- Email: [email protected]
Increasing trend of syphilis and infection resistance: a retrospective study
International Journal of Infectious Diseases 17 (2013) e971–e976
Contents lists available at SciVerse ScienceDirect
International Journal of Infectious Diseases journal homepage: www.elsevier.com/locate/ijid
Increasing trend of syphilis and infection resistance: a retrospective study Jinping Gao a,b, Jinhua Xu c, Yujun Sheng a,b, Xiaoguang Zhang a,b, Change Zhang a,b, Yang Li a,b, Bo Liang a,b, Liangdan Sun a,b, Sen Yang a,b,*, Xuejun Zhang a,b,c a b c
Department of Dermatology and Venereology, No. 1 Hospital, Anhui Medical University, Meishan road 81, Hefei 230032, Anhui, China Institute of Dermatology and Key Laboratory of Dermatology, Ministry of Education, Anhui Medical University, Hefei, Anhui, China Institute of Dermatology, Huashan Hospital of Fudan University, Shanghai, China
A R T I C L E I N F O
S U M M A R Y
Article history: Received 26 February 2013 Received in revised form 10 May 2013 Accepted 13 May 2013
Objectives: To assess the epidemic trends of syphilis and to investigate syphilis infections after exposure to infectious patients. Methods: A total of 17 211 syphilis patients from the period January 1999 to September 2012 were enrolled in this study. A variety of syphilis prevalence measures were evaluated. We analyzed the characteristics of 2954 cases using available information. Of these patients, 535 early syphilis cases were identified as index patients and the status of their sexual partners was monitored. All sexual partners were followed for 6 months to 1 year through serological testing and clinical examinations. Results: The proportion of syphilis-positive clients at the sexually transmitted disease (STD) clinic increased annually, with a five-fold increase from 1999 to 2011 (from 6.1% to 30.0%). The highest increase in syphilis infection occurred among patients in the 20–29 years age group. Male and female cases increased at the same rate between 1999 and 2007, but female cases increased at a greater rate than male cases from 2008 to 2012. Of the 535 sexual partners in the study, 330 (61.7%) were infected with syphilis and 205 (38.3%) were seronegative without any symptoms. Gender may influence disease infection rates (p = 0.008), but not at different stages of early syphilis. Conclusions: There was an increasing trend of syphilis infection in Hefei, China. A proportion of highly exposed individuals could be resistant to syphilis infection. ß 2013 International Society for Infectious Diseases. Published by Elsevier Ltd. All rights reserved.
Corresponding Editor: Eskild Petersen, Aarhus, Denmark Keywords: Syphilis Epidemiology Resistance Infection
1. Introduction Syphilis is a typical sexually transmitted disease (STD) with diverse and wide-ranging manifestations and is caused by Treponema pallidum.1 It is a serious public health concern worldwide due to its global reach and its associated complications.2 Even with the availability of relatively sensitive tests and effective treatments, syphilis is re-emerging with increasing prevalence in many countries. The effective control of syphilis remains a formidable challenge. In 1999, the World Health Organization estimated that there were 12 million new cases of syphilis, with more than 90% of those cases occurring in developing countries (http://www.who.int/hiv/pub/sti/who_hiv_aids_2001. 02.pdf). Syphilis has had a dramatic resurgence following recent economic reforms in China, and its prevalence has increased
* Corresponding author. Tel.: +86 551 5161002; fax: +86 551 5161016. E-mail address: [email protected] (S. Yang).
sharply in the past 5 years.3,4 Presently, syphilis is ranked as the third most prevalent notifiable infectious disease in China.5 The rapid spread of syphilis is likely due to a combination of biological and social factors. Previous studies have suggested that the spread of sexually transmitted infections is related to economic development,6 and have found a higher burden of syphilis in more developed provinces of China.3 With recent economic growth, the capital of Anhui Province, Hefei, has experienced a substantial syphilis epidemic over the past two to three decades. Understanding the dynamic pattern of syphilis is important to determine how to design effective interventions to reduce the resurgence of syphilis epidemics. In China, assessing the epidemiology of syphilis largely relies on the surveillance from case reports submitted by hospitals or clinics through a website-based case-reporting system.3 A measure that is commonly used to interrupt disease transmission is to monitor and follow the sexual partners of syphilis patients,7 because the infection is almost always transmitted through sexual contact with
1201-9712/$36.00 – see front matter ß 2013 International Society for Infectious Diseases. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijid.2013.05.007
e972
J. Gao et al. / International Journal of Infectious Diseases 17 (2013) e971–e976
an infected person.8 However, the infectiousness of syphilis remains unclear. There is little available information in the literature on this important aspect of syphilis. It is thus essential to evaluate why not all individuals are infected with syphilis after direct sexual contact with someone suffering from the disease. In this study, we assessed the epidemic trends and clinical characteristics of syphilis during the period January 1999 to September 2012. We explored the relationship between T. pallidum exposure and the incidence of syphilis infection among the sexual partners of patients. 2. Methods 2.1. Study sample This retrospective study was conducted using data from the case-based surveillance system at the STD Medical and Therapy Center of Anhui Medical University (AHMU) in Hefei, China, which is a the major medical institution that provides high-quality medical services to STD patients. Outpatient visits to this center account for the majority of total STD outpatient visits in Hefei. Since the establishment of the center, basic information has been collected (e.g., age, gender, occupation, and serological test results) for each attended syphilis patient who received outpatient followup. Syphilis cases were defined by positive diagnostic tests, a history of high-risk sexual behavior, and the characteristics of clinical manifestations.9 Each patient received a non-treponemal test (rapid plasma reagin test, RPR), followed by a treponemal test (Treponema pallidum particle agglutination test, TPPA) to confirm the diagnosis. From 2009 to 2011, an epidemiological investigation and clinical characteristic analysis of syphilis patients was performed at this center. All information was collected through face-to-face interviews that were conducted by clinical staff. The questionnaire-based survey provided socio-demographic information (including gender, year of birth, ethnicity, level of education,
reason for visit, and risk factors) and clinical information (e.g., manifestations, serologic test results, and stages of syphilis). In total, 2954 syphilis patients agreed to participate in this study and completed the questionnaire survey for the analysis of clinical characteristics. We monitored sexual partners to determine whether they had been infected after direct unprotected sexual contact with a syphilis patient. Only those cases who met the following criteria were included in the study: heterosexual early syphilis patients (ESPs), 18 years or older, who were not pregnant and did not have a connective tissue disease. Additionally, to qualify for the study, these ESPs had to report having had unprotected sex (no condom use) with their regular sexual partner before receiving treatment. The early stage of syphilis is potentially infectious and includes primary, secondary, and early latent syphilis.8,10 The primary stage is defined by a painless chancre at the site of infection. The secondary stage is characterized by a polymorphic eruption, lymphadenopathy, and other systemic manifestations. Early latent syphilis is defined as occurring within 1 year of infection and is an asymptomatic stage of syphilis.8 After obtaining written informed consent from both patients and their regular sexual partners, 535 ESPs, whose initial RPR titer ranged from 1:4 to 1:256 and who met the enrollment criteria, were recruited as index patients among the 2954 patients. All of their regular sexual partners underwent a clinical examination and serologic tests upon enrollment and at 1,3,6 months or 1 year follow-up. If a patient and his or her sexual partner tested positive for syphilis or exhibited clinical symptoms, they were diagnosed as having a concordant syphilis infection. However, if a patient’s sexual partner tested negative for syphilis and exhibited no clinical manifestation, it was considered a discordant syphilis infection. A diagram of the study design and sampling strategy is displayed in Figure 1. The study was approved by the institutional ethics committee of AHMU and was conducted in accordance with the principles of the Declaration of Helsinki.
