The intensity and duration of Borrelia novyi infection in rats depends upon the dose of spirochetes administered. The spirochetemia in response to an inoculum of 1 x 108 spirochetes/kg. characteristically reaches a peak of 4 to 5 million organisms per c.mm. of blood at about 72 hours. Resolution of the primary infection occurs within 48 hours after the peak counts have been observed, with crisis ending at 100 to 120 hours after inoculation.

Treatment of the acute infection, so standardized, by 10 ml./kg. of crisis plasma intraperitoneally delays the onset of the spirochetemia 76 to 140 hours, and reduces the maximal spirochetal count 10–3 to 10–6 that of the unmodified controls. This is evidence that inhibitor is present in the plasma at the time of crisis and plays a role in limiting the primary infection and subsequent relapses. The activity of crisis plasma is not destroyed by freezing, or after storage at 4°C. for 4 days. Fractions I + II + III and IV + V+ VI from crisis plasma, obtained by method 10 of Cohn et al., were also effective in suppressing the acute spirochetemia, but differed in duration and effectiveness. The first of these was about as potent as the undiluted whole plasma, and the second about as potent as plasma diluted 1:4.

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