In 2006, a new variant of Chlamydia trachomatis (nvCT) was discovered in Sweden. It had a deletion in its plasmid that included the target sequences used by 2 diagnostic tests, Abbott m2000 (Abbott Laboratories, Des Plaines, IL) and Amplicor/COBAS Amplicor/TaqMan48 (Roche, Molecular Systems, Branchburg, NJ).1 The proportion of nvCT recorded was between 20% and 65% in counties using Abbott/Roche test systems in 2006,2 and a statistical analysis from Örebro county indicated that nvCT may have been present in sizeable numbers in 2002.3 A conservative calculation with a 5% nvCT proportion of all chlamydia cases in 2002 in counties using Abbott/Roche methods and an increase to 25% in 2006, would result in 19,000 estimated false-negative tests in Sweden during that period. Abbott and Roche subsequently designed dual-target assays that could also detect nvCT. The third test system used in Sweden at that time was ProbeTec from Becton Dickinson (BD; Franklin Lakes, NJ). This system has always been able to detect nvCT because it uses another target region on the plasmid. The proportion of nvCT in counties using BD was between 7% and 19% during the same period.2
The proportion of nvCT in 2 counties unable to detect this variant in 2006 (using Roche) and in 2 counties with effective detection systems (using BD) has been followed.4 The aim of the current study was to investigate trends in C. trachomatis, including proportion of nvCT, in these 4 counties.
Because of economical and logistical limitations, 4 counties were selected to investigate differences over time in nvCT prevalence in Swedish counties using Abbott/Roche and BD methods, respectively. Dalarna county was chosen because the highest nvCT prevalence was reported from that county in late 2006, and Örebro was chosen as a complimentary Roche county with a lower nvCT rate. Norrbotten was chosen as a BD county because it is a remote county in the north of Sweden, and it did not have a neighbor county using an Abbott/Roche method, that is, it was supposed to have lower nvCT prevalence. Uppsala was chosen as a complimentary BD county surrounded by several Roche counties; thus, it was likely to have a relatively higher nvCT prevalence. In the 2 Roche counties, the COBAS TaqMan48 detection method was replaced by ompA-based methods, the Artus C. trachomatis LC PCR Kit (Qiagen, Hinden, Germany) and LightMix 480HT PCR (TIB MOLBIOL, Berlin, Germany) in November 2006 and January 2007, respectively. The new version COBAS TaqMan CT Test v2.0, that detects nvCT, was implemented in 2008.
We collected consecutive C. trachomatis positive specimens (one per patient) for each county in 2007, 2008, 2009, and 2011. The time periods differed between counties from 30 to 90 days from November 2006 to April 2007. In the other years, collection periods varied from 60 to 110 days from January to May. The length of collection periods was chosen to show differences in nvCT prevalence between counties using Roche and BD methods and was based on initial data from the counties. DNA extraction methods for the years 2007 to 2009 have been described earlier,4 and evaluations have shown that the change of DNA extraction method in the 2 Roche counties has not had any considerable impact on detection performance when compared with the Roche method used until 2006. The methods were the same in 2011, except in Uppsala where DNA was purified by Roche COBAS 4800. Specific detection of nvCT and wild-type C. trachomatis was based on size differentiation in a PCR method targeting the plasmid by use of primers from a method by Ripa and Nilsson,5 as previously described.4 We calculated differences in proportions of nvCT with 95% confidence intervals (CI) and compared them using a χ2 test (Minitab 16, Minitab Inc, State College, PA, (USA)). The study was approved by the Regional Ethical Review Board in Uppsala, Sweden (Dnr2007/312).
In total, 3648 specimens were analyzed; 196 (5.3%) were untypeable and were excluded from analysis. The number of specimens from each county ranged from 115 to 317 each year. The proportion of nvCT significantly decreased in counties using Roche from 2007 to 2009 and leveled out in 2011 (Fig. 1A). Detailed information regarding the nvCT trends is provided in Table 1.
Trends differed in counties where the BD system was used. In Norrbotten, there was no overall change between 2007 (9%) and 2011 (11%), but there was, however, an increase in the proportion of nvCT to 13% in 2008 and 19% in 2009, followed by a decrease to 11% in 2011. In Uppsala, there was an overall decline; percentages were 24% in 2007, 27% in 2008, 18% in 2009, and 12% in 2011.
