Volume 17, Number 11—November 2011
Group A Streptococcus emm Gene Types in Pharyngeal Isolates, Ontario, Canada, 2002–2010
Suggested citation for this article
Group A Streptococcus (GAS) is a gram-positive bacterial pathogen responsible for ≈600 million cases of pharyngitis each year worldwide (1). The widespread prevalence of this disease results in considerable costs, estimated to exceed $200 million annually in the United States alone (2). In addition to acute pharyngitis, GAS causes several other human diseases, ranging from relatively mild to more severe, such as necrotizing fasciitis, soft tissue infections, glomerulonephritis, acute rheumatic fever, and streptococcal toxic shock syndrome. Thus, infections caused by GAS are a major public health concern in the United States and Canada and throughout the world.
AbstractGroup A Streptococcus (GAS) is a human-adapted pathogen that causes a variety of diseases, including pharyngitis and invasive infections. GAS strains are categorized by variation in the nucleotide sequence of the gene (emm) that encodes the M protein. To identify the emm types of GAS strains causing pharyngitis in Ontario, Canada, we sequenced the hypervariable region of the emm gene in 4,635 pharyngeal GAS isolates collected during 2002–2010. The most prevalent emm types varied little from year to year. In contrast, fine-scale geographic analysis identified inter-site variability in the most common emm types. Additionally, we observed fluctuations in yearly frequency of emm3 strains from pharyngitis patients that coincided with peaks of emm3 invasive infections. We also discovered a striking increase in frequency of emm89 strains among isolates from patients with pharyngitis and invasive disease. These findings about the epidemiology of GAS are potentially useful for vaccine research.
GAS strains are classified mainly on the basis of variation in a cell-surface molecule known as M protein, encoded by the emm gene (3,4). M protein is a critical virulence factor and a major site of the human antibody response against GAS. M type–specific immunity develops in persons recovering from some GAS infections (5,6). As a result, the portion of the emm gene that encodes the amino-terminal 100 residues of M protein is under strong diversifying selection pressure, and this region is hypervariable in terms of GAS types (7). Currently, >120 distinct emm types of GAS are recognized.
Despite the considerable diversity of emm types of GAS isolates, epidemiologic studies have found that relatively few emm types tend to predominate within a local population; most isolates are composed of a small number of emm types (8,9). In distinct geographic areas, the predominant emm types often vary in frequency from year to year for reasons not fully understood. In addition, sizeable outbreaks can be caused by strains of a single emm type or of a small number of emm types. Overall, this combination of factors results in a complex epidemiologic situation for GAS pharyngitis.
Recently, vaccine candidates have been identified in an effort to reduce the prevalence of GAS disease and the number of human deaths it causes (10). Some of these experimental vaccines are based on the amino-terminus of M protein because of the type-specific immunity that may develop after GAS infection. A multivalent vaccine has been developed that exploits the amino-terminus of the M protein from many different emm types (11). In principle, the effectiveness of this type of M-protein vaccine may be highly dependent on how well the M proteins selected for the vaccine match the emm types of locally circulating strains. Thus, a more complete understanding of geographic and temporal variation in emm type may be useful for vaccine design. Furthermore, the emergence of new variants of known M types has been documented. Knowledge of the rate and patterns of emergence of distinct emm types and their alleles may be critical for understanding how GAS may “escape” the immune response generated by a vaccine based on the amino-terminus of M protein.
We investigated the distribution of GAS emm types causing pharyngitis in Toronto, Ontario, Canada, during 2002–2010. We also examined the temporal change in emm types in pharyngitis cases and compared this distribution with data from a comprehensive population-based study of GAS emm types that were causing invasive infections in Ontario. Finally, we studied the emm types causing pharyngitis in multiple geographic locations across the province of Ontario in 2009 and 2010.
Group A Streptococcus emm Gene Types in Pharyngeal Isolates, Ontario, Canada, 2002–2010 - Vol. 17 No. 11 - November 2011 - Emerging Infectious Disease journal - CDC