Rabbits immunised with virulent Treponema paraluis-cuniculi were challenged intradermally with graded doses of Treponema pallidum at three, five, seven, 12, and 30 months to ascertain the level of protection to T pallidum at various intervals after immunisation.
Rabbits challenged at three months after immunisation 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 five and seven months after immunisation, 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 months after immunisation but a third, presumably still immunosuppressed, developed lesions significantly faster than the control rabbits after challenge. At 30 months after immunisation one 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 three months after infection; in one rabbit this may have been 12 months. In most immunised rabbits, however, limited cross-protection to low challenge doses of T pallidum developed by five months and was also detectable at seven and 12 months. Only one rabbit was completely resistant to challenge with 104T pallidum after 30 months 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.