In 2009 in the 2 Roche counties, the nvCT proportion was 24% (97/405) compared with 18% (72/391) in the 2 BD counties. In 2011, the nvCT proportion remained at 24% (98/404) in the Roche counties but declined to 11% (64/559) in the BD counties; thus, the difference between Roche and BD counties in 2011 was 13% (95% CI: 11, 15%, P < 0.001).
The proportion of nvCT was compared by age and sex (data not shown). The persons infected with nvCT were significantly younger in counties using Roche, both in 2007 and 2008 (P < 0.05), but not in 2009 and thereafter. In counties using BD, nvCT was not associated with age-group. There was no sex difference in proportions of nvCT.
The total number of cases of C. trachomatis, diagnosed from January 2007 to June 2007 per 1,00,000 inhabitants, was higher in Dalarna than in Norrbotten, Uppsala, and Örebro (Fig. 1B). The case rate in Dalarna declined dramatically in 2008; differences between the 4 counties were reduced in 2009 and 2011 and were not associated with diagnostic method.
The proportion of nvCT was reported to be higher in counties that used Abbott/Roche compared with counties using BD,2 and the level of nvCT in Roche counties rapidly decreased after re-establishment of effective detection systems in 2006/2007.4 The proportion of nvCT in all 4 counties therefore converged as selective diagnostic advantage for nvCT in the Roche counties was lost. Data from this study in 2011 do not suggest continuing convergence. The nvCT proportion is now twice as high in the Roche counties. The proportion in the 2 Roche counties is the same in 2009 and 2011, and it may have reached a steady state level.
Recent characterization of nvCT, including genome sequencing and several phenotypical traits, indicates that it has no biologic fitness advantage compared with the wild type.6 It is therefore more difficult to explain the declining nvCT proportion between 2009 and 2011 in the 2 BD counties. It may be argued that the number of sexual contacts with nvCT from Roche counties has decreased and therefore also result in declining nvCT proportion in BD counties.
The reported number of C. trachomatis infections has increased in Sweden since 1997, but the trend was interrupted by the emergence of nvCT. After implementation of adequate detection systems, a sharp increase was seen in 2007, partly because of an increase in the number of people tested. The reported case rate per 1,00,000 inhabitants has fallen in 2008–2010 but is still higher than when nvCT emerged. Trends in the 21 counties vary and do not seem to be consistently related to the type of detection method (Abbott/Roche or BD) in 2006.7
In summary, the proportion of nvCT and the reported case rate of C. trachomatis have converged in the 4 counties studied in Sweden after the reestablishment of adequate detection systems. The remaining differences in nvCT proportion in Roche counties and BD counties require further investigation to determine whether nvCT has reached a steady state level.
1. Ripa T, Nilsson P. A variant of Chlamydia trachomatis
with deletion in cryptic plasmid: Implications for use of PCR diagnostic tests. Euro Surveill 2006; 11:E061109 061102.
2. Herrmann B, Törner A, Low N, et al.. Emergence and spread of Chlamydia trachomatis
variant, Sweden Emerg. Infect Dis 2008; 14:1462–1465.
3. Jurstrand M, Olcén P, Magnuson A, et al.. Emergence of the new variant of Chlamydia trachomatis in a defined area of Sweden before 2002? Sex Transm Infect 2010; 86:337–341.
4. Klint M, Hadad R, Christerson L, et al.. Prevalence trends in Sweden for the new variant of Chlamydia trachomatis
. Clin Microbiol Infect 2011; 17:683–689.
5. Ripa T, Nilsson PA. A Chlamydia trachomatis strain with a 377-bp deletion in the cryptic plasmid causing false-negative nucleic acid amplification tests. Sex Transm Dis 2007; 34:255–256.
© Copyright 2012 American Sexually Transmitted Diseases Association
6. Unemo M, Seth-Smith HM, Cutcliffe LT, et al.. The Swedish new variant of Chlamydia trachomatis
: Genome sequence, morphology, cell tropism and phenotypic characterization. Microbiology 2010; 156:1394–1404.