The Evolving Epidemiology of Chlamydial and Gonococcal Infections in Response to Control Programs in Winnipeg, Canada d ,, ,in,5,P ASS ;S Objectives. The v stdy was t describe and transmission t na a d gonoh i Canada, and to as inm control programs. Methods. Chiasydia ;:rhea surveillance case-r through 1995) and con reports (1991 through examined. Results. High -ide :both chlamyd and;g80D detlie inY tOWe ::Z1n Aia most pprominen in cases while for gon .:lar in ce and niono James F Blanchard, MD, PhD, Stephen Moses, MD, MPH, Christina Greenaway, MD, FRCP, Pamela Or,; MD, MSc, FRCP, Greg W Hammond, MD, FRCP, and Robert C. Brunham, MD, FRCP Bacterial sexually transmitted diseases (STDs) remain an important public health problem worldwide."2 Most bacterial STDs :4 are curable with relatively short courses of antimicrobials and have therefore become the focus of major disease control efforts.3 STD control strategies should be based on a sound understanding of the population- level transmission dynamics of STD pathogens. Previous theoretical and empiri- gs cal findings have provided valuable insight into the transmission dynamics of STDs.4'2 These findings have identified the impor- > tance of core groups in the spread of STDs within populations. The theoretical underpin- ning of the core group concept is derived from the basic reproductive number, which defines the ecologic success for any infec- tious disease as the average number of sec- ondary infections arising from infected indi- viduals in a fully susceptible population.5 The basic reproductive number is the prod- uct of 3 population parameters: transmissi- bility of the disease, contact rate between infective and susceptible individuals, and duration of infectivity. Only when the basic reproductive number is greater than 1 does an infectious agent successfully spread in a population. Estimates from various popula- tions demonstrate that the average rate of partner change (contact rate) in most seg- ments of the population is not sufficient to sustain STDs.'2 Instead, subpopulations with higher rates of partner change (core groups) are required for the spread and maintenance of STDs in the entire population. Various approaches have been used to identify STD core groups. At the individual g level, members of a core group can be defined according to risk behaviors such as rate of partner change.4" 0 However, identify- ing individuals within core groups can be difficult for public health practitioners and unfairly stigmatizing for individuals. There- fore, spatial analyses have been used to define geographic "core areas" within which STD transmission rates are highest.8'9 These geographic core areas are hypothesized to contain a higher proportion of the core group members and thus provide a rational target for focusing control efforts. 1"7",3 However, Wasserheit and Aral point out that the char- acteristics and roles of core groups in STD transmission evolve under the influence of control programs.'4 Furthermore, they sug- gest that these changes unfold in a stereo- typic pattern and that control programs should be responsive to such changes. In this paper, we analyze the epidemiol- ogy of Chlamydia trachomatis (chlamydia) and Neisseria gonorrhoeae (gonorrhea) infection in Winnipeg, Canada. By examin- ing the role of geographic core area popula- tions in the spread of these 2 STDs over time, we provide evidence suggesting that the epidemiology of chlamydia and gonor- rhea has evolved in response to control pro- grams, as proposed by Wasserheit and Aral.'4 James F. Blanchard is with the Epidemiology Unit and Public Health Branch, Manitoba Health, and the Department of Medical Microbiology, University of Manitoba, Winnipeg, Manitoba, Canada. Stephen Moses, Christina Greenaway, Pamela Orr, Greg W. Hammond, and Robert C. Brunham are with the Departments of Medical Microbiology and Internal Medicine, University of Manitoba. Stephen Moses and Greg W. Hammond are also with the Public Health Branch, Manitoba Health. Requests for reprints should be sent to James Blanchard, MD, PhD, Epidemiology Unit, Manitoba Health, 405-800 Portage Ave, Winnipeg, Manitoba, Canada R3G ON6 (e-mail: jamiebggov.mb.ca). This paper was accepted January 26, 1998. October 1998, Vol. 88, No. 10 ..!: I '. A,9 "'i'90'