IMMUNOHISTOCHEMICAL IDENTIFICATION OF PORCINE REPRODUCTIVE AND RESPIRATORY SYNDROME VIRUS (PRRSV) ANTIGEN IN THE HEART AND LYMPHOID SYSTEM OF 3-WEEK-OLD COLOSTRUM-DEPRIVED RIGS

Porcine reproductive and respiratory syndrome virus (PRRSV) antigens w ere detected by a streptavidin-biotin complex technique in tissues of 3-week-old colostrum-deprived pigs that had been inoculated intranasal ly with PRRSV and had developed moderate respiratory disease. Moderate , multifocal, tan-colored consolidation of the lungs and severe enlarg ement of the lymph nodes were noted at necropsy. Severe interstitial p neumonia characterized by type 2 pneumocyte proliferation, septal infi ltration with mononuclear cells, and accumulation of macrophages and n ecrotic cells in alveolar spaces was observed at 4 and 9 days postinoc ulation. Moderate multifocal perivascular lymphohistiocytic myocarditi s was observed at 9 days postinoculation. Marked lymphoid follicular h yperplasia and follicular necrosis in the tonsil, spleen, and lymph no des was observed. A monoclonal antibody that recognizes a conserved ep itope of PRRSV nucleocapsid protein was used as primary antibody for i mmunohistochemistry. Antigen was readily detected in alveolar macropha ges in the lung and in endothelial cells and macrophages in the heart. Macrophages and cells resembling dendritic cells in tonsil, lymph nod es, thymus, and spleen also stained intensely positive for viral antig en. PRRSV appears to replicate primarily within macrophages in the res piratory and lymphoid systems of the pig.