Rising reported rates of chlamydia among young women in Canada: what do they tell us about trends in the actual prevalence of the infection?(Research Update)(Report)

Abstract: This article explores possible explanations for the rise in reported chlamydia rates among young women in Canada between 1997 and 2004 and considers whether rising rates can be used to infer a parallel increase in the actual prevalence of the infection. The transition to more sensitive testing methods is among the factors that could have contributed to the rise in reported rates. In contrast to Canada, the United States (US) monitors trends in the prevalence of chlamydia among young women as well as reported rates. The US data indicate that while reported rates of chlamydia among young women rose during the same time period, prevalence levels, when adjusted for increased use of more sensitive testing methods, remained relatively stable. While available data are insufficient to draw definitive conclusions about prevalence trends in Canada, existing studies do point to unacceptably high prevalence levels. The establishment of a sentinel chlamydia surveillance system would provide a mechanism to track prevalence trends and allocate resources for chlamydia prevention and control.

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Background

Of the sexually transmitted infections (STI) reported to the Public Health Agency of Canada (PHAC), chlamydia is the most frequently reported with the highest rates being among 15- to 24-year-old females (PHAC, 2007a). Chlamydia, which is often asymptomatic, is of particular concern because, if left untreated, it can lead to pelvic inflammatory disease, ectopic pregnancy, tubal infertility, chronic pelvic pain and increased susceptibility to HIV infection (Cates & Wasserheit, 1991; Steban, 2004). Given its significance for the sexual and reproductive health of young Canadian women, it is important to track trends in the prevalence of chlamydia in this population in order to effectively target prevention and control strategies.

We have observed that reports about chlamydia rates by the health promotion community and the media often draw the conclusion that there has been an increase in chlamydia infection among Canadian young people in recent years (e.g., "Chlamydia in teens jumps 50%": Globe and Mail, Feb. 13, 2008; "Safe-sex complacency boosts STDs" Ottawa Citizen, April 30, 2008). The common assumption is that the rising trends Seen in chlamydia rate data published by the PHAC and local jurisdictions are a valid reflection of trends in the percentage of young people in Canada who are actually infected with chlamydia. In other words, it is assumed that increases in reported chlamydia rates indicate parallel increases in the percentages of youth infected (i.e., that the prevalence of chlamydia is increasing). The rise in reported rates is thus taken as a sign that the sexual health of Canadian young people is being increasingly compromised. However, that assumption appears to conflate reported rates with prevalence and is questionable on a number of other important grounds as we will discuss below.

Calculating chlamydia rates

In Canada, the surveillance of chlamydia is primarily based on the tracking of newly diagnosed cases reported to provincial ministries of health and then to the PHAC which compiles and publishes national statistics on notifiable STI (see PHAC, 2007a). In other words, when a person is tested for chlamydia and is found to be infected the result is reported to public health authorities. Public health authorities use the total number of newly diagnosed cases to calculate a rate per 100,000 of the population group in question. If the person is not infected, no report is made. It is important to note that the total number of tests done is not reported. Hence, we do not know the percent positive among those tested.