Deeper Invasive infections caused by group A strep.
Very rarely, streptococci acquired on the skin can go on to
cause more serious disease. Once on the skin, group A streps
can enter the deeper parts of the body, by “by-passing” the
body’s normal immune defences and by using special ‘tools’
to invade through the barriers covering our body, such as the
skin. They are often helped by co-existing conditions which
might cause skin breaks, such as chicken pox, or a surgical
wound, or even a tiny crack between the toes, such as those
sometimes seen with athletes foot.
Wherever the bacteria settle, there will be local inflammation.
The body will recognise the bacteria as a danger, and the
immune system will send in white blood cells to fight the
infection, leading to inflammation. It is the inflammation that
alert us to the presence of an infection.
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Who is at risk of invasive group A strep infections?
Although there are recognised groups of people at risk from
invasive group A strep infection (skin conditions which allow
bacteria in, or medical illnesses which put them at greater risk),
almost one third have no risk factors whatsoever.
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Types of invasive group A strep infection, shown as proportions of all invasive group A strep infections.
Cellulitis and Erysipelas:
If the bacteria settle in the layers just under the skin surface,
the skin will appear reddened and swollen and fever may ensue.
This is known as “cellulitis”; a special type of infection is sometimes
seen on the face, known as “erysipelas”. The group A streps will
continue to grow (by dividing) unless they are killed by the
body’s own immune defences or by antibiotics. The skin and
tissues under the skin are arranged in layers; the rapidly dividing
streptococci simply spread along these layers, resulting in the
redness spreading up and down.
This type of infection must be treated with antibiotics; some
doctors will try tablet antibiotics first, but usually if there is fever
or other signs of severe illness, it is sensible to give antibiotics in
the form where they can act quickly, that is, intravenously,
which normally requires hospital. It is very difficult to grow
bacteria from cases of cellultis, so often the exact bug is not
identified. The group A strep is believed to be the commonest
cause of cellulitis (except where there had been a boil or insect
bite recently). Other bacteria can cause cellulitis, and
antibiotics will be tailored to fit the likely infections in any given
patient. Sometimes, cellulitis in the leg can mimic a deep vein
thrombosis (blocked vein), so doctors will perform scans to
check for that.
Once antibiotics are started, doctors will look for signs of
improvement (reduction in size of the red area, reduction in
fever or pain, improvement in blood tests). If any of these
worsen, or if the skin begins to blacken or blister on treatment,
the doctors may increase the antibiotic treatment and
consider whether the infection could have spread to deeper
tissues, or whether there are dead tissues which need to be
removed, and whether the antibiotics are targeted at the
correct bugs.
Sometimes surgery is needed to explore these possibilities;
tissue taken by the surgeons can be tested in the labs for
bacteria, and can be examined for evidence of dead tissue.
Any dead tissue (‘necrotic”) must be removed surgically,
otherwise it will continue to generate a destructive
inflammatory response in the body and proved a good
‘culture medium’ for the bacteria to thrive in. Cellulitis can
occasionally be complicated by bacteremia (bacteria
spreading into the bloodstream) and septic or toxic shock.
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