No 35 - 2024

Chlamydia 2023

• In 2023, the number of laboratory-confirmed chlamydia cases was 37,111.

• The number of testing episodes increased by 3% from 2022 to 2023, and the number of chlamydia cases decreased by 11%.

• Men accounted for 42% of all confirmed chlamydia cases, while the positivity rate was higher among men (14%) than among women (8%).

• The incidence of chlamydia among young people aged 15-29 years decreased from 2022 to 2023.

• 86% of all chlamydia cases occurred in the age group 15-29 years.

• In 2023, men were tested significantly less than women, with 3,872 testing episodes per 100,000 men compared to 8,798 testing episodes per 100,000 women.

• In 2019, doxycycline was recommended for the treatment of uncomplicated chlamydia. This was due to the high prevalence of azithromycin resistance among Mycoplasma (M.) genitalium in Denmark, presumably because azithromycin had previously been recommended as the standard treatment for uncomplicated chlamydia.

• Extracts from the Prescription Register suggest that azithromycin is still used as a treatment for chlamydia in about half of the cases. Its use should be further reduced.

As in previous years, data for the preparation of the chlamydia report for 2023 was obtained via the Danish Microbiology Database (MiBa) and includes all positive and negative test results for oculo-genital infection caused by Chlamydia (C.) trachomatis. In this report for 2023, figures from 2022 are provided for comparison in several places. In 2024, the method by which data on chlamydia is collected from MiBa was revised and updated as part of a larger effort for standardized data extraction, with the main difference between the two methods being that individuals can have more than one negative testing episode annually, which results in more testing episodes and thus significantly affects positivity rates from previous reports.

A detailed epidemiological description of the incidence of chlamydia for 2023 can be read here.

Since 2021, the number of testing episodes for chlamydia has increased by 3%, and the number of confirmed chlamydia cases has decreased by 11%. The positivity rate has thus decreased by 1.5 percentage points to 9.8%.

Except for the age groups <1 year and 45+ years, more than twice as many women as men are tested for chlamydia. However, the positivity rate among men is higher than among women in almost all age groups. In 2023, men accounted for 42% of confirmed chlamydia cases, which is identical to the figures from 2022.

In total, 86% of all chlamydia cases were seen in the 15-29 age group. For men in this age group, the number of tests is significantly lower than for women, which is why preventive efforts should be strengthened with information on safe sexual behavior and partner notification upon confirmed chlamydia.

There is no obvious explanation for the significant decrease in the incidence of chlamydia. A sudden increase in condom use does not seem plausible. A theoretical possibility is a reduced tendency to have rapidly changing sexual partners, but there is no data to support or refute this.

Previously, in Denmark, azithromycin 1 g orally as a single dose was recommended for many years for the treatment of uncomplicated chlamydia. However, co-infection with C. trachomatis and M. genitalium is assumed to be relatively common, and since a significantly higher prevalence of azithromycin resistance was found among M. genitalium in Denmark than in Sweden, where doxycycline is used for the treatment of chlamydia, it was decided in 2019 to change the recommendation for the treatment of chlamydia. The first-line treatment for uncomplicated chlamydia, including uncomplicated rectal chlamydia, was then oral doxycycline 100 mg twice daily for seven days. For LGV in the rectum, the same dose is used, but for 21 days. Guidelines for the treatment of chlamydia in pregnant women and in individuals with complicated chlamydia cases, including epididymitis and salpingitis, can be found in the Danish Dermatological Society’s guidelines on Sexually Transmitted Infections.

(T.R. Pedersen, S. Hoffmann, Department of Bacteria, Parasites & Fungi, L. Nielsen, S.F. Ravn, N. Bindslev, Data Integration and Analysis)