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 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 as 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 few 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 of 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
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