Abstract. Sixteen of 17 sheep with spontaneous listeric encephalitis had neuritis characterized by diffuse and focal intrafascicular and perineural accumulations of lymphocytes, plasma cells, macrophages and neutrophils in one or more cranial nerves. Nine sheep had extensive trigeminal neuritis which was usually unilateral. Brain lesions were mainly in the stem and were foci of macrophages or neutrophils or both, malacia, neutrophilic neuronophagia, vascular cuffing, and meningitis. Lesions in the brain and trigeminal ganglia were most severe on the same side as the affected trigeminal nerve. Gram-positive bacilli were in proximal parts of cranial nerves in foci of inflammatory cells and occasionally in morphologically intact nerve fibers. Organisms in the brain were in phagocytes in areas of inflammation and in scattered neurons and axons. The results were consistent with centripetal migration of the infectious agent along one or more branches of the trigeminal nerve to the brain and dissemination in the brain stem occurring, at least partly, along fiber tracts. Intraaxonal movement of bacteria probably is a mechanism involved in the pathogenesis of this disease.Listeriosis in sheep may be manifested as abortion, neonatal septicemia, or neurologic disease. The neurologic syndrome (circling disease) occurs mainly as a sporadic encephalitis of adult sheep, is frequently fatal and rarely occurs concurrently with either of the other two syndromes.Listeric encephalitis in sheep is characterized by microabscesses, focal gliosis and perivascular cuffing. Although there are reports of a few cases of diffuse meningoencephalitis [20], the lesions usually are confined to the brain stem, especially the pons and medulla oblongata. This regional distribution is not consistent with a hematogenous route of infection [12], which usually results in disseminated lesions in the cerebrum as well as the brain stem. Focal invasion of brain parenchyma via a nonvascular route could explain partially the distribution. Lesions in the proximal parts of branches of the trigeminal nerve occur in the spontaneous disease in sheep, cattle and goats. It has been suggested that the infective organism reached the brain by migration along these nerves [1, 2,24]. Other studies in sheep did not show extensive inflammation of trigeminal nerves [5] and migration of infection along nerves was not considered likely. In addition, reports of isolation of Listeria monocytogenes from visceral organs of several sheep with spontaneous listeric 297

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