Figure 1. Flow diagram of the study design and sampling strategy.
J. Gao et al. / International Journal of Infectious Diseases 17 (2013) e971–e976
e973
Figure 2. The increasing trends in syphilis at the STD Medical and Therapy Center of AHMU from January 1999 to September 2012. There was an upward trend in syphilis infections, as observed by the proportion syphilis-positive of the total STD clinic patients. The same increasing trend was observed for male and female cases from 1999 to 2007, but the female cases gradually increased more than the male cases from 2008 to 2012.
2.2. Statistical analysis A descriptive analysis was conducted to determine the statistical significance of differences in demographic characteristics (age, gender, and education level), clinical stage of syphilis, initial RPR titer, and HIV co-infection. Categorical variables were analyzed by Pearson Chi-square test or Fisher’s exact test. The rates of infection and clinical characteristics of patients and their sexual partners were reported as percentages. A p-value of <0.05 (twotailed) was used to determine statistical significance. All statistical analyses were conducted using SPSS version 11.0 for Windows (SPSS Inc., Chicago, IL, USA). 3. Results 3.1. Trends in the syphilis epidemic from 1999 to 2012 During the period January 1999 to September 2012, a total of 17 211 cases of syphilis infection were recorded at the STD Medical and Therapy Center. There were 8025 (46.7%) male syphilis cases and 9186 (53.3%) female syphilis cases. The proportion of syphilispositive patients increased over the course of the 14-year study, with a five-fold increase from 1999 and 2011 (6.1% and 30.0%, respectively) (Figure 2). Male and female cases increased at the same rate between 1999 and 2007. However, female cases
increased at a greater rate than male cases from 2008 to 2012 (Figure 2). With regard to age, the highest rate of syphilis infection was observed in the 20–29 years age group (Figure 3). In addition, the number of syphilis cases in the other age groups also increased over the course of the 14-year study. This finding illustrates an upward trend in syphilis infections over the years in Hefei, China, from January 1999 to September 2012. 3.2. Demographic and clinical characteristics of syphilis cases An epidemiological survey was conducted among 2954 syphilis patients (1318 male and 1636 female) from 2009 to 2011(Table 1). The median age of patients was 35.8 years, with a range from newborn to 89 years. Of the 2954 patients enrolled in the survey, 2485 (84.1%) were between 20 and 50 years old. Of the 1636 female patients, 42.9% of the syphilis cases were found among the patients who were aged 20–29 years. The number of syphilis cases among men and women in diverse age groups was significantly different (p < 0.0001). There was also a significant difference in the level of education of enrolled patients (p < 0.0001); 1624 (55%) individuals attended only junior middle school. The initial RPR titer did not differ between male and female patients (p > 0.05). Among all 2954 cases, only 17 (0.6%) were co-infected with HIV. Of the infected patients, 1955 (66.2%) were infected with latent syphilis. All data are shown in Table 1.
Figure 3. The trends in syphilis cases in different age groups. Syphilis infection increased most rapidly among patients who were aged 20–29 years. Elders (over 50 years) and patients who were less than 20 years old experienced the slowest annual increase in infection rate.
J. Gao et al. / International Journal of Infectious Diseases 17 (2013) e971–e976
e974
Table 1 Characteristics of syphilis patients who attended the STD clinics from 2009 to 2011 Characteristics
Male, n (%) (N = 1318)
Race Age, years Median Range <15 15–19 20–29 30–39 40–49 50 Education level Illiterate Primary school Junior middle school Senior/vocational high school Diploma college/university Initial RPR titer 1:4 >1:4 HIV Positive Negative Stage of syphilis Primary Secondary Tertiary Latent Neurosyphilis Syphilis with pregnancy
Chinese Han 38.9 0–89 13 (1.0%) 20 (1.5%) 357 (27.1%) 346 (26.3%) 324 (24.6%) 258 (19.6%)
Female, n (%) (N = 1636)
Total, n (%) (N = 2954)
33.3 0–83 8 (0.5%) 37 (2.3%) 702 (42.9%) 455 (27.8%) 301 (18.4%) 133 (8.1%)
35.8 0–89 21 (0.7%) 57 (1.9%) 1059 (35.8%) 801 (27.1%) 625 (21.2%) 391 (13.2%)
(5.3%) (11.7%) (41.3%) (28.8%) (13.0%)
(4.8%) (10.7%) (39.5%) (28.5%) (16.5%)
<0.0001
56 (4.2%) 125 (9.5%) 491 (37.3%) 371 (28.1%) 275 (20.9%)
86 191 675 471 213
785 (59.6%) 547 (41.5%)
941 (57.5%) 681 (41.6%)
1726 (58.4%) 1228 (41.6%)
>0.05
9 (0.7%) 1309 (99.3%)
8 (0.5%) 1628 (99.5%)
17 (0.6%) 2937 (99.4%)
>0.05
173 (13.1%) 219 (16.6%) 13 (1.0%) 897 (68.1%) 16 (1.2%) 0 (0.0%)
130 272 22 1058 4 150
303 491 35 1955 20 150
<0.0001
(7.9%) (16.6%) (1.3%) (64.7%) (0.2%) (9.2%)
142 316 1166 842 488
p-Value
(10.3%) (16.6%) (1.2%) (66.2%) (0.7%) (5.1%)
<0.0001
STD, sexually transmitted diseases; RPR, rapid plasma reagin.
