DISTRIBUTION OF VIRAL-ANTIGEN IN TISSUES OF NEWBORN 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 of eight experimentally infected new-born lambs and in four new-born lambs that died of RVF during the 1974-75 RVF epidemi c. The eight experimentally infected lambs were euthanazed at 6, 12, 1 8, 24, 30, 33, 48 and 51 h post-infection (p.i.), respectively. Immuno histochemical staining utilized polyclonal hyperimmune mouse ascites f luid to RVF virus and peroxidase-diaminobenzidine as substrate. Virus antigen was most prominent in the liver and was detected as early as 1 8 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 o r adjacent to small fool of hepatocellular necrosis. At 48-51 h p.i. a nd in one of the field cases, positive staining was widespread and mos t consistently present in the cytoplasm of large numbers of degenerate d or necrotic hepatocytes and in a few acidophilic bodies. Immunohisto chemical staining was rarely observed in hepatocyte nuclei. Almost dif fuse histochemical staining was observed in disintegrated cells and in the cytoplasm of necrotic hepatocytes throughout the liver in the oth er three field cases with pan necrosis;only the primary foci of necros is and a narrow periportal rim of intact hepatocytes did not stain. No staining was observed in bile duct epithelium, endothelial and Kupffe r 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, prescap ular lymph node, lungs and kidneys.