Evaluation of serovar-independent ELISA antigens of Actinobacillus pleuropneumoniae in pigs following vaccination or experimental challenge with respiratory pathogens and natural A. pleuropneumoniae serovar 1 challenge.

Abstract

OBJECTIVE: To compare the sensitivity and cross-reactivity of six serological enzyme-linked immunosorbent assays (ELISAs) based on serovar-independent antigens of Actinobacillus pleuropneumoniae (App). DESIGN: An experimental pig trial using direct or natural challenge with App and direct challenge or vaccination using other common respiratory pathogens. 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 direct challenge with App, Pasteurella multocida or Haemophilus parasuis, after vaccination with these organisms and after natural App infection. Clinical and necropsy findings were evaluated. RESULTS: The 39-kDa ELISA had high sensitivity, but cross-reactivity, following P. multocida challenge. ELISAs using ApxIVA N terminus antigens were clearly more sensitive than C terminus antigens for detection of App-induced disease. Although affinity-purified ApxIVA-NP antigen detected marginally more diseased pigs than the -N and -NPS ELISAs, these assays only detected 41-47% of 17 pigs with lung lesions and microbiological evidence of App based on sampling up to 4-5 weeks after natural (13 pigs) or 5 weeks after direct App serovar 1 challenge (4 pigs). CONCLUSIONS: The 39-kDa ELISA readily detects App exposure and infection, but is adversely affected by P. multocida infection. ApxIVA-N-based ELISAs can be used to evaluate the App status of commercial herds, but a proportion of infected and diseased animals are seronegative at 1 month after likely exposure to aerosol transmission of App from clinical cases.