Spontaneous Listeric Encephalitis in Sheep

The brainstems of four sheep with spontaneous listeric encephalitis had scattered small foci of inflammatory cells (neutrophils or macrophages, or both) with scattered fragments of degenerating nerve fibers and glial cells. In extensive areas of malacia in the pons and medulla oblongata, there was loss of parenchyma with massive accumulation of macrophages, a few neutrophils, lymphocytes and plasma cells. In both types of lesions, phagocytes contained debris of myelin and axons, lipid vacuoles and occasionally bacteria. Neutrophils contained bacteria in phagocytic and digestive vacuoles. No bacteria were detected in macrophages but were detected in neurons and in one axon in tissue previously used for paraffin sections. A cardinal feature of listeric encephalitis in sheep is many nodules of macrophages or neutrophils or both in the parenchyma of the brain stem, especially the pons and medulla oblongata. The proportion of macrophages to neutrophils in these inflammatory foci varies greatly in the same brain. Also, the predominant type of inflammatory cell (macrophage or neutrophil) varies between different brains [4]. In light microscopic studies, bacteria occurred more frequently in neutrophils than macrophages [4]. No reason was found for this apparent greater phagocytosis by neutrophils. Although the importance of cell mediated immunity in listeric infections is well established [16], there are no reports of the ultrastructural features of the inflammatory response and phagocytosis of bacteria in listeric encephalitis. Experimental studies indicate that some proteins and viruses (herpes simplex virus, infectious bovine rhinotracheitis virus, pseudorabies virus, canine herpes virus) travel to the brain via axons of peripheral nerves [3, 5 , 7-11, 17, 181. Viruses also may spread within the brain via neuronal processes [7]. A recent study of listeric encephalitis in sheep suggested that bacteria migrated along peripheral nerves to the brain and that migration in the brain occurred, at least partly, along fiber tracts [4]. A few neurons and neuronal processes contained organisms which suggested that neuronal cytoplasm is a vehicle for movement of the infectious agent