Pathological, immunohistochemical, and in-situ hybridization studies of natural cases of postweaning multisystemic wasting syndrome (PMWS) in pigs

Fifteen pigs from five farms on which there had been a previous clinical an d histopathological diagnosis of postweaning multisystemic wasting syndrome (PMWS) were investigated. At necropsy, enlargement of lymph nodes was the most obvious lesion; other lesions were non-collapsed lungs, ulceration of the gastric pars oesophagica, and cranioventral pulmonary consolidation. Mi croscopical lesions attributable to PMWS were found in lymphoid organs (inc luding lymph nodes, tonsil, Peyer's patches and spleen), liver, kidney and lungs. Varying degrees of lymphocellular depletion, affecting both lymphoid follicles and parafollicular zones, and progressive multifocal to diffuse infiltration of lymphoid tissue by large histiocytic cells were the charact eristic lesions. Syncytial cells were seen frequently, especially in lympho id organs. A prominent finding was the presence of sharply demarcated, sphe rical, basophilic, cytoplasmic inclusions in histiocytic cells. The lymphoi d lesions were suggestive of immunosuppression. Non-lymphoid lesions includ ed interstitial pneumonia, periportal mononuclear inflammatory infiltration of the liver in varying degrees, and interstitial nephritis. Porcine circo virus (PCV) antigen and nucleic acid were regularly found in lymphoid organ s, lung, liver and, to a lesser degree, kidney. Target cells for PCV replic ation included monocyte/macrophage lineage and antigen-presenting cells. To a lesser extent, epithelial cells such as renal tubular, bronchial and bro nchiolar cells, endothelial cells, hepatocytes and lymphocytes were also la belled. One pig did not show PCV nucleic acid; sequence differences among d ifferent viral isolates are discussed as the probable cause of this lack of labelling by the in-situ hybridization PCV-specific probe. (C) 1999 W.B. S aunders Company Limited.