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To disclose the mechanism of cellular injury following porcine circovirus (PCV) infection, 12 pigs were examined by the terminal deoxynucleotidyl transferase-mediated dUTP-nick end labeling (TUNEL) method and immunohistochemistry. Histologically, the lymphoid tissues were characterized by marked apoptosis of lymphocytes, lymphocyte depletion, and macrophages and giant cells containing numerous inclusion bodies with or without apoptotic bodies. Immunohistochemically, there were many lysozyme-positive macrophages in the lymphoid follicles, while the number of CD79a-positive B lymphocytes was scanty. Apoptotic cells, which were proved to be TUNEL positive, revealed CD79a positivity. Although detectable mainly in the cytoplasm of macrophages, PCV antigens were found also in the nuclei of macrophages and apoptotic lymphocytes. Ultrastructurally, the presence of PCV virions was confirmed in apoptotic bodies phagocytosed by macrophages. These findings suggested that lymphocyte depletion with apoptotic death of B lymphocytes was caused by PCV, and that some of the inclusion bodies were phagolysosomes derived from the apoptosis. Thus, PCV may trigger the development of wasting disease syndrome by producing an immunocompromised state in pigs.

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Ten cases of bovine lymphohematopoietic neoplasms were investigated histologically, histochemically, and immunohistochemically, and were classified into eight histologic types on the basis of the origin and morphology of tumor cells. Case 1, a precursor B lymphoblastic leukemia, was positive for CD79a and terminal deoxynucleotidyl transferase. Case 2, a thymic B cell lymphoma, was also positive for these markers, but there were cytologic differences between the two cases. Cases 3-5 were diagnosed as pleomorphic B cell lymphomas, which were characterized by cytologic pleomorphism and expression of CD79a and CD5, and were etiologically associated with bovine leukosis virus (BLV). A case of diffuse large B cell lymphoma of the cerebrum (case 6) also showed a positive result for CD79a and CD5. However, the lymphoma was composed of a homogeneous population of large neoplastic cells, and was considered to be unrelated to BLV. The other B cell cases were categorized into immunoblastic (case 7) and lymphoplasmacytic (case 8) lymphomas, in which immunoglobulin-producing lymphoma cells were observed. In a cutaneous γδ T cell lymphoma (case 9), the neoplastic cells cytologically resembled those in case 6, but expressed CD3 and WC1. In case 10, an acute basophilic leukemia, some leukemia cells had intracytoplasmic granules that were metachromatic and tryptase positive but negative for naphthol AS-D chloroacetate esterase. Bovine lymphohematopoietic malignancies are classifiable into discrete histologic types according to immunophenotype. The classification is more scientific than the traditional one, the latter being based on the age of affected animals and/or the site of tumor formation.

Five types of lymphoid neoplasms in calves are described. Four cases with clinicopathologic features of the "calf form of lymphoma" were diagnosed as precursor B or T lymphoblastic leukemia. These neoplasms were positive for terminal deoxynucleotidyl transferase (TdT), a marker for immature lymphocytes. The fifth case (thymic B cell lymphoma) was also TdT positive, but was characterized by massive neoplastic involvement of the thymus. The sixth case involved bovine leukemia virus (BLV). As in the majority of previously reported BLV-associated cases, the neoplastic cells expressed CD5, and atypical giant cells were detected. Instead of enzootic bovine leukosis, in which at least two immunophenotypically distinct entities are included, a diagnosis of BLV-associated pleomorphic B-1 B cell lymphoma was made. The last case, involving an epitheliotropic γδ T cell lymphoma, was characterized by epitheliotropism in the gastrointestinal tract and WC1 expression. Since several histologic types of lymphoid neoplasm occur in calves, the term "calf form of lymphoma" should be abandoned for accurate diagnosis.

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ABSTRACT. An epizootiological survey with histopathological methods was conducted for porcine circovirus in 220 diseased pigs (1-200 days old) in 49 farms from 1985 to 1999. Histopathological lesions containing PCV antigen were detected mainly in the lymphoid tissues from 42 of 189 diseased pigs (22.2%) in 4 of 45 farms (8.9%) from 1990 to 1999. The rate of positive pigs gradually increased from 1997 onward and PCV infection was found in 50% of diseased pigs in 1999. Histopathologically, the lesions in the lymphoid tissues (including lymph nodes, Peyer's patches, tonsil and spleen) were highly correlated with the presence of numerous spherical basophilic intracytoplasmic inclusion bodies with PCV antigen, and consisted of lymphocellular depletion and infiltration of macrophages. Although most affected cells showed cytoplasmic reactivity for PCV, intranuclear antigen was also seen in the lymphocytes, macrophages and ileal epithelial cells. Ultrastructurally, macrophages and giant cells contained electron-dense, round to ovoid lysosomal bodies, in which there were concentric circle or paracrystalline arrays of small nonenveloped icosahedral viral particles, approximately 15-17 nm in diameter. Other consistent infectious agents were present in 90.5% of cases, and porcine reproductive and respiratory syndrome virus infection was in 52.4% of the cases with PCV. The histopathological findings suggested that PCV induced systemic immunosuppression in the infected pigs and made them more susceptible to infection of the organisms. Because of the presence of PCV antigens in the intestinal epithelium, feces may play a significant role in dissemination of PCV.-KEY WORDS: epizootiological survey, porcine, porcine circovirus, postweaning multisystemic wasting syndrome.