3.3. The status of sexual partners after exposure The diagnoses of and test results for regular sexual partners after sexual contacts with syphilis-positive patients are shown in Table 2. Of the 535 regular sexual partners who agreed to participate in this study, 330 (61.7%) had syphilis; 205 (38.3%) were seronegative without any symptoms. Of the 330 syphilispositive partners, 164 (67.1%) female partners and 166 (58.7%) male partners had syphilis. These differences were significant at the 5% level (p = 0.008). With regard to the different stages of syphilis in the serologically positive, 37 of 61 (60.7%) partners were in the primary stage, 80 of 112 (71.4%) were in the secondary stage and 213 of 362 (58.8%) were in the early latent stage, showing no significant difference among the three groups (p = 0.056). 4. Discussion Our study demonstrated an elevated epidemic trend of syphilis in Hefei, an inland city of China, based on the surveillance data collected at the STD Medical and Therapy Center of AHMU over the past 14 years. The increase in the spread of syphilis that we found is consistent with previous findings indicating that syphilis has been increasing in most Chinese cities.3,11 According to the national
surveillance data, the reported incidence of primary and secondary syphilis was 11.7 cases per 100 000 residents in 2009 and had increased by 2.1 times since 2005.12 A higher syphilis case burden may be attributable to changing sexual attitudes, the reemergence of prostitution, large numbers of migrants who practice unsafe sexual behaviors, and an increase in homosexual men. Evidence from the epidemiological trend suggests that syphilis should be considered an important public health concern in Hefei. There is an urgent need to improve the current surveillance system and to develop comprehensive intervention strategies. We found that those individuals within the 20–29 years age group contributed to the largest increase in the proportion of the syphilis cases from 1999 to 2012. A previous study suggested that the elders (over 50 years) may have a higher risk of STD acquisition.13 An increased proportion of elders who had syphilis in our study and national epidemiological data14 support this finding. The increasing trend among elders could be attributed to the burgeoning aging population in China, the development of medicines for erectile dysfunction, and the greater chance of sexual contact with female sex workers (FSWs).6 However, the reason why there has been a relatively smaller increase in infected individuals aged less than 20 years might be due to the low percentage of adolescents visiting the STD clinics.
Table 2 The ratio of infected vs. uninfected with syphilis in sexual partners Characteristic
Index patients
Concordant, infected Sexual partners
Discordant, uninfected Sexual partners
Number Sex Female Male Stage of syphilis Primary Secondary Early latent
535
330 (61.7%)
205 (38.3%)
242 male index cases 293 female index cases
164 (67.1%) (female) 166 (58.7%) (male)
78 (32.9%) (female) 127 (41.3%) (male)
0.008
61 112 362
37 (60.7%) 80 (71.4%) 213 (58.8%)
24 (39.3%) 32 (28.6%) 149 (41.2%)
0.056
p-Value
J. Gao et al. / International Journal of Infectious Diseases 17 (2013) e971–e976
In addition, a large proportion (55%) of syphilis patients only had high school educational attainment, which may explain their lack of awareness of how to protect themselves from risky sexual behaviors. The lack of basic medical knowledge may have influenced syphilis transmission and resulted in a substantial number of syphilis patients diagnosed at a latent stage. A higher syphilis prevalence was observed in female patients than in male patients. For example, within the 20–29 years age group, the percentage of female cases was higher than male cases. This finding may be due to an increase in the number of FSWs in China, which could explain the resurgence of syphilis.15 However, previous studies have found that gender could influence disease rates.16 In our study, an analysis of infection after exposure to syphilis revealed that the infection rate of female partners differed from the infection rate of male partners. This finding suggests that females are more susceptible to syphilis infection than males, which may lead to higher infection among females. In this study, the percentage (5.7%) of syphilis–HIV co-infection was lower than the percentage found in other studies of syphilis–HIV co-infection in China.17 However, due to increasing rates of HIV infection among syphilis patients, routine periodic screening among highrisk groups is strongly recommended. It is critical for all patients with a new diagnosis of syphilis to undergo HIV testing.18 Considering the results of this study, 61.7% of regular sexual partners could be infected if they have unprotected sex with an infectious syphilis patient. These data suggest that there is a high transmission rate in syphilis contacts. Therefore, it is imperative to take measures to prevent syphilis infection. The early detection and treatment of syphilis cases are essential parts of syphilis control strategies and can interrupt the transmission chain. Contacting sexual partners, facilitated through partner notification services, is an effective measure to help to control communicable diseases. Intriguingly, a proportion (38.3%) of sexual partners remained persistently seronegative during the study. The phenomenon whereby not everyone exposed to an infectious syphilis case is equally susceptible to infection is difficult to explain. Early studies of syphilis infection among European heterosexual and homosexual partners found that only approximately 50% to 60% of those at risk developed syphilis.19 If all people were equally susceptible to infection, then statistically, these seronegative partners would be infected with syphilis. Differences in risk factors such as safer sexual behaviors and the forms and frequency of sexual contact may explain this phenomenon. The incidence of other sexually transmitted infections, such as syphilis–HIV co-infection, could result in cases with a negative syphilis serologic test and no apparent symptoms.20 However, in this study each pair of patients and their sexual partners admitted that they had had unprotected sexual contacts or rarely used condoms. In seronegative sexual partners, none of them was infected with HIV during the follow-up period. This may not be a chance phenomenon; why were a few partners uninfected after direct sexual contacts? A possible explanation is that the seronegative sexual partners could be resistant to syphilis infection due to the existence of syphilisresistant genetic factors. It is also possible that a natural immune response mediates a potentially syphilis-resistant mechanism. People infected with T. pallidum in the early stage of syphilis could develop a Th1 predominant local cellular response to activate macrophages to phagocytose T. pallidum and resolve the lesion.21,22 The major proteins of T. pallidum could induce T cells to respond to syphilis infection with elevated reactivity.23 During the early infection, T. pallidum could elicit a vigorous inflammation to trigger an adaptive immune response.24 A typical cell-mediated immune response could result in the local clearance and resolution of the primary lesions.21,24 The major mechanism of T. pallidum clearance responses is T cell infiltration into lesions to activate
e975
macrophages to phagocytose opsonized T. pallidum.25 Different syphilis infection stages suggest that the ability of the immune response to clear T. pallidum is different among the individuals after exposure to T. pallidum. We speculated that a heterogeneity and variability in susceptibility to syphilis infection might exist in the general population, similar to HIV-1 infection.26 A characteristic feature of many human infections is that only a proportion of exposed individuals develop clinical disease. Heritable factors have long been considered to play an important role in interpreting individual variations in susceptibility.27 Recent genome-wide association studies have identified several genetic factors/susceptibility loci that confer risk to infectious diseases such as HIV,28 malaria,29 and leprosy.30 These studies suggest that genetic variation among hosts plays a key role in determining an individual’s susceptibility and resistance to infectious disease. Genetic studies of other infectious diseases, in turn, have highlighted the hypothesis that some naturally unsusceptible or resistant genetic factors exist for syphilis. Confirmation of the mechanisms that determine why some people who are exposed to T. pallidum do not develop disease is needed to understand protective immunity against T. pallidum. In summary, our study revealed an increasing epidemic trend of syphilis in Hefei, China, and that this trend may continue in the future. It is imperative to adopt effective measures to control and prevent this disease. Further studies are needed to understand the mechanism of syphilis-specific immune responses in highly exposed subjects who are seronegative, which will provide new insights into disease mechanisms and will ultimately aid in controlling syphilis epidemics. Acknowledgements The authors would like to acknowledge the patients and their sexual partners who participated in this project. Funding sources: Supported by funding from the Ministry of Education, China (IRT-1046). Ethical approval: The study was approved by the institutional ethics committee of AHMU and was conducted in accordance with the principles of the Declaration of Helsinki. Conflict of interest: No conflict of interest to declare.
