1. The characteristics of a filterable virus obtained from mice found spontaneously paralyzed and showing lesions of encephalomyelitis are described.

2. The course of the disease in mice, following intracerebral inoculation, is briefly as follows: After an incubation period varying from 7 to over 30 days a flaccid paralysis of one of the limbs appears. This paralysis usually spreads rapidly until all four limbs are affected. Young mice are more susceptible than older ones, and very young mice, less than 4 weeks of age, usually die without showing signs of paralysis.

3. Adult mice often show no signs of infection after an intracerebral inoculation of virus. A number of these mice, although showing no signs of paralysis, nevertheless have become infected, a fact which is demonstrated by recovery of the virus from the mice as well as by histopathological studies.

4. Intranasal instillation of the virus is the only other method of producing the infection. This method, however, produces paralysis in only a small percentage of the mice. Following intranasal instillation of the virus, there often develops a slight immunity to a subsequent intracerebral injection of virus.

5. The paralysis in the surviving mice recedes gradually, but a permanent residual paralysis, usually of the hind legs, is almost invariable. Such mice, however, are virus carriers, as the virus can be recovered from the spinal cord for 1 year after infection.

6. Paralyzed mice are immune to a subsequent intracerebral injection of the virus. There is evidence that neutralizing substances are present in the immune mice. A considerable proportion of the mice which have remained well after an intracerebral injection of virus are immune to a second injection.

7. The virus resists the action of 50 per cent glycerine at from 2–4°C. for at least 150 days. It passes all grades of Berkefeld filters with ease. By the use of graded collodion filters, the size of the virus particle has been determined to be probably about 13 to 19 mµ.

8. The virus of mouse encephalomyelitis is not pathogenic for rhesus monkeys. No evidence of immunological relationship with the virus of human poliomyelitis has been obtained.

9. The anatomical basis of the paralysis is an acute necrosis of the ganglion cells of the anterior horn of the spinal cord. Isolated ganglion cells of the cerebrum also undergo necrosis. Following the acute necrosis of the ganglion cells, there is a marked neuronophagia. A perivascular infiltration is observed in the brain and spinal cord.

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