Louping-ill encephalomyelitis in the sheep: VII. Influence of immune status on neuropathology

Abstract

Prior to the intracerebral injection of virulent louping-ill virus, 3 groups of 7 or 8 sheep were vaccinated, or passively immunized with specific antiserum or given control preparations. All except 2 animals succumbed to challenge, although the presence of circulating antibody in the first 2 groups prevented viraemia and extraneural replication of virus. Concentrations of virus in the nervous tissue and the cerebrospinal fluid from moribund sheep were highest in the controls, lower in those immunized passively and least in vaccinated animals. These differences were significant and were mirrored in the numbers of nerve cells containing viral antigen and the extent of neuron necrosis in histological preparations. Perivascular cuffing, meningitis and neuronophagia were, however, most severe in vaccinated sheep, slightly less in passively immunized animals and minimal in controls. Perivascular cuffs consisted mainly of macrophages and lymphoid cells, which did not stain with fluorescent antisera to sheep IgM or IgG. Cells involved in neuronophagia were all macrophages. Antibody was found in the cerebrospinal fluid in only 2 cases: both were vaccinated sheep, one surviving with generalized inflammatory changes throughout the central nervous system. Severe inflammation, though not the primary cause of death, evidently contributed to the onset of neurological symptoms.