ARCHIVED - Sub-regional variations in the epidemiology of Neisseria gonorrhoeae in a large urban region in Alberta, Canada: Results from spatial analyses using routinely collected surveillance data

15 February 2006

Introduction

In 1996, Health Canada, with input from an Advisory Committee and ExpertWorking Group, identified the elimination of locally transmitted Neisseria gonorrhoeae in Canada by the year 2010 as a goal for the control of sexually transmitted infections (STIs)⁽¹⁾. In 1997 the reported rate of gonorrhea in Canada was 14.9 per 100,000 population compared to 219.8 per 100,000 population in 1980⁽²⁾. However reported rates of gonorrhea in Canada during 2000, 2001, 2002, and 2003 were 20.1, 21.6, 23.0, and 24.9 per 100,000 population, respectively^(3-5). In the province of Alberta, reported rates of gonorrhea have also been increasing from 18.5 per 100,000 population in 1998⁽⁶⁾ to 32.3 per 100,000 population in 2003⁽⁵⁾. These increases have been particularly apparent in the Capital Health Region, a large urban region located in central Alberta, Canada⁽⁷⁾.

In this report, we present a descriptive analysis of an outbreak of gonorrhea in the Capital Health Region.We also present results of spatial analyses that we used to investigate geographic variations in the incidence of gonorrhea within smaller area units of the Capital Health Region. Our approach provided us with information that was useful in identifying epicentres of the outbreak. It also provided us with some insight into various stages of the epidemic and the geographic variability of incidence rates across the 17 sub-regions of the Capital Health Region where prevention and control activities could be targeted.

Methods

Laboratory procedures

Prior to 2000, N. gonorrhea infections were primarily detected by culture. In 2000, the Roche Cobas Amplicor was introduced for screening certain specimens for gonorrhea infections. This test was confirmed using real time polymerase chain reaction (PCR, Roche Light Cycler) with primers directed to the 16S ribosomal RNA sequence. By 2003, 20% to 25% of all specimens were processed by culture and the rest by real time PCR or GenProbe.

Data collection

The database from the Disease Control Branch of Alberta Health and Wellness was used to identify all cases of gonorrhea diagnosed in the Capital Health Region. This region comprises approximately one million residents living in four urban centres including the city of Edmonton and semi-urban (primarily industrial) areas. Since infection with gonorrhea is a notifiable condition in Alberta, health care providers and laboratories are required to nominally report all gonorrhea infections to Alberta Health and Wellness. Public health nurses contact the attending physician for further information on reported cases. The nurses also initiate contact tracing on those individuals who have not completed treatment, as indicated by the attending physicians. We used the STI database to obtain demographic data, and where available, clinical and epidemiological data used in the analysis. Demographic data included gender, age at diagnosis, ethnicity, city and postal code of residence at the time of diagnosis. Clinical data included date of diagnosis, specimen collection site, and agency where diagnostic services were sought. Epidemiological data included sexual contact history and where available, sexual risk exposures and partner information.We assumed that among females, all cases were infected through heterosexual contact. Among the males we collected missing information on sexual exposure for those individuals seeking testing at the Capital Health Region's STI Centre through medical chart reviews.

Analysis

The incidence rates of reported gonorrhea cases between 1998 and 2003 were calculated for the Capital Health Region using the mid-year populations as the denominator.We present crude rates for overall incidence along with age and sex standardized with trends over time in the overall rates. We analyzed changes in trends in selected variables by means of a chi-squared test for linear trend using the number of cases diagnosed per year as the dependant variable and the proportion of cases identified during 1998 as the reference. The variables examined included gender, age at diagnosis, ethnicity, sexual risk exposure, and diagnostic centre.

For the spatial analysis presented in this report, we assigned confirmed cases diagnosed during the study period to one of 17 sub regional boundaries within the Capital Health Region based on their residential postal code. For each year, we calculated age-sex standardized rates for each sub-region using the indirect method⁽⁸⁾.We present maps of standard scores which give a more direct indication of sub-regional variation in relation to the region-wide rate. The standard scores were calculated using the following formula:

• i is a sub-region

• R is the region-wide rate

• SE is the sub-regional standard erro

Results

The number of cases by month and year of diagnosis is shown in Figure 1. The absolute number of incident gonorrhea cases increased almost 2.6 fold from 187 cases diagnosed during 1998 to 480 cases during 2003. Table 1 gives the age and gender specific rates of gonorrhea between 1998 and 2003. Overall, the incidence rates increased from 23.5 per 100,000 population in 1998 to 55.4 per 100,000 population during 2003 (p = 0.003). Among males the incidence rate increased from 22.7 per 100,000 population during 1998 to 65.6 per 100,000 population during 2003 (p = 0.001). Among females the corresponding rates were 24.0 per 100,000 population during 1998 and 45.1 per 100,000 population during 2003 (p = 0.017, Table 1). The highest age-specific incidence was among those between 15 and 29 years at diagnosis. Among females, 82.7 percent were in this age category whereas 50.9 percent of males belonged in this category (p = 0.025).

