Necrotising fasciitis can be caused by a number of bacteria, but the group A strep is a leading cause in about half of all cases which arise in the community setting.

If group A streptococci spread below the thick layers under the outer skin surface, they will reach the connective tissue or “fascia” These are the fibrous bands of tissue which separate muscle bundles. Arranged in layers, with blood vessels and fats within them, these fascia provide a “freeway’ for group A streptococci to spread. Sometimes, the group A streptococci arrive in the connective tissues via the bloodstream, almost silently. It is believed that these bacteria acquire a more invasive nature, enter the bloodstream perhaps via the throat, and then ‘seed’ previously damaged soft tissues. Some patients have a memory of a seemingly trivial injury that subsequently becomes a focus for infection. The immune response will attempt to contain the infection but, in some cases, the bacteria will succeed in establishing an infection.

As the infection is deep and well below the outer skin, redness and swelling of the skin may not occur (unless there is cellulitis as well). The classical symptom is severe pain, in association with a fever or other ‘flu-like symptoms. Skin changes may begin to occur in the later stages of illness and include blistering and colour change. This is usually a clear sign that tissue in the fascia has died as a result of infection. Antibiotics will normally be administered quickly to assist in killing bacteria, but the main aim of treatment will be to remove as much dead or dying tissue as possible.

This will involve surgery, which may be carried out repeatedly, as surgeons attempt to identify dead areas of tissue and preserve parts which are not dead. Sometimes surgery is quite disfiguring, due to the part of the body involved, or the extent of disease. There is a difficult balance between preserving normal body shape and performing surgery which may be life-saving. Plastic surgery may be required in the convalescent period. Some cases of necrotising fasciitis due to group A strep are complicated by bacteremia (bacteria spreading into the bloodstream) and septic or toxic shock.

Patients who are particularly ill may be nursed in the intensive care unit. Sometimes, patients are given intravenous immunoglobulin (“IVIG”) which is an infusion of antibodies purified from large numbers of blood donations. Antibodies are proteins made by the immune system to fight infection. IVIG might help the body’s immune response to the streptococcal toxins and bacteria, although clinical trials to prove that IVIG provides extra benefit are hard to conduct.

Group A strep tends to affect the arms, legs, or less commonly, the trunk of the body. It is important to recognise that other bacteria can also cause necrotising fasciitis. Roughly half of all cases of NF are due to other bacteria which act as a ‘team’ to cause a similar aggressive infection which results in the death of tissues in the fascia. These cases often follow surgery or injury to the abdominal wall. Diabetics are more prone to the condition.

Bacteria from the bowel or groin area team up with other common skin bugs to invade the tissues of the abdominal wall (sometimes known as “Meleney’s synergisitc gangrene”) or in the groin (sometimes known as “Fournier’s Gangrene”). In these cases, the layers of tissue immediately under the skin are often involved as well, and the infection is often more obvious.

Distinguishing a simple post-surgical wound infection from necrotising fasciitis can sometimes be difficult but worsening of symptoms, blood tests, and skin changes provide good evidence. As with group A strep necrotisng fasciitis, surgery to remove all dead tissue is essential, along with antibiotics which kill all the likely bacteria.

Myositis

Occasionally, bacteria spread from the fascia to the muscle bundles themselves. The symptoms are very similar to necrotising fascitiits, 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.

Pneumonia

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 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.

N.B. Group B strep infection is a different type of streptococcal infection, which is important in pregnancy because it can 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.

Rarer deep infections caused by S. pyogenes

Occasionally, the group A strep is found to cause infections like meningitis (infection of the lining of the brain), infection of a joint (septic arthritis), bone (osteomyelitis), abdominal cavity (peritonitis), eye (ophthalmitis), or heart valve (endocarditis). Sometimes, if pus (abscess) or dead tissue has collected, needle aspiration or surgery is needed alongside antibiotics. All of these infections can be associated with bacteremia (bacteria spreading into the bloodstream; “blood poisoning”) and septic or toxic shock.

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 many cases of meningitis arise.