A RNA intertypic recombinant, v3/al-25, which has the 5′ and 3′ halves of the genome derived from the neurovirulent type 3 poliovirus strain 452/62 3D and the attenuated type 1 poliovirus strain LSc-gr3, respectively, was previously shown to cause severe paralytic poliomyelitis after intracerebral inoculation of monkeys. To ascertain whether the illness was caused by the recombinant itself or by temperature-resistant RNA mutants that might have arisen in the inoculated monkeys, five independent virus strains have been isolated from the spinal cord of the diseased animals. While two of these isolates exhibited RNA and RNA phenotypes, respectively, the other three strains retained the parental RNA character. Except for the RNA strain, the RNase T1 oligonucleotide maps of the genomes of all the isolates revealed only a minimal deviation from the parental pattern. These results were interpreted to mean that v3/a1-25 is intrinsically neurovirulent despite the presence of a RNA mutation(s) in the 3′ half of its genome. Nevertheless, this mutation, or other peculiarities of the 3′ half of the recombinant genome, may somewhat alleviate the pathogenicity of the virus. This notion was inferred from the fact that, when used in a relatively small dose (about 10 p.f.u.), v3/a1-25 appeared to exhibit a lower level of neurovirulence compared to either the wild-type parent 452/62 3D, or a closely related intertypic recombinant having the genome 3′ half derived from a neurovirulent RNA type 1 poliovirus strain. The problem of genetic determination of poliovirus neurovirulence and attenuation is briefly discussed.


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