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Sexually Transmitted Diseases:
doi: 10.1097/OLQ.0b013e31818653b0

Chlamydia Trachomatis Infections in Norway, 1986 to 2006, Surveillance Data

Kløvstad, Hilde MPH, RN; Aavitsland, Preben MD

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From the Department for Infectious Disease Epidemiology, Norwegian Institute of Public Health, Oslo, Norway

Supported by all laboratories reporting C. trachomatis data in Norway. Andrej Grjibovski for support with the analysis.

Correspondence: Hilde Kløvstad, MPH, RN, Norwegian Institute of Public Health, Pb 4404 Nydalen, 0403 Oslo, Norway. E-mail:

Received for publication November 21, 2007, and accepted July 6, 2008.

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Introduction: The number of Chlamydia trachomatis infections in Norway has been increasing for the last years. A new and enhanced surveillance system was implemented in 2005 to meet the challenges faced by the increasing number of cases.

Methods: The new surveillance system is laboratory based. Data are collected once a year from all laboratories on the total number of test performed, and all diagnosed cases from the preceding year. For each case the following variables are reported: date of diagnosis, birth year, sex, and municipality of residence.

Results: By 2006 all laboratories reported data as required. We have observed an increase in yearly diagnosed C. trachomatis cases in Norway during the last years. In 2006, the incidence was 4.6 per 1000 population. The proportion positive tests have increased from 6.0% in 2000 to 7.7% in 2006. In the same period the annual number of tests increased by 13.5%. Surveillance data from 2006 showed that the highest incidence rates were found in women between the ages of 15 to 24 in men aged 20 to 24 and in 2 northernmost regions of the country.

Conclusion: The year 2006 had the highest level of diagnosed cases ever in Norway. To better interpret the observed trend, a voluntary system will be introduced in 2007 to collect test rates by age, gender and geography. There is a need to evaluate current and new strategies to target the group of asymptomatic and untreated young people.

CHLAMYDIA TRACHOMATIS INFECTION IS the most common bacterial sexually transmitted infection (STI) in Norway1 and in Europe.2 C. trachomatis infection can easily be treated with antibiotics. However, if left untreated it is a major cause of pelvic inflammatory disease that may lead to complications such as infertility, ectopic pregnancy, and chronic pelvic pain in women.3–5

To meet the challenge faced by the increasing numbers of diagnosed cases, a new and enhanced surveillance system for anal and urogenital C. trachomatis infection was implemented in 2005. This article describes the past and present surveillance system and the epidemiology of C. trachomatis infections in Norway from 1986 to 2006. An anal or urogenital C.trachomatis infection is throughout the article referred to as Chlamydia.

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The Past Surveillance System (1986–2004)

Until 2002, surveillance of Chlamydia infections was based on voluntary aggregate reporting from all the laboratories in Norway performing Chlamydia diagnostics. The number increased from 16 laboratories in 1986 to 24 laboratories in 2002. In 2006, the number was down to 22. We collected only the total number of tests performed and total number of diagnosed cases (positive test results) for the preceding year. The system was based on active surveillance as The Norwegian Institute of Public Health (NIPH) once a year contacted all laboratories asking for these 2 numbers. The contact was repeated by e-mail and telephone until all data were collected.

In 2003, Chlamydia infections were made mandatory notifiable and part of the Norwegian surveillance system of infectious diseases, which is regulated by law, owned by the Ministry of Health and operated by NIPH. We got the legal right to collect a limited amount of information on the individual patients, including birth year, sex, municipality of residence, and localisation of infection. We then started working on a new system, which was implemented for the year 2005.

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New Enhanced Surveillance (2005)

The overall objective of the new, enhanced surveillance system is to have a continuous overview of the spread of Chlamydia in Norway to target preventive measures and plan resources needed.

The population under surveillance is all residents of Norway (currently 4.6 million inhabitants) and other people present in Norway temporarily, including foreign tourists, students, workers, and immigrants.

