Abstract:
A clear distinction can be made regarding the susceptibility to and the severity of lesions in young lambs when compared to adult
sheep. In particular, there are important differences in the lesions and tropism of Rift Valley fever virus (RVFV) in the liver, kidneys,
and lymphoid tissues of young lambs. A total of 84 lambs (<6 weeks old), necropsied during the 2010 to 2011 Rift Valley fever
(RVF) outbreak in South Africa, were examined by histopathology and immunohistochemistry (IHC). Of the 84 lambs, 71 were
positive for RVFV. The most striking diagnostic feature in infected lambs was diffuse necrotizing hepatitis with multifocal liquefactive
hepatic necrosis (primary foci) against a background of diffuse hepatocellular death. Lymphocytolysis was present in all
lymphoid organs except for the thymus. Lesions in the kidney rarely progressed beyond hydropic change and occasional pyknosis
or karyolysis in renal tubular epithelial cells. Viral antigen was diffusely present in the cytoplasm of hepatocytes, but this labeling
was noticeably sparse in primary foci. Immunolabeling for RVFV in young lambs was also detected in macrophages, vascular
smooth muscle cells, adrenocortical epithelial cells, renal tubular epithelial cells, renal perimacular cells, and cardiomyocytes.
RVFV immunolabeling was also often present in capillaries and small blood vessels either as non-cell-associated viral antigen, as
antigen in endothelial cells, or intravascular cellular debris. Specimens from the liver, spleen, kidney, and lungs were adequate to
confirm a diagnosis of RVF. Characteristic lesions were present in these organs with the liver and spleen being the most consistently
positive for RVFV by IHC.
