Defined virulent and avirulent strains of Semliki Forest virus were used to infect young and old Porton mice by respiratory or intraperitoneal routes. The infectivity of the blood, brain and spleen from individual mice showed very wide variations; levels of infectivity in blood were apparently unrelated to those which developed later in brain.

Initial patterns of infection in brain and blood following avirulent or virulent infection were indistinguishable. By 24 to 36 hr after virulent infection the fastest responding mice showed higher incidence and level of infectivity in brain and spleen. By 48 hr after respiratory infection or 72 hr after intraperitoneal infection virus replication was suppressed in the brains of avirulent infected mice but not in those of virulent infected mice. For young adult mice of 25 to 32 days old, virulent and avirulent infections were further distinguished at 72 hr by the levels of infectivity in brain and spleen. Virulent infections leading to death were associated with brain infectivities above a critical level and spleen infectivities below a critical level. Avirulent infections were associated with the reversed condition.

These results are discussed in terms of the differential triggering of distinct lethal and protective responses.


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