Figure 1. Number of N. gonorrhea cases reported to Alberta Health and Wellness from the Capital Health Region between 1 January, 1998 and 31 December, 2003

Table 1. Gender and age-specific incidence rates (per 100,000 population) of N. gonorrhea in the Capital Health Region between 1 January, 1998 and 31 December, 2003

The characteristics of individuals diagnosed with gonorrhea between 1998 and 2003 and trends over this time period are shown in Table 2. Overall there were significant increases in proportion of incident gonorrhea cases diagnosed among both men and women (p = 0.04 for both sexes), among those between 20 to 29 years of age at diagnosis (p = 0.05) and among those of Aboriginal ethnicity (p = 0.03). Although during this time period, the majority of cases (81.1%) continued to seek diagnostic services through family physicians or at the regional STI Centre located in the city of Edmonton, over the study period, there was a significant increase in the use of publicly funded walk-in centres for these services (p = 0.003, Table 2). We assumed that all female cases (n = 841, 43%) were infected through heterosexual exposure. The risk exposures for 686 of 1,116 (61.5%) male cases were known. Due to missing information on the gender of contacts we could not predict risk exposure of the remaining males. Among males attending the STI Centre 19.4% reported male-to-male sex as the primary exposure category during 2000. This proportion decreased to 9% during 2002 but then increased to 20.2% during 2003 (p = 0.97, Table 2). The trend was reversed among males reporting heterosexual contact as the primary exposure category, where the proportion of cases increased from 74.2% during 2000 to 90% during 2002 but then declined to 73.7% during 2003 (p = 0.88).

Table 2. Characteristics of cases diagnosed with N. gonorrhea in the Capital Health Region between 1 January, 1998 and 31 December, 2003.

1. Includes mixed and unknown ethnicities.

2. Includes remand centres and armed forces bases.

3. Only includes sexual risk exposures among males attending the STI Centre.

Maps of age and gender-standardized rates of gonorrhea infections in the Capital Health Region are shown in Figures 2A and 2B. Between 1998 and 2003, the cumulative incidence rates of N. gonorrhea ranged from 5.0 to 285.5 per 100,000 population (Figure 2A). The central sub-region 4, which forms the urban core of the Capital Health Region, had the highest age-sex standardised incidence rates (63.2 to 285.5 per 100,000 population). Surrounding sub-regions had incidence rates between 15.6 and 63.2 per 100,000 population. The lowest incidence rates were observed in the sub-regions on the periphery of the Capital Health Region, specifically, those sub-regions which were furthest away from the Central sub-region 4 (Figure 2A).

The geographical variability in the incidence of gonorrhea was more apparent when we analyzed the smaller sub-regions within the Capital Health Region by year (Figure 2B). Overall, the incidence rates in the central sub-region 4 were significantly higher than the rates in the other 16 sub-regions and these rates persisted resulting in less observable variation over time. During 1998, incidence rates were higher in the central and north western sub-regions compared to the Capital Health Region as a whole. Between 1999 and 2000, there was a shift from the north/north-west to increased incidence rates being observed in the south-western sub-regions. By 2003, it was largely the central sub-region 4 and sub-regions 3, 6 and 7 that immediately surround the central sub-region that continued to have incidence rates at or above those of the Capital Health Region as a whole.

Figure 2a. Cumulative age-sex standardized rates (per 100,000 population) of N. gonorrhea diagnosed between 1 January, 1998 and 31 December, 2003 among residents in 17 sub-regions of the Capital Health Region

Figure 2b. Standard scores depicting sub-regional variation in relation to the Capital Health Region-wide variation in N. gonorrhea incidence rates (clockwise, 1998-2002).