References 1. Ho EL, Lukehart SA. Syphilis: using modern approaches to understand an old disease. J Clin Invest 2011;121:4584–92. 2. Lafond RE, Lukehart SA. Biological basis for syphilis. Clin Microbiol Rev 2006;19:29–49. 3. Chen ZQ, Zhang GC, Gong XD, Lin C, Gao X, Liang GJ, et al. Syphilis in China: results of a national surveillance programme. Lancet 2007;369:132–8. 4. Tucker JD, Yin YP, Wang B, Chen XS, Cohen MS. An expanding syphilis epidemic in China: epidemiology, behavioural risk and control strategies with a focus on low-tier female sex workers and men who have sex with men. Sex Transm Infect 2011;87(Suppl 2):ii16–8. 5. Chen XS, Wang QQ, Yin YP, Liang GJ, Jiang N, Yang LG, et al. Prevalence of syphilis infection in different tiers of female sex workers in China: implications for surveillance and interventions. BMC Infect Dis 2012;12:84. 6. Yang LG, Tucker JD, Yang B, Shen SY, Sun XF, Chen YF, et al. Primary syphilis cases in Guangdong Province 1995-2008: opportunities for linking syphilis control and regional development. BMC Public Health 2010;10:793. 7. Centers for Disease Control and Prevention. Recommendations for partner services programs for HIV infection, syphilis, gonorrhea, and chlamydial infection. MMWR Recomm Rep 2008; 57:1-83; quiz CE1-4. 8. Singh AE, Romanowski B. Syphilis: review with emphasis on clinical, epidemiologic, and some biologic features. Clin Microbiol Rev 1999;12:187–209. 9. Ratnam S. The laboratory diagnosis of syphilis. Can J Infect Dis Med Microbiol 2005;16:45–51. 10. Dourmishev LA, Dourmishev AL. Syphilis: uncommon presentations in adults. Clin Dermatol 2005;23:555–64. 11. Chen XS, Yin YP, Tucker JD, Gao X, Cheng F, Wang TF, et al. Detection of acute and established HIV infections in sexually transmitted disease clinics in Guangxi, China: implications for screening and prevention of HIV infection. J Infect Dis 2007;196:1654–61.
e976
J. Gao et al. / International Journal of Infectious Diseases 17 (2013) e971–e976
12. Chen XS, Peeling RW, Yin YP, Mabey DC. The epidemic of sexually transmitted infections in China: implications for control and future perspectives. BMC Med 2011;9:111. 13. Schmid GP, Williams BG, Garcia-Calleja JM, Miller C, Segar E, Southworth M, et al. The unexplored story of HIV and ageing. Bull World Health Organ 2009;87. 162–162A. 14. Pearline RV, Tucker JD, Yuan LF, Bu J, Yin YP, Chen XS, et al. Sexually transmitted infections among individuals over fifty years of age in China. AIDS Patient Care STDS 2010;24:345–7. 15. Tucker JD, Chen XS, Peeling RW. Syphilis and social upheaval in China. N Engl J Med 2010;362:1658–61. 16. Wong SP, Yin YP, Gao X, Wei WH, Shi MQ, Huang PY, et al. Risk of syphilis in STI clinic patients: a cross-sectional study of 11,500 cases in Guangxi, China. Sex Transm Infect 2007;83:351–6. 17. Chow EP, Wilson DP, Zhang L. HIV and syphilis co-infection increasing among men who have sex with men in China: a systematic review and meta-analysis. PLoS One 2011;6:e22768. 18. Workowski KA, Berman SM. Centers for Disease Control and Prevention. Sexually transmitted diseases treatment guidelines, 2006. MMWR Recomm Rep 2006;55:1–94. 19. Schober PC, Gabriel G, White P, Felton WF, Thin RN. How infectious is syphilis? Br J Vener Dis 1983;59:217–9. 20. Blum L, Bachmeyer C, Caumes E. Seronegative secondary syphilis in an HIVinfected patient. Clin Exp Dermatol 2005;30:158–9. 21. Leader BT, Godornes C, VanVoorhis WC, Lukehart SA. CD4+ lymphocytes and gamma interferon predominate in local immune responses in early experimental syphilis. Infect Immun 2007;75:3021–6.
22. Van Voorhis WC, Barrett LK, Koelle DM, Nasio JM, Plummer FA, Lukehart SA. Primary and secondary syphilis lesions contain mRNA for Th1 cytokines. J Infect Dis 1996;173:491–5. 23. Arroll TW, Centurion-Lara A, Lukehart SA, Van Voorhis WC. T-cell responses to Treponema pallidum subsp. pallidum antigens during the course of experimental syphilis infection. Infect Immun 1999;67:4757–63. 24. Babolin C, Amedei A, Ozolins D, Zilevica A, D’Elios MM, de Bernard M. TpF1 from Treponema pallidum activates inflammasome and promotes the development of regulatory T cells. J Immunol 2011;187:1377–84. 25. Morgan CA, Molini BJ, Lukehart SA, Van Voorhis WC. Segregation of B and T cell epitopes of Treponema pallidum repeat protein K to variable and conserved regions during experimental syphilis infection. J Immunol 2002; 169:952–7. 26. Fowke KR, Nagelkerke NJ, Kimani J, Simonsen JN, Anzala AO, Bwayo JJ, et al. Resistance to HIV-1 infection among persistently seronegative prostitutes in Nairobi. Kenya Lancet 1996;348:1347–51. 27. Chapman SJ, Hill AV. Human genetic susceptibility to infectious disease. Nat Rev Genet 2012;13:175–88. 28. Fellay J, Shianna KV, Ge D, Colombo S, Ledergerber B, Weale M, et al. A wholegenome association study of major determinants for host control of HIV-1. Science 2007;317:944–7. 29. Timmann C, Thye T, Vens M, Evans J, May J, Ehmen C, et al. Genome-wide association study indicates two novel resistance loci for severe malaria. Nature 2012;489:443–6. 30. Zhang FR, Huang W, Chen SM, Sun LD, Liu H, Li Y, et al. Genomewide association study of leprosy. N Engl J Med 2009;361:2609–18.
