Limited protection of rabbits against infection with Treponema pallidum by immune rabbit sera.

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Abstract
After intradermal infection in rabbits with 3 .times. 106 T. pallidum (Melbourne 1 strain) samples of serum were taken at 1, 2, 3, 4 and 6 mo. after infection. Normal rabbits were passively immunized with these sera, challenged with intradermal doses (104, 103, 102, 10) of T. pallidum and the latent periods of infection, lesion diameters and the number of inoculation sites developing into lesions were observed. The sera taken at 3, 4 and 6 mo. reduced the number of intradermal inoculation sites that developed into syphilitic lesions after challenge with 10 T. pallidum. The same 3 sera also increased the latent period of infection after challenge with 104 T. pallidum. The transfer of 50 ml of immune serum/rabbit over a 9 day period before challenge had very little effect on the course of the challenge infection. Only a low level of immunity in rabbits to this strain of T. pallidum appears to be mediated by immune serum but this small degree of protection increased with time after infection. Enhanced growth of T. pallidum in the serum-recipient rabbits did not occur, thus suggesting that none of the sera was immunosuppressive.