Epidemiology of Treponema pallidum, Chlamydia trachomatis, Neisseria gonorrhoeae and Trichomonas vaginalis among forcibly displaced populations: a global systematic review and meta

Discussion This systematic review and meta-analysis presents, to our knowledge, the first comprehensive assessment of the epidemiology of four STIs—syphilis, C. trachomatis, N. gonorrhoeae and T. vaginalis—among forcibly displaced populations worldwide. Syphilis was the most frequently studied infection, with findings revealing a considerable burden and marked regional variation. Data on the other infections were more limited, underscoring persistent gaps in surveillance and research. The pooled mean prevalence of probable current syphilis infection was 0.94%, comparable to a global meta-analysis estimate of 1.11% (95% CI 0.99 to 1.22) in the general population,60 but higher than the WHO 2020 global estimate of the population at large at 0.6% (95% CI 0.5% to 0.7%).2 Methodological differences may explain some variation between these estimates; however, the pooled prevalence suggests active transmission within forcibly displaced populations. This may reflect high-risk sexual exposures in the context of displacement-related social, legal and economic vulnerabilities but could also be driven by the ongoing spread of untreated, often asymptomatic infections due to limited access to STI screening and treatment.26 30 Global shortages in benzathine penicillin may have further exacerbated this issue in recent years.61 Notably, syphilis prevalence was found to be declining over time at an annual rate of 6%. This decline may reflect global improvements in antenatal syphilis screening and treatment,62 as well as the routine inclusion of syphilis testing—alongside HIV screening—in health programmes for refugees and other displaced populations in several high-income host countries, from which the majority of data are derived.14 15 Still, the lack of universal STI screening practices, particularly within resource-limited settings where most displaced populations are resettled, constrains the interpretation of these observed trends. Calendar time, country of origin and host country collectively explained 60% of the variation in syphilis prevalence. Displaced individuals from the WHO African region had a significantly higher prevalence, likely reflecting the region’s background syphilis prevalence, which is the highest globally at 1.7% in 2020.2 This may also be partly due to cross-reactivity with antibodies against non-venereal treponematoses—such as yaws, bejel and pinta—endemic in several African countries.63 Higher syphilis prevalence has similarly been reported among economic migrants from the African region.30 In parallel, we found that refugees settled in low-income countries had significantly higher syphilis prevalence than those in high-income countries, reflecting heightened vulnerability to poor sexual health in these settings. As forcibly displaced individuals resettle, their risk of syphilis infection becomes shaped by various host-country factors, particularly in protracted displacement situations. These include local syphilis prevalence, sexual network dynamics, structural vulnerabilities and limited access to sexual health services.64 The pooled prevalence of C. trachomatis (0.87%) and N. gonorrhoeae (0.14%) was, on the other hand, substantially lower than the WHO 2020 global estimates of 3.2% (95% CI 2.7 to 3.9%) and 0.7% (95% CI 0.5 to 1.1%), respectively.2 Interpreting these lower prevalence estimates within the context of high vulnerability and displacement is challenging. Similar patterns have been observed for HIV, in which a lower HIV prevalence was paradoxically found in conflict-affected populations in some Sub-Saharan African countries, despite the well-documented increase in the social and structural determinants of STI transmission and severity in humanitarian settings.65 This may be due to social isolation and a decline in mobility and urbanisation during periods of war.64–66 However, such conditions may not be applicable to all settings, such as in the Eastern Mediterranean region, where millions of displaced people have resettled in urban settings within neighbouring countries over the years.9 67 68 Several other factors may contribute to the observed low prevalence of C. trachomatis and N. gonorrhoeae. Most studies reporting on these infections were conducted in high-income host countries, where sexual health services are more accessible. In LMICs where antimicrobial stewardship is often inadequate, the widespread overuse of oral antibiotics may have contributed to the lower infection burden observed.4 69 Additionally, C. trachomatis and N. gonorrhoeae prevalence tend to be highest among younger individuals aged 15–24 years,70–72 whereas the subset of studies that reported participants’ age indicated a slightly older population (average of 26 years, data not shown). Despite the relatively large number of identified studies reporting over 1.1 million test results, significant knowledge gaps remain regarding the burden of curable STIs among forcibly displaced populations. Most data were on syphilis, allowing for more robust prevalence estimates, while evidence for N. gonorrhoeae, C. trachomatis and T. vaginalis was limited. For C. trachomatis and N. gonorrhoeae, the lack of widely available rapid tests and reliance on syndromic management in many low-resource and humanitarian settings likely contributed to the scarcity of prevalence data. Although some point-of-care options exist for T. vaginalis, they are not widely deployed in humanitarian programmes, and the infection is often overlooked. Only three studies reported on T. vaginalis prevalence, precluding a reliable pooled estimate. As such, while our conclusions are most reliable for syphilis, the limited data on the other infections suggest a variable and possibly underestimated burden, pointing to the urgent need to strengthen diagnostic capacity and generate more representative data for these infections. Most data were collected over the past 15 years, aligning with the rapid increase in forcibly displaced populations in recent decades. However, the regional distribution of available prevalence measures does not reflect global displacement patterns, and as a result, our findings may not be representative of broader forcibly displaced populations. For instance, only 21.6% of prevalence measures were determined in LMICs, despite these countries hosting 75% of the world’s displaced population.4 Refugees from the Eastern Mediterranean region were also under-represented, although three out of the six countries from which the majority of refugees originate—Afghanistan, Syria and Sudan—are situated in this region.4 Notably, only seven studies reported on Syrian refugees and three on Afghani refugees, the two largest refugee populations globally.4 Syrian refugees in particular have been living in protracted displacement for over a decade. Although the number of studies identified in the systematic review increased over time, this population was not captured in very recent literature, underscoring a critical gap in available data. Despite comprising the vast majority of forcibly displaced populations globally,4 IDPs are also severely under-represented in available