Contents lists available at SciVerse ScienceDirect
International Journal of Infectious Diseases journal homepage: www.elsevier.com/locate/ijid
Increasing trend of syphilis and infection resistance: a retrospective study Jinping Gao a,b, Jinhua Xu c, Yujun Sheng a,b, Xiaoguang Zhang a,b, Change Zhang a,b, Yang Li a,b, Bo Liang a,b, Liangdan Sun a,b, Sen Yang a,b,*, Xuejun Zhang a,b,c a b c
Department of Dermatology and Venereology, No. 1 Hospital, Anhui Medical University, Meishan road 81, Hefei 230032, Anhui, China Institute of Dermatology and Key Laboratory of Dermatology, Ministry of Education, Anhui Medical University, Hefei, Anhui, China Institute of Dermatology, Huashan Hospital of Fudan University, Shanghai, China
A R T I C L E I N F O
S U M M A R Y
Article history: Received 26 February 2013 Received in revised form 10 May 2013 Accepted 13 May 2013
Objectives: To assess the epidemic trends of syphilis and to investigate syphilis infections after exposure to infectious patients. Methods: A total of 17 211 syphilis patients from the period January 1999 to September 2012 were enrolled in this study. A variety of syphilis prevalence measures were evaluated. We analyzed the characteristics of 2954 cases using available information. Of these patients, 535 early syphilis cases were identified as index patients and the status of their sexual partners was monitored. All sexual partners were followed for 6 months to 1 year through serological testing and clinical examinations. Results: The proportion of syphilis-positive clients at the sexually transmitted disease (STD) clinic increased annually, with a five-fold increase from 1999 to 2011 (from 6.1% to 30.0%). The highest increase in syphilis infection occurred among patients in the 20–29 years age group. Male and female cases increased at the same rate between 1999 and 2007, but female cases increased at a greater rate than male cases from 2008 to 2012. Of the 535 sexual partners in the study, 330 (61.7%) were infected with syphilis and 205 (38.3%) were seronegative without any symptoms. Gender may influence disease infection rates (p = 0.008), but not at different stages of early syphilis. Conclusions: There was an increasing trend of syphilis infection in Hefei, China. A proportion of highly exposed individuals could be resistant to syphilis infection. ß 2013 International Society for Infectious Diseases. Published by Elsevier Ltd. All rights reserved.
Corresponding Editor: Eskild Petersen, Aarhus, Denmark Keywords: Syphilis Epidemiology Resistance Infection
1. Introduction Syphilis is a typical sexually transmitted disease (STD) with diverse and wide-ranging manifestations and is caused by Treponema pallidum.1 It is a serious public health concern worldwide due to its global reach and its associated complications.2 Even with the availability of relatively sensitive tests and effective treatments, syphilis is re-emerging with increasing prevalence in many countries. The effective control of syphilis remains a formidable challenge. In 1999, the World Health Organization estimated that there were 12 million new cases of syphilis, with more than 90% of those cases occurring in developing countries (http://www.who.int/hiv/pub/sti/who_hiv_aids_2001. 02.pdf). Syphilis has had a dramatic resurgence following recent economic reforms in China, and its prevalence has increased
* Corresponding author. Tel.: +86 551 5161002; fax: +86 551 5161016. E-mail address: [email protected] (S. Yang).
sharply in the past 5 years.3,4 Presently, syphilis is ranked as the third most prevalent notifiable infectious disease in China.5 The rapid spread of syphilis is likely due to a combination of biological and social factors. Previous studies have suggested that the spread of sexually transmitted infections is related to economic development,6 and have found a higher burden of syphilis in more developed provinces of China.3 With recent economic growth, the capital of Anhui Province, Hefei, has experienced a substantial syphilis epidemic over the past two to three decades. Understanding the dynamic pattern of syphilis is important to determine how to design effective interventions to reduce the resurgence of syphilis epidemics. In China, assessing the epidemiology of syphilis largely relies on the surveillance from case reports submitted by hospitals or clinics through a website-based case-reporting system.3 A measure that is commonly used to interrupt disease transmission is to monitor and follow the sexual partners of syphilis patients,7 because the infection is almost always transmitted through sexual contact with
1201-9712/$36.00 – see front matter ß 2013 International Society for Infectious Diseases. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijid.2013.05.007
e972
J. Gao et al. / International Journal of Infectious Diseases 17 (2013) e971–e976
an infected person.8 However, the infectiousness of syphilis remains unclear. There is little available information in the literature on this important aspect of syphilis. It is thus essential to evaluate why not all individuals are infected with syphilis after direct sexual contact with someone suffering from the disease. In this study, we assessed the epidemic trends and clinical characteristics of syphilis during the period January 1999 to September 2012. We explored the relationship between T. pallidum exposure and the incidence of syphilis infection among the sexual partners of patients. 2. Methods 2.1. Study sample This retrospective study was conducted using data from the case-based surveillance system at the STD Medical and Therapy Center of Anhui Medical University (AHMU) in Hefei, China, which is a the major medical institution that provides high-quality medical services to STD patients. Outpatient visits to this center account for the majority of total STD outpatient visits in Hefei. Since the establishment of the center, basic information has been collected (e.g., age, gender, occupation, and serological test results) for each attended syphilis patient who received outpatient followup. Syphilis cases were defined by positive diagnostic tests, a history of high-risk sexual behavior, and the characteristics of clinical manifestations.9 Each patient received a non-treponemal test (rapid plasma reagin test, RPR), followed by a treponemal test (Treponema pallidum particle agglutination test, TPPA) to confirm the diagnosis. From 2009 to 2011, an epidemiological investigation and clinical characteristic analysis of syphilis patients was performed at this center. All information was collected through face-to-face interviews that were conducted by clinical staff. The questionnaire-based survey provided socio-demographic information (including gender, year of birth, ethnicity, level of education,
reason for visit, and risk factors) and clinical information (e.g., manifestations, serologic test results, and stages of syphilis). In total, 2954 syphilis patients agreed to participate in this study and completed the questionnaire survey for the analysis of clinical characteristics. We monitored sexual partners to determine whether they had been infected after direct unprotected sexual contact with a syphilis patient. Only those cases who met the following criteria were included in the study: heterosexual early syphilis patients (ESPs), 18 years or older, who were not pregnant and did not have a connective tissue disease. Additionally, to qualify for the study, these ESPs had to report having had unprotected sex (no condom use) with their regular sexual partner before receiving treatment. The early stage of syphilis is potentially infectious and includes primary, secondary, and early latent syphilis.8,10 The primary stage is defined by a painless chancre at the site of infection. The secondary stage is characterized by a polymorphic eruption, lymphadenopathy, and other systemic manifestations. Early latent syphilis is defined as occurring within 1 year of infection and is an asymptomatic stage of syphilis.8 After obtaining written informed consent from both patients and their regular sexual partners, 535 ESPs, whose initial RPR titer ranged from 1:4 to 1:256 and who met the enrollment criteria, were recruited as index patients among the 2954 patients. All of their regular sexual partners underwent a clinical examination and serologic tests upon enrollment and at 1,3,6 months or 1 year follow-up. If a patient and his or her sexual partner tested positive for syphilis or exhibited clinical symptoms, they were diagnosed as having a concordant syphilis infection. However, if a patient’s sexual partner tested negative for syphilis and exhibited no clinical manifestation, it was considered a discordant syphilis infection. A diagram of the study design and sampling strategy is displayed in Figure 1. The study was approved by the institutional ethics committee of AHMU and was conducted in accordance with the principles of the Declaration of Helsinki.
