Improving our Understanding of Mycoplasma Genitalium Epidemiology: A Re-Analysis of Two Cohort Studies

INTRODUCTION: Sexually-transmitted M. genitalium has recently gained increasing attention as a major cause of urethritis, cervicitis, pelvic inflammatory disease, infertility, and increased HIV risk. Unravelling the natural history of M. genitalium is crucial to understand its transmission dynamics to inform control policy. 

METHODS: We fitted a transmission model to infection data collected among a cohort of female students from London (2378 baseline and 900 follow-up samples; Oakeshott, 2010). We determined maximum likelihood estimates for the force of infection of a low-risk group (<2 partners last year), λ_low, recovery rate, γ, and ratio between the force of infection of the high- and low-risk groups. We also determined if γ from the London student data was consistent with infection data from a cohort of Ugandan sex workers (1027 participants, five measurements; Vandepitte, 2013). 

RESULTS: From the London student data, we estimated λ_low=0.0147/year, λ_high/ λ_low=3.0, and γ=0.87/year. The 95% confidence interval (CI) for γ was [0.52/year, 2.23/year]. Neither the estimated recovery rate (0.87/year) nor the upper CI was consistent with the Ugandan data: unless we assume a test sensitivity of maximally 0.56 (γ=0.87/year) or 0.80 (γ=2.23/year) for the Ugandan data, which seem unlikely, we cannot explain the Ugandan data with that recovery rate. 

CONCLUSIONS: Both cohort studies suffer from biases that potentially have affected our estimates. More information (e.g., partnership duration) would help to design more accurate models. The difference in recovery rate estimates is probably partly due to the sex workers having more frequent antibiotic treatment or perhaps developing immunity promoting faster clearance. Additionally, for some of the students, apparently persistent infection might have been due to reinfection occurring from long-term partners. Further data controlling for confounders are needed.