These observations are concerned with the chance occurrence of latent mouse encephalomyelitis, either the paralytic TO or encephalitic FA type, in normal stock Swiss mice.In the course of experiments utilizing about 40,000 mice purchased chiefly from two animal dealers, the TO type of encephalomyelitis was encountered spontaneously in 4 instances, and following non-specific inoculations in 3 instances. The FA virus was found to be the cause of encephalomyelitis occurring in one instance spontaneously, and in 7 instances following intracerebral inoculation of non-infectious material or of murine-adapted strains of human poliomyelitis virus.In the latter instances, the inoculated murine-adapted virus and the contaminating FA strain were separated into pure lines of each strain. The contaminated murine-adapted human strains were freed of FA virus by treatment with FA hyper-immune serum or by passage through the monkey, a host which is not susceptible to the FA virus. FA virus was readily obtained from the mixture of strains by serial passage of brains of mice showing only encephalitic signs or by passage through chick embryos.The virus strains were identified by complement fixation and/or virus neutralization tests as well as by their host spectrum and “clinical” signs.There was no evidence that the interference phenomenon prevented the simultaneous passage of the Lansing and the FA strains through several serial transfers in mice.
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