Detection of A. pleuropneumoniae by immunostaining. Detection in different body tissues during acute infection using a Horseradish-Peroxidase-(HRP)-polymer based detection system with DAB chromogen and Hematoxylin Gill 2 counterstain. A Liver (20-fold magnification): Note oligofocal detection of coccoid bacteria (brown) with A. pleuropneumoniae antigen within sinuisoids. B Pericard (20-fold magnification): Note detection of coccoid bacteria (brown) with A. pleuropneumoniae antigen within superficial exudate containing karyolytic leukocytes some of which are elongated (so called oat cells) which is characteristic of A. pleuropneumoniae. C Spleen (40-fold magnification): Note neutrophilia of the red pulp and detection of A. pleuropneumoniae antigen (brown) within two macrophages. D Kidney (4-fold magnification): Note colony of small rod-shaped bacteria with A. pleuropneumoniae antigen (brown) within the pelvis. Inlet: higher (40-fold) magnification of the bacterial colony.