A new agent, DA, has been isolated from a spontaneously paralyzed mouse. Its biological properties and the pathology of lesions in experimental infection indicate close relationship with the Theiler group of encephalomyelitis viruses.

Serological studies were inconclusive. The intracerebral neutralization test failed to reveal measurable antibody and other routes of inoculation were unsuitable because of low invasiveness of the agent. Repeated vaccination of females did not render their offspring resistant to homologous intracerebral challenge.

The occurrence of early viremia is reported following intracerebral inoculation of the DA strain and also the known Theiler strains, 4727, FA, and GD-VII.

The pathology of mice experimentally infected with DA and with known members of the Theiler group is described. Attention is called to the demyelinating lesion of the cord in mice surviving for several months and to the persistence of virus in the CNS of such animals.

Another characteristic feature of the pathology was the degeneration of ventral nerve roots and in some mice of the peripheral nerves. Similar changes were not seen with strains of Theiler virus other than DA.

Spheroidal bodies of undetermined significance were found in the lesions.

Finally the occurrence of myositis after intracerebral inoculation, not only mice infected with the DA virus, but also with the 4727 and FA strains, is described and discussed.

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