Although Brazil is classified as an upper-middle-income economy, the northern region is the poorest in the country and the economy ranges from low to lower-middle income in most cities, including those where this study took place. This study is one of the first epidemiological reports on syphilis in FSWs in the Brazilian Amazon region. In this region, the Amazon rainforest occupies large areas of the northern Brazil, which causes relative isolation, resulting (or because of) limited infrastructure and public services and low human development . This scenario has a negative impact on sexual behaviour, increasing risk-taking activities [12,13,14]. Additionally, gender empowerment is an issue on the population of such regions, which are still male-centred, and women are marginalized in a social context. Subsistence is much harder for women, which can be pushed towards sex work and must succumb to male will.
In fact, acquisition and transmission of T. pallidum are related to social, economic, cultural and behavioural factors that influence the occurrence of syphilis in the population, especially among FSWs . The high prevalence of syphilis among FSWs has been reported in low-income countries, such as the Dominican Republic (5.1–11.1%), Honduras (1.3–6.0%), Guatemala (1.1–11.8%), El Salvador (2.7–15.0%) and Peru (2.2–4.1%) .
The prevalence of syphilis was high (14.1%), which was 65% higher than a recent study that evaluated a similar population of FSWs from 12 Brazilian cities (8.5%) , and 3.5 higher than the prevalence found in the city of Botucatu (4%), southeast Brazil  using rapid qualitative test and ELISA. These results indicate the disparity on syphilis prevalence among FSWs in Brazil, as well as the need for syphilis control and preventive measures in marginalized populations, such as the observed in this Amazon region and other populations worldwide. This data reinforces the need for specific approaches in order to control syphilis spread in these populations.
Unfavourable socioeconomic conditions, and consequently low living standards, may favour initiation and involvement in sex work, causing a negative influence self-care and vulnerability to STIs, such as syphilis [15, 18, 19]. Recently, unprotected sex has been linked to as an advantage to get higher payment on sex work reported by FSWs, which was associated to high rates of HIV and HBV infections [12, 14].
This study has shown that low educational level and practice of anal sex are independent risk factors for syphilis infection. Schooling is one of the most important variables to measure the socio-economic status and its effects on the health status of a population. We found that FSWs who had attended up to the primary school showed a significantly higher prevalence of syphilis positivity. This may reflect the low overall awareness of these individuals about the risks involved and sex work and measures to prevent STIs [20, 21].
Having anal sex with high frequency (> 50% of sexual encounters) during the last year was another factor that increased the prevalence of syphilis infection. Anal intercourse increases the risk of STIs because of the characteristic of the rectum, which shows a delicate tissue that can be easily damaged, which is aggravated by the limited lubrification, giving access to bacteria and virus to the bloodstream [22,23,24]. Studies have shown that anal intercourse carries one of the highest-risk of sexual activity for getting HIV. Our results suggest that anal intercourse not only increases the risk of HIV acquisition but also from T. pallidum, and the same infection mechanism can be applied to men who have sex with men and heterosexuals [14, 23, 24].
Although several epidemiological studies have indicated anal sex as a risk factor for acquisition and transmission of T. pallidum among men who have sex with men [25,26,27], low evidence is found among heterosexual couples . Indeed, this is one of the first investigations to report an association of anal sex between men and women and syphilis infection. This result indicates that not only the use of condoms is important to the spread of STIs, but also the type of sexual practice among heterosexual couples. Thus, more studies are required to clarify the risk of anal intercourse on syphilis transmission in heterosexual sex that was found on this investigation.
Infectious transmissible diseases are a threat not only for the more vulnerable marginalised populations but can spread to an entire society, especially when associated with sexual activities. Controlling the infection in populations at risk also helps to control the spread of diseases in the entire population. Thus, public health institutions should perform several actions with key populations such as (i) educational program to present and discuss the risks involved in sexual practices; (ii) offering rapid syphilis tests and possibilities of self-testing (iii); stimulating public awareness about syphilis transmission and disease signs and symptoms (iv); continually offer treatment and reinforces treatment adherence; (v) conduct regular vaccination campaigns against other associated pathogens such as HBV and HPV; (vi) regularly offer methods for sexual protection, such as condoms and lubricants for anal sex.
Sex work is usually a matter of need, not of choice. So, governments and other human organizations can help risk groups, such as FSWs, by creating ways to promote self-esteem, self-confidence, and stimulate awareness of social rights and benefits, considering their vulnerability that place them in a group of risk, and also alerting them how they can be a key player in disease spread/control [13, 14]. As an example, a comprehensive health program was planned and implemented with FSW in southern China, resulting in several public health benefits, including in the prevention and control of syphilis .
This study has some limitations. Firstly, limited sample size and restriction to three cities, which makes the results not necessarily representative of the FSW population of the state of Pará, Brazil. Another limitation is that the diagnosis was exclusively based on serological tests, no clinical examinations were performed in order to check signs of active disease. Also, recently acquired infections can present a small concentration of antigens and antibodies that aren’t detected yet by either VDRL and ELISA and therefore may have been diagnosed as negative. In addition, although convenience sampling has been found to be adequate for quasi-representative sampling in hidden populations, other sampling methods could have been used to improve representativeness. Finally, bias is always a risk in data from self-reported forms and the cross-sectional design of this study limits its capacity to establish causality.