Rapid Emergence of a New Clone Impacts the Population at Risk and Increases the Incidence of Type Group A Invasive Disease.

Background: Invasive group A (iGAS) disease caused by type strains has been increasing worldwide, driven by the emergence of an epidemic clonal variant (clade 3 ). The clinical characteristics of patients with iGAS disease, and in particular with clade 3 iGAS disease, are poorly described.

Methods: We used population-based iGAS surveillance data collected in metropolitan Toronto, Ontario, Canada during the period 2000-2014. We sequenced the genomes of 105 isolates representing all iGAS disease cases in the area during the period and 138 temporally matched iGAS isolates collected elsewhere in Ontario.

Results: Clades 1 and 2 and clade O, a newly discovered genetic variant, caused most cases of iGAS disease in metropolitan Toronto before 2008. After rapid emergence of new clade 3, previously circulating clades were purged from the population and the incidence of iGAS disease significantly increased from 0.14 per 100000 in 2000-2007 to 0.22 per 100000 in 2008-2014. Overall, organisms caused significantly more arthritis but less necrotizing fasciitis than strains of the more common type . Other clinical presentations were soft tissue and severe respiratory tract infections. Clinical outcomes did not differ significantly between clades overall. However, clade 3 iGAS disease was more common in youth and middle-aged individuals.

Conclusions: The rapid shift in iGAS strain genetics in metropolitan Toronto has resulted in a significant increase in the incidence of iGAS disease, with noticeably higher rates of clade 3 disease in younger patients.