Behind Bars and at Risk: What Drives Syphilis Among Newly Admitted Prisoners

Syphilis, a bacterial infection caused by Treponema pallidum, continues to pose a serious global public health challenge, and few settings illustrate this challenge more clearly than prisons. People entering correctional facilities often carry a heavy burden of infectious disease, shaped by years of limited healthcare access, high-risk sexual behavior, and substance use. A new case–control study from Thailand has now identified precisely which factors are most strongly associated with syphilis infection among newly admitted prisoners, offering correctional systems a data-driven roadmap for targeted screening and prevention. The findings, published in PLOS One, arrive at a moment when public health experts are increasingly calling for prisons to be treated not as isolated institutions but as critical nodes in the broader network of community health.

The research team, led by Thanaset Hiranbandit and Wutthichai Jariya, conducted their investigation across three correctional facilities in a province of Thailand. Rather than enrolling every incoming prisoner, the investigators used a case–control design, a method particularly well suited to studying relatively uncommon outcomes such as syphilis infection. Cases were newly admitted prisoners who tested positive for syphilis during routine screening at prison entry, while controls were newly admitted prisoners who tested negative. Both groups were randomly selected from individuals who had undergone syphilis screening between April 1, 2025, and September 30, 2025, a six-month window that captured a representative cross-section of the incoming prison population.

Data collection relied on validated instruments, including case record forms and structured questionnaires, ensuring that the information gathered was consistent and comparable across participants. The researchers then applied multivariable logistic regression, a statistical technique that allows investigators to estimate the independent effect of each risk factor while controlling for the influence of all the others. This approach is essential in observational studies of this kind, because many risk behaviors cluster together; without statistical adjustment, it would be impossible to determine whether an association reflects a genuine independent effect or merely the overlap of correlated exposures. Statistical significance was set at a p-value below 0.05, the conventional threshold in epidemiological research.

The final analysis included 51 cases and 204 controls, a four-to-one ratio that maximizes statistical power in case–control designs. The results were striking. Five factors emerged as independently and significantly associated with syphilis infection among newly admitted prisoners. Homosexual or bisexual orientation carried an adjusted odds ratio of 9.59, meaning that individuals with this orientation had nearly ten times the odds of syphilis infection compared with heterosexual participants after controlling for other variables. A history of sexually transmitted infections proved even more predictive, with an adjusted odds ratio of 38.03, indicating that prior STI diagnosis was one of the strongest markers of current syphilis infection in this population.

Behavioral factors related to sexual practice also weighed heavily in the analysis. Anal intercourse was associated with an adjusted odds ratio of 4.47, while sexual intercourse without condom use showed an adjusted odds ratio of 13.82. These findings align with well-established understanding of syphilis transmission, which occurs through direct contact with syphilitic lesions during vaginal, anal, or oral sex. Condoms, when used consistently and correctly, substantially reduce but do not eliminate transmission risk, and anal intercourse carries particular anatomical vulnerability because the rectal mucosa is more susceptible to microabrasions through which the bacterium can enter the bloodstream.

Perhaps the most dramatic association in the study involved injection drug use, which carried an adjusted odds ratio of 45.60, with a confidence interval ranging from 2.88 to 722.12. While the wide confidence interval reflects the relatively small number of participants reporting this behavior, the magnitude of the association underscores the profound overlap between substance use and infectious disease risk in carceral settings. Injection drug use is a recognized driver of bloodborne and sexually transmitted infections worldwide, both through direct needle sharing and through the social and economic circumstances that often accompany dependence, including transactional sex and inconsistent condom use.

The biological logic behind these associations is worth unpacking. Syphilis progresses in distinct stages, beginning with a painless ulcer known as a chancre at the site of infection, which may go unnoticed, particularly in rectal or vaginal locations. Untreated, the infection can progress to secondary and latent stages and, in a minority of cases, to devastating tertiary disease affecting the heart, brain, and nervous system. Because early syphilis can be asymptomatic, individuals may harbor the infection unknowingly for months or years, making screening the only reliable way to detect and treat it in populations with elevated prevalence. A prior history of any sexually transmitted infection serves as a powerful proxy for sustained exposure to the conditions that facilitate syphilis transmission, which explains why it dominated the statistical analysis.

The study’s conclusions carry direct operational significance for correctional health systems. The researchers argue that prison entry represents a critical opportunity for syphilis screening, a moment when a captive and accessible population can be systematically evaluated and treated. Rather than screening every incoming prisoner uniformly, the authors advocate for risk-based screening strategies that prioritize individuals with high-risk sexual behaviors and substance use. Such targeted approaches could concentrate resources where they are most needed, reducing syphilis transmission both within correctional facilities and, by extension, in the communities to which prisoners eventually return. This is particularly important given that most prisoners serve finite sentences, and untreated infections acquired before or during incarceration can seed further transmission after release.

From a broader public health perspective, the Thai findings resonate with global concerns about syphilis resurgence. Many countries have witnessed rising syphilis rates in recent years, particularly among men who have sex with men and among populations with limited healthcare access. Prisons concentrate many of the structural drivers of infectious disease, including overcrowding, interrupted continuity of care, and the social marginalization that shapes risk behavior long before admission. Treating correctional entry screening as a strategic public health intervention, rather than a routine administrative procedure, could yield substantial returns in infection control. The study’s identification of injection drug use and prior STI history as the two strongest predictors offers correctional systems a practical triage framework: incoming prisoners reporting these factors warrant immediate and thorough evaluation, while those reporting condomless sex or anal intercourse should also be flagged for enhanced screening and preventive counseling.

As syphilis continues to circulate globally, studies like this one demonstrate how rigorous epidemiological methods can translate into actionable prevention strategies for vulnerable populations. The Thai case–control study provides correctional facilities with clear, quantified evidence of which factors matter most, and it reinforces a growing consensus that prisons must be integrated into national infectious disease control efforts rather than treated as separate from them. With targeted screening at admission, prompt treatment of detected infections, and linkage to care upon release, correctional health systems have a genuine opportunity to interrupt syphilis transmission chains that extend far beyond prison walls.

Subject of Research: Syphilis risk factors among newly admitted prisoners in Thailand

Article Title: Risk factors for syphilis among newly admitted prisoners: A case–control study

Article References: Hiranbandit, T., & Jariya, W. (2026). Risk factors for syphilis among newly admitted prisoners: A case–control study. PLOS One, 21(10), e0360956. https://doi.org/10.1371/journal.pone.0360956

Image Credits: AI Generated

DOI: 10.1371/journal.pone.0360956

Keywords: syphilis, prisoners, case-control study, Thailand, sexually transmitted infections, injection drug use, prison health, public health, epidemiology, infectious disease, Risk, factors