Evaluation of serovar-independent ELISA antigens of Actinobacillus pleuropneumoniae in pigs, following experimental challenge with A. pleuropneumoniae, Mycoplasma hyopneumoniae and Pasteurella multocida.

Abstract

OBJECTIVE: To compare the sensitivity and specificity of six serological enzyme-linked immunosorbent assays (ELISAs) based on serovar-independent antigens of Actinobacillus pleuropneumoniae (App) and investigate cross-reactivity in disease-free pigs challenged with Mycoplasma hyopneumoniae and Pasteurella multocida. DESIGN: Five experimental pig trials using direct challenge with App serovars 1, 7 or 15 or direct challenge with M. hyopneumoniae and/or various dose rates of P. multocida. PROCEDURE: A 39-kDa outer membrane protein antigen and five recombinant antigens from the apxIVA gene of App were evaluated. The latter were derived from the ApxIVA N-terminus (ApxIVA-N, ApxIVA-NP, ApxIVA-NPS) or C-terminus (ApxIVA-C, ApxIVA-CP). Pigs were sampled after challenge and clinical and necropsy findings evaluated. RESULTS: The 39-kDa ELISA had high sensitivity but lacked specificity, with significantly increased cross-reactivity following P. multocida challenge. ELISAs based on ApxIVA N-terminus antigens were significantly more sensitive than C-terminus antigens for the detection of App-induced disease. Although ApxIVA-N and ApxIVA-NP ELISAs had increased reactivity following P. multocida challenge, they retained high specificity for App-induced disease (90-93%). Affinity purified ApxIVA-NP antigen had marginally better specificity than ApxIVA-N, without reduced sensitivity. Mycoplasma hyopneumoniae did not affect serological cross-reactivity. In disease-free pigs, the specificity of the ApxIVA-NPS ELISA may be adversely affected by nasal carriage of apparently low-virulence App strains. CONCLUSIONS: ApxIVA-N-based ELISAs can be used for evaluating App status in commercial herds, but some appear limited by high carriage rates of low-virulence App. The 39-kDa antigen is only of merit in exclusion of App disease by negative serology.