Volume 23-16
August 15, 1997

[Table of Contents]

SURVEILLANCE OF INVASIVE MENINGOCOCCAL DISEASE IN CANADA, 1995-1996

Introduction

Invasive meningococcal disease (IMD) is a nationally reportable disease in Canada. Since 1985, the Laboratory Centre for Disease Control (LCDC) has enhanced IMD surveillance to correlate case-by-case information provided by the provinces and territories with detailed laboratory studies done at the National Laboratory for Bacteriology at LCDC. This report provides information on IMD from 1 January 1995 to 31 December 1996.

Methodology

Provincial and territorial ministries of health, and provincial and federal laboratories provide data for meningococcal surveillance. Both laboratory-confirmed cases and cases meeting the clinical case definition are reported to LCDC. Meningococcal serotyping and subtyping is routinely completed at LCDC by the National Laboratory for Bacteriology. Multilocus enzyme electrophoresis is completed as part of the routine surveillance of serogroup C meningococcal isolates. Data have been recorded and analyzed using Epi-Info version 6.04. All incidence rates are per 100,000 population per year.

Incidence

There were 304 cases of IMD reported across Canada during 1995, for an incidence of 1.0 per 100,000 population. During 1996, there was a 13% decrease with 265 cases reported for an incidence of 0.9. Figures 1 and 2 show the number of cases and the incidence of IMD reported across the country during these 2 years. During the past decade, there have been fluctuations in the Canadian incidence of IMD (Figure 3). The incidence reached a peak of 1.6 per 100,000 population during 1989 and 1990, and gradually decreased to 0.9 in 1996, the lowest rate in 11 years.

Seasonal Distribution

IMD showed a clear seasonal distribution with approximately one-third of cases occurring during one quarter of the year. Thirty-one percent of cases occurred from January to March in 1995, and 34% occurred during these months in 1996. In contrast, only 17% of IMD cases in 1995 and 16% in 1996 occurred during the warmer months of July to September.

Figure 1 Distribution of invasive meningococcal disease, Canada, 1995 and 1996

Figure 2 Incidence of invasive meningococcal disease, Canada, 1995 and 1996

Figure 3 Invasive meningococcal disease, Canada, 1984-1995

Age Distribution

As in previous years, the incidence rates of IMD varied inversely with age (Figure 4). Infants < 1 year of age had the greatest age-specific incidence (13.6 in 1995 and 11.1 in 1996). The incidence declined with age until the 15- to 19-year-age group, where there was a second smaller peak of 2.6 in 1995 and 2.0 in 1996. This can be compared to the much lower incidence in adults (0.5 in 1995 and 0.4 in 1996).

Case-Fatality Rates

During 1995 there were 21 deaths from IMD for a case-fatality rate (CFR) of 6.9%. During 1996, the CFR decreased slightly to 6.5% (17 deaths), the lowest rate in 11 years (Figure 5). The CFR varied by serogroup. The CFR among persons with serogroup B disease was 5% (seven deaths) in 1995 and 4% (four deaths) in 1996; whereas the CFR among persons with serogroup C disease was 12% (11 deaths) and 9% (eight deaths) in 1995 and 1996, respectively.

Serogroups

Figure 6 shows the distribution of meningococcal serogroups. Of the 304 reported cases of IMD in 1995, 11% (34 cases) were diagnosed based on the clinical case definition. Serogroup results were available for 266 isolates. Serogroups B and C were the two most commonly isolated, accounting for 48% and 38% of confirmed cases, respectively. During 1996, 17% of the 264 cases were diagnosed clinically. Serogroup results were available for 218 cases. Serogroup B accounted for 46% of isolates and serogroup C for 42%.

The age distribution of serogroup B and serogroup C diseases varied greatly. Infants with meningococcal disease were significantly more likely to be infected with serogroup B disease than serogroup C disease in both 1995 (RR = 1.9; p < 0.05) and 1996 (RR = 2.0; p < 0.05). There was no difference in gender distribution among persons with serogroup B or C disease.

Serotype and Subtype

Meningococcal strains are designated by serogroup:serotype:subtype. Serotyping and subtyping were available for 120 of the 129 serogroup B isolates from 1995 and 90 of the 100 isolates from 1996. The two most common serogroup B strains isolated during both 1995 and 1996 were B:NT:P1. - (non-serotypable, non-subtypable; 23 isolates in 1995 and 20 in 1996) and B:4:P1. - (20 isolates in 1995 and 11 in 1996). Serotyping and subtyping were available for 96 of the 101 serogroup C isolates from 1995 and 84 of the 92 isolates from 1996. Serogroup C serotypes and subtypes were more homogeneous than serogroup B. The three most common serogroup C strains isolated during both 1995 and 1996 were C:2a:P1.2.5 (41 isolates in 1995 and 36 in 1996), C:2a:P1.2 (25 isolates in 1995 and 10 in 1996), and C:2a:P1. - (20 isolates in 1995 and 22 in 1996).

Figure 4 Incidence of invasive meningococcal disease, by age, Canada, 1995 and 1996

Figure 5 Case-fatality rate from invasive meningococcal disease, Canada, 1985-1996

Figure 6 Distribution of meningococcal serogroups, Canada, 1995 and 1996

Electrophoretic Typing Electrophoretic typing was available for all of the serogroup C meningococcal isolates that had been serotyped (96 isolates in 1995 and 84 in 1996). It is noteworthy that in both years 92% of the isolates belonged to a single electrophoretic type, ET15, or its variants.

Acknowledgements

We would like to thank our colleagues from the provincial and territorial ministries of health and from the National Laboratory for Bacteriology for providing epidemiologic and laboratory data for this report.

Source : S Deeks, MD, MHSc, D Kertesz, MD, Division of Respiratory Diseases, Bureau of Infectious Disease; A Ryan, W Johnson, PhD, F Ashton, PhD, National Laboratory for Bacteriology, Bureau of Microbiology, LCDC, Ottawa, ON.