Bovine herpesvirus meningoencephalitis association with infectious bovine rhinotracheitis (IBR) vaccine.

During 1992, on a farm in the Tokachi district of Hokkaido, Japan, approximately 20 Holstein-Friesian calves showed neuroparalysis and died within 7-10 days after routine vaccination. Six male calves, aged about 1.5 months, were submitted to our laboratory for pathological examination and diagnosed as acute or subacute necrotizing meningoencephalitis due to bovine herpes virus (BHV) infection. The main necropsy findings included a few hemorrhages or clots, and malacic lesions localized in the cortical to subcortical area of the cerebrum. Histopathological brain lesions were characterized by laminar or focal necrosis of neurons, accompanying macrophages, polymorphonuclear cell infiltration, severe astrogliosis, and perivascular cuffing in all six calves. Nuclear basophilic inclusion bodies, which showed positive reaction with immunocytochemical staining of BHV antigen, were observed in the necrotic neurons, astroglia and oligodendroglia in five affected calves. BHV antigens were also seen in the cell bodies and cell processes of the necrotic neurons, which was indicative of cell-to-cell propagation of infection. There was a general tendency for more severe lesions to be located at the cortex to subcortex of the cerebrum. Milder lesions were observed in the cerebellum and brain stem. These findings suggest that the infectious route to the cerebrum in the present cases was through the olfactory bulbs and/or along the meninges beginning from the ethmoid bone, rather than through the trigeminal ganglia route as had been emphasized in studies dealing with experimental infection.