The virus of equine encephalomyelitis (eastern strain) evokes in the horse, calf, sheep and dog an unusually intense encephalomyelitis characterized by acute primary degeneration of nerve cells, the appearance in neurons of the brain stem and elsewhere of nuclear inclusions resembling those in Borna disease and poliomyelitis, polymorphonuclear infiltration in the nervous tissues with early microglial proliferation, and perivascular cuffing with mononuclears and polymorphonuclears in varying proportions. The grey matter is affected more than the white. Lesions may be less marked in the striatum, brain stem and cord than in the cerebral cortex, thalamus and hypothalamic region, and are always of low grade in the cerebellum. Meningeal infiltration is secondary.
Similar changes produced in the horse by the western strain of virus are less intense and extensive.
In the guinea pig, rabbit and mouse, the eastern virus causes an acute encephalomyelitis which, as is usual in neurotropic virus diseases of these lowly species, has a special tendency to affect the higher olfactory centers. In addition to inclusions in the nerve cells, tiny oxyphilic bodies occur with less frequency in the glial and mesodermal nuclei of the guinea pig. In this animal, too, interstitial or bronchopneumonia may complicate the picture.
In the guinea pig the disease resulting from infection with the western virus may be indistinguishable from that due to the eastern.