The pathogenicity of the DIs virus was compared with those of three other vaccinia virus strains, Lister (Elstree), Ikeda and IHD, by intracerebral inoculation into cynomolgus monkeys. By inoculation of a large amount of the virus into the thalamus and cisterna, the animals receiving the DIs virus remained normal, whereas those receiving the other viruses showed severe clinical symptoms such as anorexia, apathy, coma, and paralysis, and were found moribund or dead within a few days. Histological studies also revealed a big difference in the severity of lesions induced by the DIs virus and those by the other three vaccinia viruses, although the main common finding was leptomeningitis purulenta acuta.
Immunization of cynomolgus monkeys with a single shot of the DIs viurs via the sc, id or im route, followed by a percutaneous challenge with the Ikeda virus, revealed that the skin resistance had not been established (Tagaya et al., 1973) . Studies on the dynamics of circulating hemagglutination-inhibiting and neutralizing antibodies, however, indicated that the immunized animals responded more readily and strongly to the challenge with the Ikeda virus than did non-immune animals. This may suggest that the DIs virus induced basic immunity in monkeys, although circulating neutralizing antibody was not detectable and skin reactions by the Ikeda virus were only slightly modified. Thermolabile vaccinia virusneutralizing activity in the serum also appeared or increased by inoculation with the DIs virus as well as with the Ikeda virus. The immunological role of this substance awaits further studies.
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