ARCHIVED - 4.0 Epidemiology of Invasive GAS Disease in Canada

Invasive GAS disease became nationally notifiable in January 2000. The most recent year for which complete national data have been published is 2001. The overall incidence of disease in 2001 was 2.7 per 100,000 population. The highest reported incidence rates occurred among adults ≥ 60 years of age (5.3 per 100,000), followed by children < 1 year of age (4.8 per 100,000) and children 1 to 4 years of age (3.6 per 100,000)^(8). Preliminary data for subsequent years show slight variation in overall incidence: 2.8 per 100,000 in 2002, 3.2 per 100,000 in 2003 and 2.6 per 100,000 in 2004 (unpublished data, Public Health Agency of Canada).

Elevated rates of invasive GAS disease have been detected among Aboriginals living in the Canadian Arctic through the population-based International Circumpolar Surveillance system. Between 2000 and 2002, no cases of invasive GAS disease were reported among non-Aboriginals in the territories, northern Quebec or northern Labrador. In contrast, among Aboriginals in northern Canada, the incidence rate of disease was 9.0 per 100,000 in 2000 (7 cases), 3.0 per 100,000 in 2001 (2 cases) and 5.0 per 100,000 in 2002 (4 cases)^(9-11). Preliminary data from 2003 also indicate a higher rate of invasive GAS disease in Aboriginal (2.6 per 100,000) compared with non-Aboriginal (1.9 per 100,000) populations (seven cases overall)⁽¹²⁾.

The NCS provides laboratory reference services for GAS associated with invasive disease. Among 2,195 isolates from blood, brain or cerebrospinal fluid (CSF) between 1993 and 1999, the most frequent M protein type identified was M1 (28%), followed by serotypes M28 (9%), M12 (8%), M3 (8%) and M4 (6%)⁽¹³⁾. With the exception of 2000-2001, M1 was consistently the most frequently encountered serotype between 1992 and 2004-2005^(13-16).The NCS also tests S. pyogenes isolates for antibiotic sensitivity. Of 817 isolates from all sites tested in 2004-2005, 11.1% demonstrated resistance to erythromycin, and 2.0% were resistant to clindamycin. No isolates demonstrated resistance to penicillin, chloramphenicol or vancomycin⁽¹⁶⁾.

Clinical data for cases of invasive GAS infection are not collected nationally. Data from the Ontario GAS Study provide further information on the epidemiology of invasive disease. In cases detected by this enhanced population-based surveillance system during 1992 and 1993, the most common clinical presentations were skin or soft-tissue infections (48%), bacteremia with no septic focus (14%) and pneumonia (11%)⁽¹⁷⁾. Thirteen percent of cases were classified as streptococcal toxic shock syndrome (STSS), and 6% had necrotizing fasciitis (NF). The overall case fatality rate (CFR) was 13%, although syndrome-specific CFRs were highest among patients with STSS (81%), NF (45%) and pneumonia (33%). Overall, 44 (14%) of infections were classified as nosocomial, including 14 cases acquired in LTCF for the elderly. The risk of invasive GAS disease was significantly associated with several underlying conditions, including HIV infection, cancer, heart disease, diabetes, lung disease and alcohol abuse. Among the isolates for which serotyping was available, the most common serotypes were M1 (24%), M12 (7.4%), M4 (6.5%), M28 (6.2%) and M3 (5.8%)⁽¹⁷⁾. More recent findings from Ontario enhanced surveillance system from 1992 through 1999 demonstrate the increasing incidence of specific clinical manifestations of invasive GAS infections. The annual incidence rate of NF increased from 0.08 per 100,000 population in 1992 to 0.49 per 100,000 in 1995 (p < 0.001)⁽¹⁸⁾. The annual incidence of GAS pneumonia increased from 0.16 per 100,000 in 1992 to 0.35 per 100,000 in 1999⁽¹⁹⁾.

Enhanced GAS surveillance in Alberta between 2000 and 2002 showed higher provincial incidence rates of disease than observed through national passive surveillance, at 5.0 (in the year 2000), 5.7 (2001) and 3.8 (2002) per 100,000, with corresponding CFRs of 10.7%, 13.2% and 6.8%, respectively. Incidence rates were highest in the metropolitan regions of Calgary (6.9 per 100,000) and Edmonton (4.8 per 100,000). Acquisition in a hospital, nursing home or LTCF was the most frequently reported risk factor (17%), followed by injection drug use (13%), pregnancy related risk factors (13%), varicella (12%) and cancer (11%). The most common serotypes were M1 (16%), M3 (12%) and PT2967 (10%). There was seasonal variation, the greatest number of cases occurring in the winter and early spring months⁽²⁰⁾.