Figure 1. Flow diagram of the study design and sampling strategy.
J. Gao et al. / International Journal of Infectious Diseases 17 (2013) e971–e976
e973
Figure 2. The increasing trends in syphilis at the STD Medical and Therapy Center of AHMU from January 1999 to September 2012. There was an upward trend in syphilis infections, as observed by the proportion syphilis-positive of the total STD clinic patients. The same increasing trend was observed for male and female cases from 1999 to 2007, but the female cases gradually increased more than the male cases from 2008 to 2012.
2.2. Statistical analysis A descriptive analysis was conducted to determine the statistical significance of differences in demographic characteristics (age, gender, and education level), clinical stage of syphilis, initial RPR titer, and HIV co-infection. Categorical variables were analyzed by Pearson Chi-square test or Fisher’s exact test. The rates of infection and clinical characteristics of patients and their sexual partners were reported as percentages. A p-value of <0.05 (twotailed) was used to determine statistical significance. All statistical analyses were conducted using SPSS version 11.0 for Windows (SPSS Inc., Chicago, IL, USA). 3. Results 3.1. Trends in the syphilis epidemic from 1999 to 2012 During the period January 1999 to September 2012, a total of 17 211 cases of syphilis infection were recorded at the STD Medical and Therapy Center. There were 8025 (46.7%) male syphilis cases and 9186 (53.3%) female syphilis cases. The proportion of syphilispositive patients increased over the course of the 14-year study, with a five-fold increase from 1999 and 2011 (6.1% and 30.0%, respectively) (Figure 2). Male and female cases increased at the same rate between 1999 and 2007. However, female cases
increased at a greater rate than male cases from 2008 to 2012 (Figure 2). With regard to age, the highest rate of syphilis infection was observed in the 20–29 years age group (Figure 3). In addition, the number of syphilis cases in the other age groups also increased over the course of the 14-year study. This finding illustrates an upward trend in syphilis infections over the years in Hefei, China, from January 1999 to September 2012. 3.2. Demographic and clinical characteristics of syphilis cases An epidemiological survey was conducted among 2954 syphilis patients (1318 male and 1636 female) from 2009 to 2011(Table 1). The median age of patients was 35.8 years, with a range from newborn to 89 years. Of the 2954 patients enrolled in the survey, 2485 (84.1%) were between 20 and 50 years old. Of the 1636 female patients, 42.9% of the syphilis cases were found among the patients who were aged 20–29 years. The number of syphilis cases among men and women in diverse age groups was significantly different (p < 0.0001). There was also a significant difference in the level of education of enrolled patients (p < 0.0001); 1624 (55%) individuals attended only junior middle school. The initial RPR titer did not differ between male and female patients (p > 0.05). Among all 2954 cases, only 17 (0.6%) were co-infected with HIV. Of the infected patients, 1955 (66.2%) were infected with latent syphilis. All data are shown in Table 1.
Figure 3. The trends in syphilis cases in different age groups. Syphilis infection increased most rapidly among patients who were aged 20–29 years. Elders (over 50 years) and patients who were less than 20 years old experienced the slowest annual increase in infection rate.
J. Gao et al. / International Journal of Infectious Diseases 17 (2013) e971–e976
e974
Table 1 Characteristics of syphilis patients who attended the STD clinics from 2009 to 2011 Characteristics
Male, n (%) (N = 1318)
Race Age, years Median Range <15 15–19 20–29 30–39 40–49 50 Education level Illiterate Primary school Junior middle school Senior/vocational high school Diploma college/university Initial RPR titer 1:4 >1:4 HIV Positive Negative Stage of syphilis Primary Secondary Tertiary Latent Neurosyphilis Syphilis with pregnancy
Chinese Han 38.9 0–89 13 (1.0%) 20 (1.5%) 357 (27.1%) 346 (26.3%) 324 (24.6%) 258 (19.6%)
Female, n (%) (N = 1636)
Total, n (%) (N = 2954)
33.3 0–83 8 (0.5%) 37 (2.3%) 702 (42.9%) 455 (27.8%) 301 (18.4%) 133 (8.1%)
35.8 0–89 21 (0.7%) 57 (1.9%) 1059 (35.8%) 801 (27.1%) 625 (21.2%) 391 (13.2%)
(5.3%) (11.7%) (41.3%) (28.8%) (13.0%)
(4.8%) (10.7%) (39.5%) (28.5%) (16.5%)
<0.0001
56 (4.2%) 125 (9.5%) 491 (37.3%) 371 (28.1%) 275 (20.9%)
86 191 675 471 213
785 (59.6%) 547 (41.5%)
941 (57.5%) 681 (41.6%)
1726 (58.4%) 1228 (41.6%)
>0.05
9 (0.7%) 1309 (99.3%)
8 (0.5%) 1628 (99.5%)
17 (0.6%) 2937 (99.4%)
>0.05
173 (13.1%) 219 (16.6%) 13 (1.0%) 897 (68.1%) 16 (1.2%) 0 (0.0%)
130 272 22 1058 4 150
303 491 35 1955 20 150
<0.0001
(7.9%) (16.6%) (1.3%) (64.7%) (0.2%) (9.2%)
142 316 1166 842 488
p-Value
(10.3%) (16.6%) (1.2%) (66.2%) (0.7%) (5.1%)
<0.0001
STD, sexually transmitted diseases; RPR, rapid plasma reagin.
