Figure 1.3 Weekly distribution of Streptococcus pyogenes bacteraemia -England, Wales, Northern Ireland and Channel Islands, 1995-2003. Source: Health Protection Agency.
others appear to result in more subtle immunoparesis. Injecting-drug users are now recognised as a major risk group for invasive GAS disease, accounting for up to a fifth of cases in some countries (Factor etal. 2003; Lamagni etal. 2004). Most infections are thought to be sporadic community-acquired infections, around 5% resulting from nosocomial infection. However, importantly, in approximately 30% of invasive disease cases there is no underlying medical or other predisposing condition. This distinguishes the GAS from other P-haemolytic streptococci.
Invasive GAS infections exhibit a distinct seasonal pattern, with striking late winter/early spring peaks (Figure 1.3). How much of this can be attributed to winter vulnerability to GAS pharyngitis and carriage and how much is likely to be the result of postviral infection susceptibility to bacterial infections is unclear.
The mortality from invasive GAS disease is high (14-27%) and rises considerably with patient age and number of organ system failures, reaching 80% in the presence of STSS (Davies etal. 1996; Efstratiou 2000). STSS complicates roughly 10-15% of all cases of invasive GAS disease, accounting for most patients with invasive GAS disease who require admission to an intensive care facility (Eriksson etal. 1998).
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