Invasion and spread of equine herpesvirus 9 in the olfactory pathway of pigs after intranasal inoculation.

The neuropathogenesis of equine herpesvirus 9 (EHV-9) in pigs was investigated by intranasal inoculation of the virus together with intramuscular administration of dexamethasone (DM). All infected pigs developed characteristic meningo-encephalitis, accompanied by basophilic intranuclear inclusion bodies in the neuronal cells. One non-DM-treated and two DM-treated pigs had prominent malacic lesions in the rhinencephalon. Associated with the encephalitic lesions, there was invariably an increase in the number of nucleated cells in the cerebrospinal fluid (CSF). EHV-9 antigen was first detected in the nasal and olfactory epithelial cells in the nasal cavity, and in the neuroglial cells in the olfactory bulb. Subsequently it was demonstrated in the amygdaloid and caudate nuclei, and putamen. The virus was not isolated from the CSF. These results suggest that, after intranasal inoculation, EHV-9 replicates in the olfactory epithelial cells, spreading to the central nervous system via the olfactory pathway.