Group A streptococcal (GAS) infections are caused by Streptococcus pyogenes, a bacterium that is commonly found on the skin or in the upper respiratory tract (nose and throat). The term group A refers to the presence of a surface antigen on the bacterium, which helps in the microbiological identification of GAS. Streptococci are grouped according to these surface antigens, each type of antigen being allocated a letter of the alphabet; hence we have Lancefield groups A to W (excluding I and J), named after Dr Rebecca Lancefield who devised this classification scheme. Streptococci are also classified into pyogenic ('pus-forming') streptococci, which include groups A, B, C, and G, and non-pyogenic (non 'pus-forming') streptococci which encompasses the mitis, anginosus, bovis, salivarius and mutans sub-groups. Streptococcus pneumoniae, otherwise known as pneumococcus, forms its own distinct group.
The most common presentations of GAS infection are a mild sore throat ('strep throat') and skin/soft tissue infections such as impetigo and cellulitis. Scarlet fever, also caused by GAS, was once a serious childhood disease but is now less common and less severe than it used to be. Rare complications of GAS infection include acute rheumatic fever and poststreptococcal glomerulonephritis (heart and kidney diseases caused by an immune reaction to the bacteria).
GAS can also cause more serious invasive infections such as bacteraemia (an infection of the bloodstream), necrotising fasciitis (a severe infection involving death of areas of soft tissue below the skin) and streptococcal toxic shock syndrome (rapidly progressive symptoms with low blood pressure and multi-organ failure).
The most common presentation of GAS infection in the UK is a mild sore throat known as 'strep throat', with approximately 15-30% of sore throats seen by doctors thought to be caused by this. Less common is scarlet fever, with around 2000 notifications received each year.
The incidence of more serious invasive infections is relatively low. Around 3 cases per 100,000 population of bacteraemia were reported in England, Wales and Northern Ireland for 2006 (approximately 1300 cases). See Health Protection Report, 2007; volume 1 no 46.
Severe respiratory tract infections account for around 17% of all invasive GAS infections, and skin/soft tissue infections around 42%. Cases of necrotising fasciitis are even less common, accounting for approximately 5% of invasive GAS infections and usually associated with patients presenting with skin trauma as the main predisposing factor.
Invasive GAS infections are most common in the elderly (> 75years of age, approximately 8 cases per 100,000 population) and the very young (The most common risk factor for invasive GAS infection is having a skin lesion (a lesion is an abnormality due to disease or injury) of some kind which provides easier entry of the organism in to the body. A small proportion of invasive GAS infections start with a respiratory tract infection.
Whilst recent childbirth is a potential predisposing factor for GAS infection, it contributes only about 4% of the total invasive GAS infections.
GAS infections are largely spread by aerosols produced in the nose and throat of infected people, or through direct contamination of wounds.
Most severe GAS infections occur sporadically in the community with only about 9% being associated with healthcare interventions.
All GAS infections may be treated with antibiotics, such as penicillin, or erythromycin if the patient is allergic to penicillin. Early treatment improves the prognosis of invasive disease. In cases of necrotising fasciitis, surgical removal (debridement) of affected tissue is essential in addition to rapid treatment with intravenous antibiotics to prevent further spread.
Approximately 15 - 25% of people diagnosed with an invasive GAS infection will, unfortunately, die. However, this will vary considerably depending on the individual's age, type of infection, general health condition before the infection, and the strain causing the infection.