A major change compared with the old surveillance system, is a clearer case definition. A case is now defined as a person with 1 or more positive laboratory tests for Chlamydia trachomatis in a urinary sample or a sample from urethra, vagina, cervix, or anus within a period of 60 days. The time period of 60 days is chosen to avoid counting positive follow-up tests as new cases. Follow-up tests are recommended to be taken of all positive cases, but only 5 to 6 weeks after treatment.

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Data Collection, Transfer, and Processing

Sampling for Chlamydia testing is available at no extra cost to the patient at all general practitioners’ offices, students’ clinics, STI clinics, hospitals, and other sites. All samples are sent to one of the diagnosing laboratories. There is no possibility of Chlamydia diagnostics in Norway outside these laboratories. In February every year, NIPH actively collects data for the preceding year from all laboratories. For each case the following variables only are reported as individual records in a simple Excel file: date of diagnoses (test received), birth year, sex, and municipality of residence. Unique individual identifiers are not used. The total number of tests performed during the year is also reported from each laboratory. The surveillance system does not include information on testing site. Diagnostic method and type of test used by the different laboratories is not routinely captured by the surveillance system, but collected sporadically.

The data file is sent to NIPH by regular mail or e-mail marked with a unique code for each laboratory and year. The different data files are checked for logical errors and merged to one database at NIPH. Data checks, entry and analysis are performed by the same person–the epidemiologist in charge of Chlamydia surveillance at NIPH.

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Data are reported to stakeholders through yearly reports in the national epidemiologic bulletin, the NIPH website (, mass media, conferences and other publications. Data are also available at the NIPHs online table generator (, where the users can create their own tables combining various variables on national and regional level.

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The number of positive test results reached a peak in the late 1980s with nearly 20,000 per year (Fig. 1). The number then decreased until 1998. The last 8 years it has increased every year and reached a new peak in 2006, with 21,259 cases. This represented an incidence rate (IR) of 4.6 per 1000 inhabitants. There was a 31% increase in diagnosed cases from 2000 (14,631 positive) to 2006 (Fig. 1).

Fig. 1
Fig. 1
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The highest number of tests performed per year for Chlamydia was in the first half of the 1990s (Fig. 2). Eighty tests were performed per 1000 population in 1991. In the same period, the proportion of positive test results was at its lowest with only 4.1% in 1993. The proportion positive tests have increased from 6.0% in 2000 when 53 tests were performed per 1000 population to 7.7% in 2006 when 59 tests were performed per 1000 population. In the same period, the total number of tests performed per year increased by 13.5% (Fig. 2). There has been no association between the number of reporting laboratories and number of tests performed throughout the period.

Fig. 2
Fig. 2
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NIPH received data on total number of test performed and total number of cases from all laboratories each year since 1986. In 2005, 22 out of 23 laboratories reported data by date of diagnosis, sex, birth year, and municipality (95% of the cases). One laboratory reported cases only by sex (1060 cases). In 2006, all 22 laboratories reported cases by date of diagnosis, sex, birth year, and municipality.

Among cases diagnosed in 2006, 61% were women and 38% were men. Mean age was 24 years for all cases, for men, 26, and for women, 23. Among cases below 25 years of age 68% were women. In the age groups above 30 years, 55% were men. The age groups below 25 represented 66% of all cases. Forty percent were in the age group 20 to 24 years and 26% in the age group 15 to 19 years.1,6 For women, the highest incidence rates were found in the age groups 15 to 19 years (31/1000) and 20 to 24 years (37/1000) with an incidence rate ratio of 8.8 [95% confidence interval (CI) 8.3–9.4] and 10.7 (95% CI 10–11.4), respectively compared with the age group 30 to 39 (Table 1). Among men, the age group 20 to 24 years had the highest incidence rate (24/1000) (Table 1), the incidence rate ratio for this age group was 6.2 (95% CI 5.8–6.6) compared with the age group 30 to 39 (Table 1).

