ASFV antigen detection by immunohistochemistry (IHC)
ASFV antigen-labelled cells were detected in various organs and majorly found in the mononuclear cells/macrophages. The result was tabulated in Table 3. The labelled cells were commonly detected in the lymphoid organs (spleen, tonsil and lymph node) as well as lung, kidney and liver tissues. Figure 6a showed the positive cells in the background of the haemorrhagic spleen. In the liver, positive cells were found in the sinusoids, and they were mostly enlarged, plump Kupffer cells (Fig 6b). In a severe case, ASFV antigen–labelled hepatocytes were found (Fig 6c). In the tonsil, positive mononuclear cells were seen in the epithelium of the tonsillar crypt (M-cell) and mostly within the diffuse lymphoid area surrounding the lymphoid follicles (Fig 6d). In the lymph node, positive cells were numerous in some cases and difficult for numeration because the cells lost their distinctive shape and blended with the background. Figure 6e showed the ASFV antigen–positive macrophages with many of them containing phagocytosed erythrocytes. In the lungs, pulmonary intravascular macrophages were commonly found positive (Fig 6f). In the cerebrum, positive cells were scarcely found. Figures 6g and 6h showed a few positive mononuclear cells in meningitis and perivascular cuffing lesions. In the epicardium of the heart, other than mononuclear cells/macrophages, many spindle-shaped mononuclear cells were positive in the haemorrhagic area (Fig 6i). In the kidney tissue, many spindle-shaped cells were stained positive in addition to plump-shaped macrophages within the haemorrhagic renal pelvis region (6j). Occasionally, a few positive cells were found in the capillary of glomeruli identified as macrophages (Fig 6k). Multifocally, the renal tubules of the kidneys showed granular cytoplasmic staining of ASFV antigen (Fig 6l). Tissue sections from negative control pig showed no ASFV antigen-labelled cells or non-specific staining in all tissues.