Invasive Group A Strep

Invasive Group A Strep 2021-11-17T16:06:22+00:00

Invasive infections caused by group A strep

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 may 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.


Occasionally, bacteria spread from the fascia to the muscle bundles themselves. The symptoms are very similar to necrotising fasciitits, and the treatment also involves surgery to remove any dead muscle plus antibiotics. Doctors can sometimes detect myositis (inflammation of the muscles) using blood tests, even if there is no obvious evidence at surgery. Myositis due to group A strep is often complicated by bacteremia (bacteria spreading into the bloodstream) and septic or toxic shock.


Group A strep can cause infection of the lung tissue; whether the bug reaches the lung directly from the throat, or whether it reaches the lung via the bloodstream is unknown (and may vary from case to case). Pneumonia due to group A strep was thought to be uncommon but cases are being described more commonly now. It can be a devastating infection, and, like necrotising fasciitis, can be associated with bacteremia (bacteria spreading into the bloodstream) and septic or toxic shock.

Puerperal (childbirth-related) sepsis

Group A strep used to commonly cause severe infections in women around the time of childbirth (“puerperium”). This is much less common nowadays, but, for reasons not completely understood, the weeks around childbirth still represent a risk period for group A strep infections. Group A strep, most commonly spread from infected sore throats, gain access to the pregnant (or recently pregnant) womb, probably via the genital tract. This can result in infection of the membranes covering the fetus (amnionitis, which may result in premature labour or even stillbirth) and/or infection of the lining of the womb (endometritis).

Either condition will cause illness in the mother, and bacteremia (bacteria spreading into the bloodstream) with septic or toxic shock can ensue. Group A Strep infection is considered a very serious infection in such women.

N.B. Group B strep infection is a different type of streptococcal infection, which is important in pregnancy because it can rarely result in devastating infection of the newborn. The group B strep is carried in the genital tract of some pregnant women, and babies acquire the infection at the time of birth.

Bacteremia and Septicemia

The appearance of bacteria in the blood stream is a serious indicator of deep-seated group A streptococcal infection. Bacteremia is more commonly known as “blood poisoning” and of course can be used to describe the appearance of any bug in the bloodstream. Doctors have to take special blood samples (blood cultures) to detect bacteria in the blood; because the bacteria have to grow in the lab in order to be detected and identified, doctors often do not know the result for a day or two.

Sometimes, doctors may guess that the group A strep is a likely cause of an illness, for example, in necrotising fasciitis, and antibiotics and surgery are planned accordingly. Often, however, the first indication that an infection is caused by group A strep in will be, that the blood culture shows a group A strep. Doctors may have started antibiotics using a ‘best guess’ approach. Fortunately, group A strep is killed by most of the antibiotics commonly used in a wide range of conditions.

It is widely believed that group A strep bacteremia can itself cause a focus of infection in the body by “seeding” abnormal areas, such as a bruised muscle, or faulty heart valve. Seeding appears to be the way in which cases of meningitis arise, although these conditions are exceedingly rare.