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The hypothesis that experimentally determined survival times of Treponema pallidum in stored donor blood could be related to the number of treponemes initially present in the treponeme-blood mixtures was investigated by inoculating rabbits with three graded doses of treponemes suspended in donor blood and stored at 4 degrees C for various periods of time. The storage periods up to which living treponemes could be detected in the testes of the inoculated rabbits as well as those storage periods up to which seroconversion occurred are related to the number of treponemes present in the treponeme-blood mixtures. Increasing numbers of treponemes present in the donor blood resulted in longer periods up to which positive results in both parameters were found. All available evidence suggests that the upper limit of seroconversion coincides with the upper limit of treponemal survival. Inoculation with 5 X 10(4) treponemes per ml of donor blood resulted in a treponemal survival time of 48 h, inoculation with 1.25 X 10(6) treponemes per ml in a treponemal survival time of 72 h and inoculation with 2.5 X 10(7) treponemes per ml donor blood in a survival time of 120 h.

The influence of storage at 4 degrees C on the survival of Treponema pallidum in donor blood, artificially infected with treponemes of the virulent Nichols strain was studied. Rabbits were inoculated in each testis with 0.5 ml of the blood-treponeme mixture, containing 5 X 10(5) microorganisms/ml, which was stored for up to 336 h. Under these circumstances, the blood-treponeme mixture appeared to be infectious for up to 96 h of storage, based on the demonstration of T. pallidum and serological findings, using FTA-ABS, TPHA and VDRL tests. The two former tests were found to be equally sensitive in detecting incubating syphilis in the rabbits, the VDRL test was significantly slower in this respect. From the absence of orchitis as well as seroconversion in rabbits, inoculated with blood-treponeme mixtures stored for 120 h or longer periods of time, it is concluded that after 120 h of storage infectivity is lost. The presently found survival time of treponemes in blood between 96 and 120 h of storage is discussed in relation with survival times found by other investigators.