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Leon type of human poliomyelitis virus has been adapted to mice from original monkey cord virus by use of an intraspinal technic of inoculation. This mouse adapted Leon virus has been serially passed through mice for 35 passages and causes flaccid paralysis, more frequently in the front legs, after an incubation period of 4 to 6 days. A second similar adaptation has been accomplished and this strain carried through 16 mouse passages. The mouse virus causes paralysis only after intraspinal inoculation of mice, being negative when given intra-cerebrally and is of low virulence thus far. Apparently this virus is still as virulent for monkeys as it is for mice. Proof that this mouse virus is indeed Leon poliomyelitis virus rests on its ability to cause typical clinical and histopathological disease after intracerebral inoculation of monkeys, cross immunization against monkey Leon virus in monkeys and cross neutralization tests in mice and in monkeys. The virus also fails to propagate in the chick embryo. It is neutralized by human gamma globulin in low dilutions. Thus far attempts to adapt the Brunhilde type of human poliomyelitis virus to mice by this same method have been completely negative.
Li et al. (1951) studied this question.