Pulmonary intravascular macrophages in lungs of pigs inoculated with African swine fever virus of differing virulence.

The role of pulmonary intravascular macrophages (PIM) in a viral infection has been studied by structural and ultrastructural methods with two strains of the African swine fever (ASF) virus: the virulent strain E70 and the attenuated strain E75. Pulmonary intravascular macrophages were the cells of the mononuclear phagocyte system of the lung most involved in the replication of strain E70, showing a marked cytopathic effect and necrosis. Concurrently, their size and number increased sharply. This stimulation of PIMs by the virulent strain of the ASF virus, together with the cytopathic effect, might be the cause of the abundant masses of cell debris found in septal capillaries and thus be related to the pulmonary oedema that characterizes the acute forms of the disease. With the attenuated strain of the ASF virus, stimulation of PIMs, as well as viral replication within them, was also intense, but the cytopathic effect was less. Later, in the course of the infection, the number of PIMs decreased sharply and then alveolar macrophages increased, bearing most of the viral replication and signalling the onset of a pneumonic process.