ABSTRACT
Objectives A new variant of Chlamydia trachomatis (nvCT) was discovered in Sweden in late 2006. The nvCT has a plasmid deletion, which escaped detection by two nucleic acid amplification tests that were being 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 the co-circulating wtCT strains.
Methods We 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.
Results Model 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 of nvCT enabled its rapid spread in the presence of high treatment rates. After detection, the proportion of nvCT decreased from 30-70% in Abbott-Roche counties and 5-20% in Becton Dickinson counties 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% in infectious duration.
Conclusions We reconstructed the course of a natural experiment in which a mutant strain of C. trachomatis spread across Sweden. Our modelling study for the first time provides support of a reduced transmissibility or infectious duration of nvCT. The results of this mathematical model, incorporating epidemiological surveillance data, has improved our understanding of the epidemic caused by nvCT in Sweden.
