Distribution of viral antigen in tissues of new-born lambs infected with Rift Valley fever virus

The distribution of Rift Valley fever (RVF) viral antigen was studied by immunohistochemistry in the liver, spleen, prescapular lymph node, lungs and kidneys ot eight experimentally infected new-born lambs and in four new-born lambs that died of RVF during the 1974-75 RVF epidemic. The eight experimentally infected lambs were euthanazed at 6, 12, 18, 24, 30, 33, 48 and 51 h post-infection (p.i.), respectively. Immunohistochemical staining utilized polyclonal hyperimmune mouse ascites fluid to RVF virus and peroxidase-diaminobenzidine was substrate. Virus antigen was most prominent in the liver and was detected as early as 18 h p.i. in the cytoplasm of hepatocytes that were sparsely scattered throughout the lobules. At 24-33 h p.i. antigen was also present in or adjacent to small foci of hepatocellular necrosis. At 48-51 h p.i. and in one of the field cases, positive staining was widespread and most consistently present in the cytoplasm of large numbers of degenerated or necrotic hepatocytes and in a new acidophilic bodies. Immunohistochemical staining was rarely observed in hepatocyte nuclei. Almost diffuse histochemical staining was observed in disintegrated cells and in the cytoplasm of necrotic hepatocytes throughout the liver in the other three field cases with pannecrosis; only the primary foci necrosis and a narrow periportal rim of intact hepatocytes did not stain. No staining was observed in bile duct epithelium, endothelial and Kupffer cells in the initial stages of Infection, supporting the contention that hepatocytes constitute the primary site of RVF virus replication in new-born lambs. Few cells stained positively in the spleen, prescapular lymph node, lungs and kidneys.