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.
To compare the sensitivity and cross-reactivity of six serological enzyme-linked immunosorbent assays (ELISAs) based on serovar-independent antigens of Actinobacillus pleuropneumoniae (App). An experimental pig trial using direct or natural challenge with App and direct challenge or vaccination using other common respiratory pathogens. 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. 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). 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.