Table 1
Table 1
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The highest incidence rates were found in the northern parts of Norway in both 2005 and 2006.1,6 The incidence rate was almost twice as high in the 2 most northern regions (8.1/1000) as the country average which was 4.6/1000 in 2006. The 5 biggest cities altogether had a higher incidence rate (5.5/1000) than the rest of the country (4.3/1000).

In the period 1996 to 1999, most laboratories changed from cell culture or ELISA methods for Chlamydia diagnostics to nucleic acid amplification methods (NAAT). There was no geographical variation in time of introduction of NAAT throughout the country, and by 2004 all tests performed were analyzed by NAAT. In 2006, 11 laboratories used the BD ProbeTec ET system, 9 used Roche (Cobas Amplicor or TaqMan 48), and 1 laboratory used their own inhouse method.

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We have observed a continuous increase in the incidence of Chlamydia in Norway since 1998. The highest incidence rates were found in women aged 15 to 24 and in men aged 20 to 24, and in the 2 most northern regions of the country. In 2005, a new and enhanced surveillance system was established. A clearer case definition was used to avoid double reporting and important epidemiologic information was provided for each diagnosed case. Despite these improvements the new system still has limitations as to fully interpret the observed trends.

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A True Increase?

A major problem in the surveillance of Chlamydia is that the infection is most often asymptomatic.7,8 Thus, the number of diagnosed cases per time unit does not necessarily reflect the incidence of new cases. The number of diagnosed cases is dependent on the number of persons who are tested, the diagnostic validity of the test and the prevalence of infection among the people who are tested. We believe that the observed increase in number of positive test results reflects a true increased incidence in the Norwegian population. The same trend has also been observed in our neighboring countries, Sweden and Denmark,9,10 and in other European countries.2 Other factors may, however, have contributed to the increase.

First, a main strategy to fight the problem of Chlamydia has been to encourage young men and women to get tested for Chlamydia after unprotected sex. This has been done through information activities and an increased availability and acceptability (urine tests) for testing. Thus, an increasing test activity may be one important explanation of the increased incidence. However, the proportion positive among the tested has also increased in the same period.

Second, the observed increase in incidence of Chlamydia may be a result of more targeted testing among the risk groups in the recent years. In the first half of the 1990s, the test activity was high also in low prevalence populations.11 Recommendations for new test indications were issued by NIPH in 1995 identifying the following groups for Chlamydia testing: individuals with clinical symptoms compatible with Chlamydia, men and women below 25 years of age after each change of sexual partner or, if no information on sexual partner, every 1 to 2 years; pregnant women and women undergoing legal abortion.12 A recent study from one Norwegian region showed in increase in testing of young people at risk in line with these recommendations.13 The increase in proportion positive since the mid 1990s may also indicate such improved targeting of testing.

Finally, more sensitive analytical methods may have influenced the result, as suggested by Burckhardt and et al.14 and by our own data. By using a Generalised Estimating Equations population average model with autoregressive correlation matrix we found an association between introduction of NAAT and proportion positive for chlamydia among the tested (coefficient 0.87 with 95% CI 0.25–1.49). However, as the number of and proportion positive cases has continued to increase even after the introduction of NAAT, secular trend, case mix, and test activity may be more important to explain the increase in number of diagnosed cases than the introduction of more sensitive analytical methods.

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Gender and Geographical Distribution and Undiagnosed Infections

Despite the recent increase in number of test performed each year, many people still do not seek health care to get tested. We believe the sex and age distribution documented by our surveillance system is merely a picture of who is getting tested and not necessarily the real distribution in the population. NIPH does not have the legal right to collect any information on individuals with a negative test result. Thus the surveillance system does not provide information on testing rates by age, gender, and municipality of residence. Testing related increases in different groups can therefore not be discriminated from a true increase in the same groups.

