Histopathological alterations and immunohistochemistry in tissue samples from horses suspected of Rhodococcus equi infection

The aim of this study is to determine the presence of Rhodococcus equi by immunohistochemistry using a highly specific monoclonal antibody (Mab10G5) targeted against virulent R. equi antigen in various archived tissues embedded in paraffin blocks from 8 horses suspected of R. equi infection and in fresh prepared tissue samples from a recently dead foal and to compare results with pathological findings. Histopathology diagnosis was based on granulomatous bronchopneumonia coupled to congestion, necrosis and cellular inflammatory response in lymphoid tissues and sometimes in intestines and liver. Immunopositivity was observed in alveolar macrophages, neutrophils and multinucleated giant cells in lung samples except in one case as well as in histiocytes, fibroblasts and endothelial cells in fibrosis areas and in lymphoid tissues (lymph nodes, spleen and thymus). Additionally, immunoreactive hepatocytes and Kupffer cells were also seen in one case (archived case no6). These results demonstrate the presence of virulent antigen from R. equi not only in lungs but also in liver and lymphoid tissues even in archived samples embedded in paraffin