Are nontreponemal tests suitable for monitoring syphilis treatment efficacy? Evidence from rabbit infection models

We aimed to characterize kinetics of non-treponamal antibody titres during the natural course of syphilis and explore their roles in monitoring syphilis treatment efficacy. Sixty New Zealand white male rabbits were challenged with Nichols or Amoy Treponema pallidum strains, and the rapid plasma reagin (RPR) test was performed to quantify non-treponemal antibody titres during the infection course. Viable T. pallidum in the challenged rabbits was assessed with rabbit infectivity tests. The RPR titres of the Nichols or Amoy strain between no benzathine penicillin G (BPG) and BPG treatment subgroups displayed a similar trend: first ascending and then descending. Compared with baseline, the proportions of fourfold decline in RPR titres in the Nichols or Amoy group presented a similar result on days 30, 60 and 180 between the no BPG and BPG treatment subgroups (0%, 0/5; 80%, 4/5; 100%, 5/5; vs. 0%, 0/5; 80%, 4/5; 100%, 5/5; p 0.999; 0%, 0/5; 80%, 4/5; 80%, 4/5; vs. 40%, 2/5; 100%, 5/5; 100%, 5/5; p 0.098, respectively). Compared with the maximum baseline titre, the proportion of fourfold decline in PRR titre also showed a similar result in the two groups on days 30, 60 and 180 between the no BPG and the BPG treatment subgroups (0%, 0/5; 100%, 5/5; 100%, 5/5, vs. 40%, 2/5; 100%, 5/5; 100%, 5/5; p 0.129; 0%, 0/5; 100%, 5/5; 100%, 5/5, vs. 80%, 4/5; 100%, 5/5; 100%, 5/5; p 0.091, respectively. Moreover, regardless of whether the RPR titres presented a fourfold decline, viable T. pallidum could be detected in untreated rabbits' lymph nodes at 30, 60 and 180 days post infection, while viable T. pallidum was not detected in any of the treated rabbits' lymph nodes. The RPR titre increased and then decreased (even became negative) during the natural course of syphilis, similar to that seen after BPG treatment. The RPR tetre is thus a questionable indicator of syphilis treatment efficacy.