ULTRASTRUCTURAL-CHANGES RELATED TO THE LYMPH-NODE HEMORRHAGES IN ACUTE AFRICAN SWINE FEVER

In order to determine the pathogenic mechanisms involved in lymph node haemorrhages in acute African swine fever (ASF), eight pigs were inoc ulated with ASF virus, strain Malawi'83. Lymph node haemorrhages were observed from three days post infection (dpi) onwards, coinciding with ASF virus replication in monocytes and macrophages adjacent to stimul ated endothelial cells, phagocytic stimulation of capillary and small- vessel endothelial cells, increase in the number of fenestrations of e ndothelial cells, and endothelial cell loss, as well as clusters of bl ood cells and necrotic material beneath the endothelium. Vascular lumi na were blocked by platelet plugs and fibrin microthrombi. These pheno mena became more marked as the disease progressed. At five dpi, virus replication was also found in circulating neutrophils. At seven dpi, l esions were more intense and were accompanied by virus replication in sinus and capillary endothelial cells, and in other cell populations i ncluding pericytes, fibroblasts, smooth muscle fibres and reticular ce lls. The results obtained in this study suggest that lymph node haemor rhages are related to endothelial stimulation and the onset of dissemi nated intravascular coagulation. Virus replication in vessel wall cell s occurs only in the final stages of the disease and plays a secondary role.