The level of viraemia was determined in serial blood samples obtained from 2 mice after the injection of Riley virus. The plasma virus titre rose rapidly to a peak value of 109 to 1010 ID50 per ml by 24 hours after infection, and then fell slowly to a level of 105 to 106ID50 per ml by the 10th day after infection, where it remained relatively stable.

Neither blockade of the RES with thorotrast, zymosan, or carbon, nor stimulation of the RES with stilboestrol or zymosan, before the injection of Riley virus, produced any observable alteration in the level of viraemia attained 24 hours after infection. However 10 days or more after infection with Riley virus blockade of the RES with thorotrast caused a transitory rise, and stimulation of the RES with stilboestrol caused a slight but prolonged fall, in the level of viraemia. Zymosan injection at this period of infection caused an initial rise, followed by a fall, in the level of viraemia; these changes correlated with the initial period of blockade and the subsequent period of stimulation of the RES observed in carbon clearance studies.

The clearance of injected Riley virus particles from the plasma over a period of 3 hours after injection was measured in previously uninfected mice and mice which had been infected with Riley virus for 3 weeks. The mice which had been infected 3 weeks before the test cleared rather more of the injected virus than the previously uninfected mice.

It is concluded that although the activity of the RES affects, and may determine, the level of viraemia, the permanence of the viraemia in Riley virus-infected mice does not appear to be due to a failure of the RES to clear virus particles from the plasma.

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