<i>Treponema pallidum</i> Infection in Asymptomatic Persons: A Puzzling Scenario in the Canary Islands (Spain) (2001-2020)
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Background and Objectives: Syphilis is an infectious disease caused by T. pallidum subsp. Pal-lidum. In high-income countries the main mode of transmission is sexual. Approximately half of infected patients are asymptomatic, which does not exclude the possibility of trans-mission. The aim of this study was to evaluate syphilis seroprevalence among asymptomatic persons in Gran Canaria (Canary Islands, Spain). Patients and Methods: Three different groups were studied from 2001 to 2020: i) the “healthy” population, based on 948,869 vol-untary blood donations; ii) undocumented African immigrants, including 1,873 recent arri-vals in Gran Canaria; and iii) people living with HIV (PLWH) , a group of 1,690 patients followed by our team. We also included a reference population representative of the overall population the Canary Islands. The evaluation included both treponemal and reaginic tests. Results: i) among blood donors, the mean seroprevalence of positive treponemal tests was 0.25% (95% CI: 0.19-0.31). Non-treponemal test positivity (RPR) ranged from 0.05 to 0.06% with titers ≤ 1:4 in all cases; ii) thirty-four of 641 undocumented African migrants (5.30%; 95% CI: 3.82-7.32%) had a confirmed positive treponemal test but only 4 had a pos-itive RPR, with titers ranging from 1:1 to 1:4; iii) 46.51% (95% CI: 44.14 - 48.89) of PLWH patients had a confirmed positive treponemal test. For factors related to HIV-syphilis coin-fection, multivariate analysis clearly showed the association with male sex and the MSM risk category. However, the results of this series call into question the overall role of immi-gration in the seroprevalence of syphilis among PLWH in our setting. Active syphilis (RPR > 1:8) was found in 20.10% of PLWH. Conclusions: In summary, syphilis is a re-emerging in-fection, and asymptomatic persons constitute a group that facilitates its transmission and spread. In our setting, seroprevalence was lowest in the healthy population, higher in re-cently arrived African migrants, and highest in PLWH, especially MSM. The presence of active syphilis however is mainly restricted to MSM. This information is of relevance for the design of syphilis control strategies.