data, accounting for less than 2% of study populations in this review. Women and adolescents are disproportionally impacted by internal displacement,4 often residing in complex humanitarian settings where ongoing conflict and heightened risk of sexual violence73 74 may drive STI transmission, while simultaneously limiting access to healthcare.75 In contrast, most studies included in this review were conducted in high-income European settings where female refugees constitute a minority of the displaced population,76 despite women comprising approximately half of refugees and asylum seekers globally.4 As a result, the female refugees examined in these studies may not be representative of the broader female refugee population. While women in forced displacement settings face increased STI-related vulnerabilities,77 they are also more likely than men to seek healthcare, particularly antenatal care,78 which presents opportunities for STI detection and treatment. This is especially relevant in high-income host countries with potentially better access to sexual and reproductive health services. Still, our findings underscore the need to further study sex- and gender-based differences in STI transmission and risk factors among forcibly displaced populations, as these patterns may vary across sociocultural contexts and economic regions of resettlement. Additionally, as the health needs of IDPs remain largely understudied and poorly characterised,79 more research is urgently needed among conflict-affected IDPs to better estimate the prevalence and determinants of curable STIs in these settings. Finally, our review did not identify any studies reporting STI prevalence among forcibly displaced individuals who are men who have sex with men, sex workers or other key populations. Given the disproportionate burden of STIs among these groups, this represents a significant gap in existing research and overlooks the intersecting vulnerabilities of forced displacement and key population status. Future studies should prioritise the inclusion of these populations to provide a more comprehensive understanding of STI epidemiology in humanitarian settings. Reliable estimation of STI burden among forcibly displaced populations is hindered by a lack of standardised surveillance systems and comprehensive, population-based data collection in both LMICs and high-income host countries. Routine programmatic data contain significant gaps due to the transient and often undocumented status of these populations, making continuous monitoring and follow-up difficult. Exclusionary policies in host countries, including restrictions on healthcare access for undocumented individuals, further limit the availability, reliability and representativeness of surveillance data. In LMICs, where 75% of the global refugee population and the overwhelming majority of IDPs are settled,4 syndromic management is often the standard of care due to resource constraints. Syndromic management, a systematic, symptoms-based approach to diagnosing and treating suspected STIs,80 fails to capture asymptomatic cases, which represent the majority of curable STIs.81 This approach can lead to both the undertreatment of infected individuals and the overtreatment of persons with symptoms due to alternate conditions, the latter of which has implications for antimicrobial resistance.82 In high-income countries, where diagnostic and surveillance infrastructures are comparatively robust,83 refugees and asylum seekers also face barriers to accessing sexual health services. These include prohibitive costs, perceived discrimination, language and communication challenges with providers, limited awareness of services, poor health literacy and stigma.84–86 Left untreated, curable STIs can result in significant morbidity, including pelvic inflammatory disease, infertility, perinatal complications and increased risk of HIV transmission.87–89 Forcibly displaced persons may be at risk of these sequelae, though data are sparse. In addition to gaps in the availability and scope of data, this study has several limitations. First, substantial heterogeneity was observed across studies in terms of population characteristics, sampling methods, diagnostic modalities, and study settings. While the meta-regression analysis accounted for 64% of the variability in syphilis prevalence, a considerable proportion of the heterogeneity remains unexplained. The relatively small number of studies that reported on C. trachomatis, N. gonorrhoeae, and especially T. vaginalis limited our ability to explore sources of heterogeneity for these infections. Second, the data span over four decades, making it challenging to distinguish true epidemiological changes from shifts in diagnostic technologies, STI treatment policies or surveillance systems. Older data may also not accurately reflect current STI risk patterns among forcibly displaced populations, given evolving migration pathways and geopolitical contexts. Another limitation is the wide array of diagnostics used for STI ascertainment, which vary in sensitivity and specificity. While we stratified our analysis into broad testing diagnostic categories reflecting current versus lifetime infections, we could not adjust for potential biases associated with individual tests within each category. Also, for syphilis specifically, the absence of non-treponemal antibody titres in most studies may have led to misclassification of past adequately treated infections as probable current infections.90 The potential biases in syphilis diagnostic interpretation are summarised in online supplemental table 2 and discussed in detail in a prior publication.37 A strong small-study effect was observed and confirmed by the analysis that demonstrated the presence of publication bias, with studies testing at least 500 participants reporting a 49% lower syphilis prevalence. However, over half of the studies included in the analysis had large sample sizes (online supplemental table 8), enhancing the precision of the estimates. Finally, most studies relied on convenience sampling from health facility attendees or immigration reception sites, potentially under-representing individuals without healthcare access or undocumented populations. In some settings, the latter often constitute a large proportion of forcibly displaced populations. For example, in Lebanon, while 755 426 Syrian refugees are registered with the United Nations, government estimates suggest a total of approximately 1.5 million.91 92 This study has several strengths. It provides the first comprehensive assessment of curable STI epidemiology among forcibly displaced populations worldwide, addressing a major gap in the existing literature. By synthesising data from over 1.1 million test results, it generates robust pooled prevalence estimates, particularly for syphilis. The use of meta-regression analysis further strengthens the analysis by identifying key sources of heterogeneity and offering insights into regional, temporal and socioeconomic influences on STI prevalence in humanitarian settings. Despite limitations in data availability and scope, this study enhances epidemiological understanding of STIs in this population and establishes a foundation for future research and targeted public health interventions in displacement contexts.