In situ hybridization for the detection of the apxIV gene in the lungs of pigs experimentally infected with twelve Actinobacillus pleuropneumoniae serotypes.

The detection of the apxlV gene in lung tissues from pigs experimentally infected with the 12 major A. pleuropneumoniae serotype (1 to 12) reference strains was studied by in situ hybridization using a non-radioactive digoxigenin-labeled DNA probe. In situ hybridization produced a distinct positive signal in all pigs inoculated with the 12 A. pleuropneumoniae serotypes. Positive hybridization typically exhibited a dark-brown to black reaction product in intracellular and extracellular locations, without background staining. A strong hybridization signal was seen in degenerated alveolar leukocytes ("oat cells") adjacent to the foci of coagulative necrosis and in the alveolar spaces. The in situ hybridization methodology developed for the detection of the apxIV gene is a valuable tool for the diagnosis of porcine pleuropneumonia caused by A. pleuropneumoniae when only formalin-fixed tissues are submitted for diagnosis.