Sequential changes in susceptibility to Treponema pallidum of rabbits previously infected with Treponema paraluis-cuniculi

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Abstract
Rabbits immunized with virulent T. paraluis-cuniculi were challenged intradermally with graded doses of T. pallidum at 3, 5, 7, 12 and 30 mo. to ascertain the level of protection to T. pallidum at various intervals after immunization. Rabbits challenged at 3 mo. after immunization showed no protection against T. pallidum and developed syphilitic lesions significantly faster than the control rabbits, which suggests that the former rabbits were immunosuppressed. Some protection was evident at 5 and 7 mo. after immunization, as fewer inoculation sites developed syphilitic lesions with challenges of 103, 102 and 10 T. pallidum and lesions developed significantly slower with 106 challenge. Two rabbits showed significant protection at 12 mo. after immunization but a 3rd, presumably still immunosuppressed, developed lesions significantly faster than the control rabbits after challenge. At 30 mo. after immunization, 1 rabbit was completely protected and developed no lesions after challenge; the other rabbit showed only partial protection against challenge with 104, 103 and 102 but complete protection against challenge with 10 T. pallidum. T. paraluis-cuniculi appeared to induce a state of immunosuppression by 3 mo. after infection; in 1 rabbit this may have been 12 mo. In most immunized rabbits, however, limited cross-protection to low challenge doses of T. pallidum developed by 5 mo. and was also detectable at 7 and 12 mo. Only 1 rabbit was completely resistant to challenge with 104 T. pallidum after 30 mo. and another was only partly immune. Thus, T. paraluis-cuniculi infection does not produce a rapid pronounced cross-immunity to T. pallidum in rabbits, which may thus limit its usefulness as a vaccine against syphilis.