ORIGINAL STUDY Genital Chlamydia, Genital Herpes, Trichomonas vaginalis and Gonorrhea Prevalence, and Risk Factors Among Nearly 70,000 Randomly Selected Women in 4 Nordic Countries Mette Tuxen Faber, MSc,* Ann Nielsen, MSc, PhD,* Mari Nygård, MD, PhD,† Pa¨r Spare´n, PhD,‡ Laufey Tryggvadottir, MSc,§¶ Bo Terning Hansen, PhD,† Kai-Li Liaw, PhD, and Susanne K. Kjaer, MD, DMSc* , ** Background: The aim of this study was to assess the prevalence of women reporting ever having genital chlamydia, genital herpes, Trichomonas vaginalis, and gonorrhea, and to identify factors associ- ated with each of these sexually transmitted infections (STIs). Methods: The study was based on a large cross-sectional survey conducted in 2004 –2005 among randomly sampled women (18 – 45 years) from the computerized population registries in Denmark, Ice- land, Norway, and Sweden. A total of 69,567 women were included in the study. Results: The overall prevalence in Denmark, Iceland, Norway, and Sweden was 1.5% for reporting ever having had Trichomonas vagina- lis, 1.9% for gonorrhea, 4.8% for genital herpes, and 17.0% for genital chlamydia. The prevalence of each of these STIs varied with birth cohort and country. In addition, they were strongly associated with lifetime number of partners and having a previous diagnosis of another sexually transmitted infection. Moreover, a diagnosis of genital chla- mydia or gonorrhea was associated with early age at first intercourse and smoking initiation. Finally, reporting genital chlamydia was asso- ciated with early age at drinking initiation, and ever use of hormonal contraceptives and condoms. Conclusion: Genital chlamydia occurs frequently among women in the Nordic countries. Risk-taking behavior, particularly sexual behavior, is strongly associated with STIs, which suggest that further information is needed about STIs and their consequences, targeting high-risk groups. There is also a need for continued monitoring of STIs in order to follow the prevalence and to gain further knowledge about risk factors. S exually transmitted infections (STIs) have a great impact on health, potentially causing severe health outcomes such as infertility, ectopic pregnancy, pelvic inflammatory disease, and even gynecologic cancers such as cervical, vulvar, and vaginal cancers. 1,2 STIs are also associated with stigmatization, anxi- ety, and psychological stress. 1,3 Moreover, STIs not only rep- resent a health problem for the individual, but also an economic burden for society. 4 In the Nordic countries, surveillance systems for bac- terial STIs are mainly based on case reporting from clini- cians and/or laboratories. Case reporting from Universal Physician is mandatory for gonorrhea in Denmark, Iceland, Norway, and Sweden. Surveillance of genital chlamydia is most frequently based on laboratory reporting. In addition, genital chlamydia is notifiable in Iceland, Norway, and Swe- den. The number of reported cases of bacterial STIs in the Nordic countries reflects a complex mixture of different practices, such as screening practices and partner notifica- tion practices. Viral STIs, such as genital herpes, are not notifiable in any of the Nordic countries. Exact data on the prevalence of bacterial as well as viral STIs are necessary for developing and evaluating strategies to control and pre- vent STIs in the population. 5–7 In order to target prevention strategies against STIs, it is also important to identify groups at particular risk of acquiring an STI. Several studies have explored possible risk factors for STIs. However, the influence of risk factors on specific STIs may vary, 8,9 and since previous studies have tended to focus only on 1 specific STI or STIs in general, 10 –12 relatively little is known about differences in risk factor profiles between different STIs. Relatively few nation-wide studies of the general pop- ulation on risk factors for STIs have been published. Most studies are limited by only including women from a certain geographical area or women attending a certain STI clinic, 10,12–14 which makes it difficult to generalize the re- sults from these studies. Consequently, it is still important to provide population-based assessment of factors associated with STIs. In this large study of nearly 70,000 women from the general population in Denmark, Norway, Iceland, and Sweden, our aim was to assess the prevalence of women reporting ever having had a clinical diagnosis of genital chlamydia, genital herpes, Trichomonas vaginalis, or gonorrhea, and to identify factors associated with each of these STIs. From the *Department of Viruses, Hormones and Cancer, Institute of Epidemiology, Danish Cancer Society, Copenhagen, Denmark; †Department of Screening-Based Research, Cancer Registry of Norway, Oslo, Norway; ‡Department of Medical Epidemiology and Biostatistics, Karolinska Institute, Stockholm, Sweden; §Ice- landic Cancer Society, Reykjavik, Iceland; ¶Department of Medi- cine, University of Iceland, Reykjavik, Iceland; Merck Research Laboratories, Merck & Co., Inc., Upper Gwynedd, PA; and **De- partment of Obstetrics and Gynecology, Rigshospitalet, Copenha- gen, Denmark Supported by Merck & Co., Inc. (grant EPO 8014.016). Correspondence: Susanne K. Kjaer, MD, DMSc, Department of Vi- ruses, Hormones and Cancer, Institute of Cancer Epidemiology, Danish Cancer Society, Strandboulevarden 49, DK-2100 Copenha- gen, Denmark. E-mail: susanne@cancer.dk. Received for publication July 8, 2010, and accepted February 8, 2011. DOI: 10.1097/OLQ.0b013e318214bb9b Copyright © 2011 American Sexually Transmitted Diseases Association All rights reserved. Sexually Transmitted Diseases ● Volume 38, Number 8, August 2011 727