Objectives The reported number of syphilis cases is increasing in Japan. In this study, we aimed to estimate both the incidence of infection and diagnosis-and-reporting rate using a mathematical model that captures the time course of infection.
Methods We analysed yearly notification data from 1961 to 2016 and stratified according to clinical stage and sex. We built a compartmental ordinary differential equations model to describe the natural epidemiological course of syphilis in which the incidence of infection and diagnosis-and-reporting rate were included as time-varying parameters that we estimated via the maximum likelihood method.
Results From 2008 to 2016, the estimated number of new syphilis cases among men and women was 1070 (95% CI 1037 to 1104) and 302 (95% CI 287 to 318), respectively, which was increased from the previous period (1999–2007) with 269 (95% CI 256 to 282) and 71 (95% CI 64 to 78) cases, respectively. The diagnosis-and-reporting rate did not vary greatly over time. We estimated the number of undiagnosed syphilis cases from either incubating or early clinical (ie, primary or secondary) infections in 2016 to be 262 (95% CI 249 to 275) and 79 (95% CI 74 to 84) for men and women, respectively.
Conclusions The recent increase in reported cases of syphilis is owing to an increase in the number of infections. We found no evidence of time-dependent improvement in detection or reporting.
- bacterial infection
- mathematical model
- incidence studies
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Contributors HN conceived the study and directed YE. YE collated and analysed the data, and TY guided YE in programming the model. YE, TY and HN drafted the manuscript and figures. HN edited earlier versions of the manuscript. All authors have read and approved the final version of the manuscript.
Funding The present study was primarily supported by a Health and Labour Sciences Research Grant (H28-AIDS-General-001). HN received funding support from the Japan Agency for Medical Research and Development (JP18fk0108050); the Japan Society for the Promotion of Science KAKENHI (Grant Numbers 16KT0130, 17H04701, 17H05808 and 18H04895); the Inamori Foundation, the Telecommunication Advancement Foundation; and the Japan Science and Technology Agency (JST) CREST programme (JPMJCR1413). TY received funding support from the Japan Society for the Promotion of Science KAKENHI (Grant Number JP19K19429).
Disclaimer The funders had no role in the study design, data collection and analysis, decision to publish or preparation of the manuscript.
Competing interests None declared.
Patient consent for publication Not required.
Provenance and peer review Not commissioned; externally peer reviewed.
Data availability statement Data are available in a public, open access repository. Data are available on reasonable request. The dataset that we used in the present study is openly accessible as the yearly notification data, and the arranged dataset is available from the corresponding author on request.
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