A study from one Norwegian microbiologic department showed that only 8% of the samples tested for Chlamydia in the period 1990 through 2003 were from men.15 A recent prevalence study among male students in 2 larger Norwegian cities, showed a prevalence of 7.8%.16 The potential of undetected and untreated male Chlamydia cases is therefore high. In Sweden, individual case reporting of Chlamydia is mandatory both for laboratories and clinicians.17 In addition, all laboratories performing Chlamydia testing report on a voluntary basis the annual number of tested and Chlamydia positive by gender and age group. In 2006, only 26% of the persons tested were men (personal communication, Inga Velicko, Swedish Institute for Infectious Disease Control).

The geographical distribution shows a clearly higher Chlamydia incidence in the 2 northernmost regions than in the rest of the country, which is the most sparsely populated area of the country. Although we lack information on testing rates by municipality we believe this is more likely to be caused by unsafe sexual practices rather than higher testing rates in the north. In line with these findings, the highest incidence rate of teenage pregnancies is also found in the same two regions.18

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Validity and Timeliness of Surveillance

Until 2005, the reported number of diagnosed cases from most laboratories equaled the total number of positive test results. This might have led to an overestimation of number of cases because it does happen that more than 1 sample (e.g., urethra and cervix) is taken from the same person, and positive follow-up tests could also have been counted as new cases. The new case definition is more specific in terms of excluding double reports either caused by multiple samples or positive follow-up tests. Sixty days is, however, a longer time period than the recommendations for follow-up tests. Thus, double reporting may also occur with the new system. The new system may, in turn, be less sensitive because an individual can be counted with an incident Chlamydia infection only once within a 60-day period. As we use no unique identifiers, reinfections will not be traced by the surveillance system. With the magnitude of diagnosed cases it is not likely that this will be introduced.

Cases are reported up to 1 year after time of diagnosis. We consider timeliness of the data to be sufficient since control measures in general are long-term and related to trends over time.

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Conclusion and Recommendations

We conclude that the Chlamydia epidemic in Norway has been increasing since 1998. In 2005, a new and enhanced system for Chlamydia surveillance was successfully established providing important indicators on the epidemiologic situation in Norway. Surveillance data are actively used by policy makers and health professionals in defining preventive strategies at a national and regional level. However, Norwegian Chlamydia surveillance data represents only the fraction of the infected who get tested, and information is only available for those with a positive test result. Trends cannot be fully interpreted without additional information on case mix among the tested. From 2007, we will introduce a voluntary system of collection of aggregated data from all laboratories on gender, age-groups, and county of living on the total number of tested. Regular population-based prevalence surveys should be considered as a supplement to surveillance data as a tool to evaluate and better target preventive strategies.

Prevention and control of Chlamydia relies on early diagnostics and treatment, including contract tracing, education on safer sexual behavior, and accessibility of condoms. Despite the increasing number of tests performed each year, there is a need to evaluate current and new strategies to reach the group of untreated young people.

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12. Aavitsland P, Lystad A. Indikasjoner for testing for seksuelt overførbare infeksjoner med Chlamydia trachomatis. Tidsskr Nor Laegeforen 1995; 115:3141–3144.

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14. Burckhardt F, Warner P, Young H. What is the impact of change in diagnostic test method on surveillance data trends in Chlamydia trachomatis infection? Sex Transm Infect 2006; 82:24–30.

15. Bakken IJ, Skjeldestad FE, Nordbo SA. Testing pattern and prevalence of Chlamydia infections among men. Tidsskr Nor Laegeforen 2005; 125:1634–1636 [Norwegian].

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17. Gotz H, Lindback J, Ripa T, et al. Is the increase in notifications of Chlamydia trachomatis infections in Sweden the result of changes in prevalence, sampling frequency or diagnostic methods? Scand J Infect Dis 2002; 34:28–34.

18. Fødsler i Norge 2003–2004. Åsrapport/Annual report. Medisinsk fødselsregister/Medical Birth Registry of Norway. Norwegian Institute of Public Health, Bergen, Norway; 2006.

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