The rise and fall of the new variant of Chlamydia trachomatis in Sweden: mathematical modelling study

ObjectivesA new variant ofChlamydia trachomatis(nvCT) was discovered in Sweden in 2006. The nvCT has a plasmid deletion, which escaped detection by two nucleic acid amplification tests (Abbott/Roche, AR), which were used in 14 of 21 Swedish counties. The objectives of this study were to assess when and where nvCT emerged in Sweden, the proportion of nvCT in each county, and the role of a potential fitness difference between nvCT and co-circulating wild-type strains (wtCT).MethodsWe used a compartmental mathematical model describing the spatial and temporal spread of nvCT and wtCT. We parameterised the model using sexual behaviour data and Swedish spatial and demographic data. We used Bayesian inference to fit the model to surveillance data about reported diagnoses of chlamydia infection in each county and data from four counties that assessed the proportion of nvCT in multiple years.ResultsModel results indicated that nvCT emerged in central Sweden (Dalarna, Gävleborg, Västernorrland), reaching a proportion of 1% of prevalent CT infections in late 2002 or early 2003. The diagnostic selective advantage enabled rapid spread of nvCT in the presence of high treatment rates. After detection, the proportion of nvCT decreased from 30-70% in AR counties and 5-20% in counties that Becton Dickinson tests, to around 5% in 2015 in all counties. The decrease in nvCT was consistent with an estimated fitness cost of around 5% in transmissibility or 17% reduction in infectious duration.ConclusionsWe reconstructed the course of a natural experiment in which a mutant strain ofC. trachomatisspread across Sweden. Our modelling study provides support, for the first time, of a reduced transmissibility or infectious duration of nvCT. This mathematical model improved our understanding of the first nvCT epidemic in Sweden and can be adapted to investigate the impact of future diagnostic escape mutants.Key messagesThe dynamics of a new variant ofChlamydia trachomatis(nvCT) that escaped testing and treatment in Sweden can be reconstructed using a mathematical transmission model.Our study for the first time provides support of a reduced transmissibility or infectious duration of the nvCT in Sweden.This mathematical model improved our understanding of the nvCT epidemic in Sweden and can be adapted to investigate the impact of future diagnostic escape mutants.