Viremia with JE virus and its relationship to pathogenicity were studied in mice. The degree of viremia and susceptibility to peripheral infection decreased with age. Mice became markedly resistant to JE virus infection between 1 and 2 weeks of age. A viremia-positive (V+) variant was isolated by rapid blood-to-blood passages of the Mukai strain in 1-week-old mice. This strain causes the highest viremia with JE virus ever reported in 3-week-old mice. The maximal titer was about 103 PFU/ml of blood when 30 PFU were inoculated subcutaneously. In general, viremia with JE virus was not detected so easily and its titer was not so high when the virus was inoculated subcutaneously in 3-week-old mice. There was a proportional relationship between the occurrence of viremia and pathogenicity. The V+ variant showed 100-fold higher pathogenicity in peripheral infections and 10-fold higher in intracerebral infection than the wild type (V-) virus in 3-week-old mice.The causal relationship between the occurrence of viremia and the production of circulating interferon was studied. V- virus produced more interferon in the blood of mice than the V+ virus, whereas the titer of viremia was higher with the V+ variant than with V- virus. So the occurrence of viremia was regulated by the ability of the virus to produce circulating interferon. No difference was found in sensitivity to interferon between V+ and V- virus.
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