Global epidemiology of Trichomonas vaginalis Danielle N Poole, 1 R Scott McClelland 2 ▸ Additional material is published online only. To view please visit the journal online (http://dx.doi.org/10.1136/ sextrans-2013-051075). 1 Department of Medicine, University of Washington, Seattle, Washington, USA 2 Departments of Medicine, Epidemiology, and Global Health, University of Washington, Seattle, Washington, USA Correspondence to Dr R Scott McClelland, Departments of Medicine, Epidemiology, and Global Health, University of Washington, Box 359909, 325 9th Avenue, Seattle, WA 98104, USA; mcclell@uw.edu Received 31 January 2013 Revised 6 May 2013 Accepted 8 May 2013 Published Online First 6 June 2013 To cite: Poole DN, McClelland RS. Sex Transm Infect 2013;89:418–422. ABSTRACT Despite having the highest prevalence of any sexually transmitted infection (STI) globally, there is a dearth of data describing Trichomonas vaginalis (TV) incidence and prevalence in the general population. The lack of basic epidemiological data is an obstacle to addressing the epidemic. Once considered a nuisance infection, the morbidities associated with TV have been increasingly recognised over the past decade, highlighting the importance of this pathogen as a public health problem. Recent developments in TV diagnostics and molecular biology have improved our understanding of TV epidemiology. Improved characterisation of the natural history of TV infection has allowed us to hypothesise possible explanations for observed variations in TV prevalence with age. Direct and indirect hormonal effects on the female genital tract provide a likely explanation for the greater burden of persistent TV infection among women compared with men. Further characterisation of the global epidemiology of TV could enhance our ability to respond to the TV epidemic. INTRODUCTION Trichomonas vaginalis (TV) is the most prevalent curable sexually transmitted infection (STI) glo- bally. 1 A number of studies have highlighted the fact that at least 80% of TV infections are asymp- tomatic. 23 However, even asymptomatic infections are a public health concern. In addition to the risk of transmission to sex partners, TV infection has been associated with as much as a 2.7-fold increase in the risk of HIV acquisition, 4–6 a 1.3-fold increase in the risk of preterm labour, and a 4.7-fold increase in the risk of pelvic inflammatory disea- se. w1 w2 (superscript “w”s are references to the WebLink references in the online supplementary document). In this review, we summarise current knowledge of the global epidemiology of TV infection. Additionally, we highlight recent and interesting advances in our understanding of the epidemio- logical correlates of TV infections. These include sex differences in the incidence and prevalence of infection, and the potentially important role of female sex hormones, and the menstrual cycle in mediating TV susceptibility and natural history. WHO has estimated that over half the 248 million new TV infections each year occur in men. 1 By contrast, 89% of prevalent TV cases are found among women. 1 Biological differences between the sexes contribute to these striking differences in the incidence and prevalence of TV infection between men and women. Recent innovations in detection, including the availability of nucleic acid amplifica- tion tests (NAATs), have improved our understand- ing of the natural history of TV infections. These advances in our understanding of TV infection have been particularly notable in men, as sensitivity of detection by wet mount is so poor in men that it is not used, while culture detection in urethral samples yields variable sensitivity. w3 w5 7 Enhanced detection of TV infection in men has facilitated the investigation of different biological mechanisms influencing persistence versus clearance of infection between the sexes. Greater availability of iron in the female genital tract due to menstrual bleeding may contribute to sex-dependent epidemio- logical patterns of TV infection. 8 One study suggests that the TV genome has adapted to existing in the setting of cyclic variation in iron availability, 9 such as that present during menstrual cycles. Additionally, oestrogen has been identified as an important deter- minant of the natural history of TV infection. w5 Recent studies add depth to our understanding of the potential role of female hormones, including both physiological hormonal cycles and hormonal contra- ceptives, in TV infection. SUMMARY OF GLOBAL ESTIMATES OF TV PREVALENCE AND INCIDENCE At the time of this publication, TV is not a report- able infection in any country. As such, there is a lack of TV case-reporting data at national and global levels. Despite this major limitation, WHO has made an effort to generate regional and global estimates of TV incidence and prevalence among adults aged 15–49 years old in 1999 and 2005 (table 1). 1 10 Remarkably, empirical data on TV incidence and prevalence were so scarce that they were not used in developing the 1999 estimates. Instead, TV prevalence among women was esti- mated to be twice the regional prevalence of Chlamydia trachomatis infection. Prevalence of TV infection among men was calculated to be one- tenth of the estimated TV prevalence among women. Estimates of TV incidence were generated by dividing the prevalence by the estimated average duration of infection (females: 1.03–1.36 years, males: 0.11–0.12 years). 10 By contrast with the 1999 estimates, WHO used data from studies conducted between 1999 and 2005 to generate an estimate of TV prevalence in 2005. Of note, the research studies contributing to the WHO estimate were not designed to measure prevalence in the overall population. Prevalence of TV infection among women was estimated as 8.08% from study data for the Africa, South-East Asia, and Western Pacific regions. 1 By contrast, TV prevalence in men was calculated to be 1.00% from study data available only for the South-East Asia region. For regions in which study data were not available, TV estimates were based on the prevalence of other STIs. When interpreting the 2005 statistics, it is important to bear in mind that the available data represent specific populations of Editor’s choice Scan to access more free content 418 Poole DN, et al. Sex Transm Infect 2013;89:418–422. doi:10.1136/sextrans-2013-051075 Review group.bmj.com on July 12, 2016 - Published by http://sti.bmj.com/ Downloaded from group.bmj.com on July 12, 2016 - Published by http://sti.bmj.com/ Downloaded from group.bmj.com on July 12, 2016 - Published by http://sti.bmj.com/ Downloaded from