Subcellular changes in the tonsils of pigs infected with acute African swine fever virus.

A study of the pathogenesis of acute African swine fever (ASF) was carried out in pigs inoculated with a highly virulent strain of ASF virus to determine the sequential development of the subcellular changes in a particular lymphoepithelial organ, the tonsil. The apoptosis of the lymphocytes and the inhibition of lymphocyte proliferation were the main changes that occurred in the tonsillar lymphoid structures. This may explain the early lymphopenia observed in acute ASF. Moreover, vascular changes, consisting of increased vascular permeability, activation of endothelial cells and loss of these cells, might have been the cause of the characteristic haemorrhages found in the lymphoid organs during this disease. Virus replication has been observed in the epithelial cells, fibroblasts and reticular cell beginning on day 5 post-infection. The activation of the endothelial cells, apoptosis of lymphocytes, decreased lymphocyte mitosis and virus replication in non-mononuclear phagocyte system (MPS) cells all occurred after an intense proliferation and activation of the tonsillar macrophages and coincide with virus replication, which occurs in the macrophages 5 days post infection.