteriological follow-up.

Analyses of data collected in the Danish Salmonella Control Program in swine herds were conducted to assess the association between bacteriological and serological findings (including the degree to which changing serological status over time modulated the relationship) and to assess the degree to which type of swine operation and age group affected the relationship between bacteriological and serological findings. Although serological status was significantly associated with bacteriological prevalence of salmonella enterica in swine herds, the association was clarified by adding information on the change in serological status over time and the age group being tested. Salmonella infection assessed by serological tests at a single time-point and data on age group tested would allow better targeting and managing control programs.