3.3. The status of sexual partners after exposure The diagnoses of and test results for regular sexual partners after sexual contacts with syphilis-positive patients are shown in Table 2. Of the 535 regular sexual partners who agreed to participate in this study, 330 (61.7%) had syphilis; 205 (38.3%) were seronegative without any symptoms. Of the 330 syphilispositive partners, 164 (67.1%) female partners and 166 (58.7%) male partners had syphilis. These differences were significant at the 5% level (p = 0.008). With regard to the different stages of syphilis in the serologically positive, 37 of 61 (60.7%) partners were in the primary stage, 80 of 112 (71.4%) were in the secondary stage and 213 of 362 (58.8%) were in the early latent stage, showing no significant difference among the three groups (p = 0.056). 4. Discussion Our study demonstrated an elevated epidemic trend of syphilis in Hefei, an inland city of China, based on the surveillance data collected at the STD Medical and Therapy Center of AHMU over the past 14 years. The increase in the spread of syphilis that we found is consistent with previous findings indicating that syphilis has been increasing in most Chinese cities.3,11 According to the national
surveillance data, the reported incidence of primary and secondary syphilis was 11.7 cases per 100 000 residents in 2009 and had increased by 2.1 times since 2005.12 A higher syphilis case burden may be attributable to changing sexual attitudes, the reemergence of prostitution, large numbers of migrants who practice unsafe sexual behaviors, and an increase in homosexual men. Evidence from the epidemiological trend suggests that syphilis should be considered an important public health concern in Hefei. There is an urgent need to improve the current surveillance system and to develop comprehensive intervention strategies. We found that those individuals within the 20–29 years age group contributed to the largest increase in the proportion of the syphilis cases from 1999 to 2012. A previous study suggested that the elders (over 50 years) may have a higher risk of STD acquisition.13 An increased proportion of elders who had syphilis in our study and national epidemiological data14 support this finding. The increasing trend among elders could be attributed to the burgeoning aging population in China, the development of medicines for erectile dysfunction, and the greater chance of sexual contact with female sex workers (FSWs).6 However, the reason why there has been a relatively smaller increase in infected individuals aged less than 20 years might be due to the low percentage of adolescents visiting the STD clinics.
Table 2 The ratio of infected vs. uninfected with syphilis in sexual partners Characteristic
Index patients
Concordant, infected Sexual partners
Discordant, uninfected Sexual partners
Number Sex Female Male Stage of syphilis Primary Secondary Early latent
535
330 (61.7%)
205 (38.3%)
242 male index cases 293 female index cases
164 (67.1%) (female) 166 (58.7%) (male)
78 (32.9%) (female) 127 (41.3%) (male)
0.008
61 112 362
37 (60.7%) 80 (71.4%) 213 (58.8%)
24 (39.3%) 32 (28.6%) 149 (41.2%)
0.056
p-Value
J. Gao et al. / International Journal of Infectious Diseases 17 (2013) e971–e976
In addition, a large proportion (55%) of syphilis patients only had high school educational attainment, which may explain their lack of awareness of how to protect themselves from risky sexual behaviors. The lack of basic medical knowledge may have influenced syphilis transmission and resulted in a substantial number of syphilis patients diagnosed at a latent stage. A higher syphilis prevalence was observed in female patients than in male patients. For example, within the 20–29 years age group, the percentage of female cases was higher than male cases. This finding may be due to an increase in the number of FSWs in China, which could explain the resurgence of syphilis.15 However, previous studies have found that gender could influence disease rates.16 In our study, an analysis of infection after exposure to syphilis revealed that the infection rate of female partners differed from the infection rate of male partners. This finding suggests that females are more susceptible to syphilis infection than males, which may lead to higher infection among females. In this study, the percentage (5.7%) of syphilis–HIV co-infection was lower than the percentage found in other studies of syphilis–HIV co-infection in China.17 However, due to increasing rates of HIV infection among syphilis patients, routine periodic screening among highrisk groups is strongly recommended. It is critical for all patients with a new diagnosis of syphilis to undergo HIV testing.18 Considering the results of this study, 61.7% of regular sexual partners could be infected if they have unprotected sex with an infectious syphilis patient. These data suggest that there is a high transmission rate in syphilis contacts. Therefore, it is imperative to take measures to prevent syphilis infection. The early detection and treatment of syphilis cases are essential parts of syphilis control strategies and can interrupt the transmission chain. Contacting sexual partners, facilitated through partner notification services, is an effective measure to help to control communicable diseases. Intriguingly, a proportion (38.3%) of sexual partners remained persistently seronegative during the study. The phenomenon whereby not everyone exposed to an infectious syphilis case is equally susceptible to infection is difficult to explain. Early studies of syphilis infection among European heterosexual and homosexual partners found that only approximately 50% to 60% of those at risk developed syphilis.19 If all people were equally susceptible to infection, then statistically, these seronegative partners would be infected with syphilis. Differences in risk factors such as safer sexual behaviors and the forms and frequency of sexual contact may explain this phenomenon. The incidence of other sexually transmitted infections, such as syphilis–HIV co-infection, could result in cases with a negative syphilis serologic test and no apparent symptoms.20 However, in this study each pair of patients and their sexual partners admitted that they had had unprotected sexual contacts or rarely used condoms. In seronegative sexual partners, none of them was infected with HIV during the follow-up period. This may not be a chance phenomenon; why were a few partners uninfected after direct sexual contacts? A possible explanation is that the seronegative sexual partners could be resistant to syphilis infection due to the existence of syphilisresistant genetic factors. It is also possible that a natural immune response mediates a potentially syphilis-resistant mechanism. People infected with T. pallidum in the early stage of syphilis could develop a Th1 predominant local cellular response to activate macrophages to phagocytose T. pallidum and resolve the lesion.21,22 The major proteins of T. pallidum could induce T cells to respond to syphilis infection with elevated reactivity.23 During the early infection, T. pallidum could elicit a vigorous inflammation to trigger an adaptive immune response.24 A typical cell-mediated immune response could result in the local clearance and resolution of the primary lesions.21,24 The major mechanism of T. pallidum clearance responses is T cell infiltration into lesions to activate
e975
macrophages to phagocytose opsonized T. pallidum.25 Different syphilis infection stages suggest that the ability of the immune response to clear T. pallidum is different among the individuals after exposure to T. pallidum. We speculated that a heterogeneity and variability in susceptibility to syphilis infection might exist in the general population, similar to HIV-1 infection.26 A characteristic feature of many human infections is that only a proportion of exposed individuals develop clinical disease. Heritable factors have long been considered to play an important role in interpreting individual variations in susceptibility.27 Recent genome-wide association studies have identified several genetic factors/susceptibility loci that confer risk to infectious diseases such as HIV,28 malaria,29 and leprosy.30 These studies suggest that genetic variation among hosts plays a key role in determining an individual’s susceptibility and resistance to infectious disease. Genetic studies of other infectious diseases, in turn, have highlighted the hypothesis that some naturally unsusceptible or resistant genetic factors exist for syphilis. Confirmation of the mechanisms that determine why some people who are exposed to T. pallidum do not develop disease is needed to understand protective immunity against T. pallidum. In summary, our study revealed an increasing epidemic trend of syphilis in Hefei, China, and that this trend may continue in the future. It is imperative to adopt effective measures to control and prevent this disease. Further studies are needed to understand the mechanism of syphilis-specific immune responses in highly exposed subjects who are seronegative, which will provide new insights into disease mechanisms and will ultimately aid in controlling syphilis epidemics. Acknowledgements The authors would like to acknowledge the patients and their sexual partners who participated in this project. Funding sources: Supported by funding from the Ministry of Education, China (IRT-1046). Ethical approval: The study was approved by the institutional ethics committee of AHMU and was conducted in accordance with the principles of the Declaration of Helsinki. Conflict of interest: No conflict of interest to declare.