Discussion

Reported incidence rates of gonorrhea are substantially higher in the province of Alberta compared to other provinces across Canada.Within Alberta, the incidence rates are highest in the Capital Health Region. Our results, which were derived from routinely collected surveillance data, suggest the presence of a generalized outbreak of gonorrhea affecting both males and females. Missing risk exposure data on approximately one-third of the males make it difficult to determine which risk behaviours are driving this outbreak. The increase in the absolute numbers and rates of infection among females and available data on sexual exposure among males from the regional STI Centre suggest that unsafe heterosexual behaviours were the primary sexual risk exposures. However, among cases diagnosed at the STI centre, there has been a significant increase in infections acquired through male-to-male sex, particularly when comparing cases diagnosed during 2002 with 2003. Of note, there are at least 12 cases among males attending the STI Centre who self-reported as bisexuals, but we have not yet been able to determine the existence of a bridge, if any, between the homosexual and heterosexual gonorrhea cases. The notably high incidence rates among persons aged 20 to 29 at diagnosis is concerning. These observed high rates among youth has been previously reported in Alberta among those attending STI clinics⁽⁹⁾. Similarly, rising incidence rates of chlamydia and syphilis have also been reported among this group. Youth, particularly young women, may lack social power or skills to engage in safer sex or to refuse sexual activity, which may in turn lead to high-risk exposure due to multiple sex partners and subsequent acquisition of STIs^(10-13). Observed high rates of gonorrhea incidence among Aboriginal groups is also cause for concern. The gaps and needs contributing to health disparities among these groups have to be addressed.

The descriptive spatial analyses presented in this paper relied on the use of data that was already being collected as a part of routine surveillance. These analyses highlight the importance of real-time data for decision-making related to STI control. Our results suggest that the high overall rates of gonorrhea incidence in the Capital Health Region are being driven primarily by the high rates found in the central sub-region 4 and to a lesser extent by the rates found in surrounding sub-regions 3, 6 and 7. The increasing incidence rates, particularly in the central sub-region 4, indicate that new STI control strategies are required to meet the unique challenges encountered by residents. These challenges include socioeconomic deprivation, high unemployment rates, a dearth of low income housing, high mobility, and substance abuse. Our findings are similar to those reported from Manitoba^(14,15). Using spatial mapping, researchers from the United States and the United Kingdom have also shown that certain groups residing in areas characterized by low socioeconomic status and poor access to health care are most at risk for STIs^(16-21). However, other underlying factors including testing behaviours and peer-pressures may be playing a role disease transmission. These factors need to be identified and investigated through additional qualitative and quantitative studies.

Given some limitations with our approach, our findings have to be interpreted with some caution. Biases are inherent in a passive surveillance system that relies on reporting from physicians and from laboratories. The diagnostic testing patterns that are used by physicians may differ according to the way in which physicians manage their practise, or the area of residence or socioeconomic status of the patient^(22-24). A selection bias may have occurred since individuals attending diagnostic facilities are mostly seeking testing for gonorrhea. Females with mild urethritis or cervicitis or individuals with asymptomatic rectal infections may not seek diagnostic services and may hence be underrepresented. While contact tracing may help identify the population not seeking care, this process relies on information provided by the case. Such information may be subject to recall bias. Demographic and epidemiological information available in the provincial STI database is incomplete particularly with respect to risk exposures. Contact tracing can serve to complete any missing information, but it is only initiated on cases that do not complete treatment.

Despite these limitations our results are concerning. The continued high incidence rates of gonorrhea in central sub-region 4 is of particular concern given the Canadian context where, under the universal health care system, diagnostic testing and treatment, partner notification and preventative education are all offered free of charge to all individuals.While a detailed discussion of the social and policy implications of socioeconomics and health in the Canadian context are beyond the scope of this report, broader measures for disease control and prevention need to be considered to control and prevent similar outbreaks from occurring in the future. There is also a clear need to develop practical epidemiological approaches that allow us to better monitor the dynamics in the epidemiology of STIs. Our approach of using routinely collected surveillance data for spatial analyses proved to be a practical approach in identifying geographic epicentres that may be driving the gonorrhea epidemic in the Capital Health Region.We must continue to use evidencebased prevention and control strategies while simultaneously developing innovative and locally relevant approaches to keep STIs under control.

Acknowledgements

The authors would like to thank Shirin Ali, Angela Kaida and Bertha Kress at Alberta Health and Wellness for compiling and extracting the data on the gonorrhea cases. Thanks also to Ross McWhirter and Dawn Coppock at the Provincial Laboratory for Public Health (Microbiology) for data on gonorrhea test statistics and testing algorithms.

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Source: GC Jayaraman, PhD, MPH, Centre for Infectious Disease Prevention and Control, Public Health Agency of Canada; AE Singh, BMBS, FRCPC, MSc, Alberta Health and Wellness, Capital Health Region; N Yiannakoulias, MA, Alberta Health and Wellness; J Gratrix, RN, MSc, B Anderson, RN, MSc, M Johnson, MD, FRCPC, Capital Health Region; L Svenson, MSc, B Kress, RN, Alberta Health and Wellness; G J Tyrrell, PhD, Provincial Laboratory for Public Health (Microbiology); T Wong, MD, FRCPC, MPH, Centre for Infectious Disease Prevention and Control, Public Health Agency of Canada, Ottawa, Ontario.