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The pathology of Rift Valley fever. I. Lesions occurring in natural cases in new-born lambs.

A widespread epizootic of Rift Valley fever occurred in the Republic of South Africa and South West Africa during 1974--75. This is a report on the gross pathology of 34 new-born lambs and the histopathology of 93 new-born lambs that died during this outbreak. The liver was affected in every case and showed the most pronounced lesions. The organ was grossly enlarged in most cases, with scattered greyish-white necrotic foci 1--2 mm in diameter and haemorrhages of varying size throughout. Haemorrhages were also frequently seen in the mucosa of the abomasum. The massive diffuse necrosis of hepatocytes (pannecrosis) associated with well-demarcated foci of primary coagulative necrosis, present in 100% of the cases examined, was characteristic of the histopathology of the new-born lamb. Bile thrombi were noticed in the livers of 31% of the lambs and intranuclear inclusions in 49% of the cases. The diagnostic significance of the microscopic liver lesions is discussed. Focal necrosis and haemorrhages were frequently seen in the adrenal cortex while generalized destruction of lymphocytes in the lymph nodes and spleen occurred in many of the animals. In addition, the following hitherto undescribed or previously not well-documented lesions are recorded: (i) mineralization of single or groups of necrotic hepatocytes in 62% of the livers: (ii) pyknosis and karyorrhexis of the cellular elements in the glomeruli and a hyalinized appearance of many of these affected glomeruli; and (iii) necrosis of the tips of the villi in the small intestine in some of the animals.