References 1. Ho EL, Lukehart SA. Syphilis: using modern approaches to understand an old disease. J Clin Invest 2011;121:4584–92. 2. Lafond RE, Lukehart SA. Biological basis for syphilis. Clin Microbiol Rev 2006;19:29–49. 3. Chen ZQ, Zhang GC, Gong XD, Lin C, Gao X, Liang GJ, et al. Syphilis in China: results of a national surveillance programme. Lancet 2007;369:132–8. 4. Tucker JD, Yin YP, Wang B, Chen XS, Cohen MS. An expanding syphilis epidemic in China: epidemiology, behavioural risk and control strategies with a focus on low-tier female sex workers and men who have sex with men. Sex Transm Infect 2011;87(Suppl 2):ii16–8. 5. Chen XS, Wang QQ, Yin YP, Liang GJ, Jiang N, Yang LG, et al. Prevalence of syphilis infection in different tiers of female sex workers in China: implications for surveillance and interventions. BMC Infect Dis 2012;12:84. 6. Yang LG, Tucker JD, Yang B, Shen SY, Sun XF, Chen YF, et al. Primary syphilis cases in Guangdong Province 1995-2008: opportunities for linking syphilis control and regional development. BMC Public Health 2010;10:793. 7. Centers for Disease Control and Prevention. Recommendations for partner services programs for HIV infection, syphilis, gonorrhea, and chlamydial infection. MMWR Recomm Rep 2008; 57:1-83; quiz CE1-4. 8. Singh AE, Romanowski B. Syphilis: review with emphasis on clinical, epidemiologic, and some biologic features. Clin Microbiol Rev 1999;12:187–209. 9. Ratnam S. The laboratory diagnosis of syphilis. Can J Infect Dis Med Microbiol 2005;16:45–51. 10. Dourmishev LA, Dourmishev AL. Syphilis: uncommon presentations in adults. Clin Dermatol 2005;23:555–64. 11. Chen XS, Yin YP, Tucker JD, Gao X, Cheng F, Wang TF, et al. Detection of acute and established HIV infections in sexually transmitted disease clinics in Guangxi, China: implications for screening and prevention of HIV infection. J Infect Dis 2007;196:1654–61.
e976
J. Gao et al. / International Journal of Infectious Diseases 17 (2013) e971–e976
12. Chen XS, Peeling RW, Yin YP, Mabey DC. The epidemic of sexually transmitted infections in China: implications for control and future perspectives. BMC Med 2011;9:111. 13. Schmid GP, Williams BG, Garcia-Calleja JM, Miller C, Segar E, Southworth M, et al. The unexplored story of HIV and ageing. Bull World Health Organ 2009;87. 162–162A. 14. Pearline RV, Tucker JD, Yuan LF, Bu J, Yin YP, Chen XS, et al. Sexually transmitted infections among individuals over fifty years of age in China. AIDS Patient Care STDS 2010;24:345–7. 15. Tucker JD, Chen XS, Peeling RW. Syphilis and social upheaval in China. N Engl J Med 2010;362:1658–61. 16. Wong SP, Yin YP, Gao X, Wei WH, Shi MQ, Huang PY, et al. Risk of syphilis in STI clinic patients: a cross-sectional study of 11,500 cases in Guangxi, China. Sex Transm Infect 2007;83:351–6. 17. Chow EP, Wilson DP, Zhang L. HIV and syphilis co-infection increasing among men who have sex with men in China: a systematic review and meta-analysis. PLoS One 2011;6:e22768. 18. Workowski KA, Berman SM. Centers for Disease Control and Prevention. Sexually transmitted diseases treatment guidelines, 2006. MMWR Recomm Rep 2006;55:1–94. 19. Schober PC, Gabriel G, White P, Felton WF, Thin RN. How infectious is syphilis? Br J Vener Dis 1983;59:217–9. 20. Blum L, Bachmeyer C, Caumes E. Seronegative secondary syphilis in an HIVinfected patient. Clin Exp Dermatol 2005;30:158–9. 21. Leader BT, Godornes C, VanVoorhis WC, Lukehart SA. CD4+ lymphocytes and gamma interferon predominate in local immune responses in early experimental syphilis. Infect Immun 2007;75:3021–6.
22. Van Voorhis WC, Barrett LK, Koelle DM, Nasio JM, Plummer FA, Lukehart SA. Primary and secondary syphilis lesions contain mRNA for Th1 cytokines. J Infect Dis 1996;173:491–5. 23. Arroll TW, Centurion-Lara A, Lukehart SA, Van Voorhis WC. T-cell responses to Treponema pallidum subsp. pallidum antigens during the course of experimental syphilis infection. Infect Immun 1999;67:4757–63. 24. Babolin C, Amedei A, Ozolins D, Zilevica A, D’Elios MM, de Bernard M. TpF1 from Treponema pallidum activates inflammasome and promotes the development of regulatory T cells. J Immunol 2011;187:1377–84. 25. Morgan CA, Molini BJ, Lukehart SA, Van Voorhis WC. Segregation of B and T cell epitopes of Treponema pallidum repeat protein K to variable and conserved regions during experimental syphilis infection. J Immunol 2002; 169:952–7. 26. Fowke KR, Nagelkerke NJ, Kimani J, Simonsen JN, Anzala AO, Bwayo JJ, et al. Resistance to HIV-1 infection among persistently seronegative prostitutes in Nairobi. Kenya Lancet 1996;348:1347–51. 27. Chapman SJ, Hill AV. Human genetic susceptibility to infectious disease. Nat Rev Genet 2012;13:175–88. 28. Fellay J, Shianna KV, Ge D, Colombo S, Ledergerber B, Weale M, et al. A wholegenome association study of major determinants for host control of HIV-1. Science 2007;317:944–7. 29. Timmann C, Thye T, Vens M, Evans J, May J, Ehmen C, et al. Genome-wide association study indicates two novel resistance loci for severe malaria. Nature 2012;489:443–6. 30. Zhang FR, Huang W, Chen SM, Sun LD, Liu H, Li Y, et al. Genomewide association study of leprosy. N Engl J Med 2009;361:2609–18.