Cellulitis

If 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. The 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, at least initially. Outpatient intravenous therapy is increasingly used for many cases. It is very difficult to grow bacteria from cases of cellulitis, 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), but group C and G streps can also cause this condition. 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.

Necrotising fasciitis

Necrotising fasciitis can be caused by a number of bacteria, though the group A strep is a leading cause in about half of all cases that occur in the community setting. Group C and G can also cause this condition, though rarely.
If 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 streptococci to spread. Again, 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 that 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 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 causing disease, although there have been no clinical trials that show this yet (and it has not particularly been used for group C or G strep infection).
Streptococci tend 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 synergistic 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 strep necrotising fasciitis, surgery to remove all dead tissue is essential, along with antibiotics that kill all the likely bacteria.

Pneumonia

Group C and G streps can cause infection of the lung tissue; whether the bugs reach the lung directly from the throat, or whether they reach the lung via the bloodstream is unknown (and may vary from case to case). Pneumonia due to these types of streptococci is very rare. Like necrotising fasciitis, can be associated with bacteremia (bacteria spreading into the bloodstream) and septic or toxic shock.

Bone and joint infection

Both group C and G streps can spread from the skin into joints , often those that are previously damaged by injury or arthritis. This may cause swelling, redness, and pain over the joint. Sometimes, the joint infection (arthritis) spreads into the bone, causing ‘osteomyelitis’. This type of infection requires prolonged antibiotic treatment, alongside drainage of any pus around the joint or bone.

Rarer deep infections caused by groups C and G

Occasionally, streps can cause infections like meningitis (infection of the lining of the brain), infection of the genital tract/womb after childbirth (puerperal sepsis), infection of the newborn, 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 Septicaemia

The appearance of bacteria in the blood stream is a serious indicator of deep-seated 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 which bug 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 due to strep will be, that the blood culture is positive and grows a strep, usually the following day or two days later. Special tests will be done to check which group the strep belongs to (i.e. group A, B, C, F, or G) Doctors may have started antibiotics using a ‘best guess’ approach. Fortunately, streps are killed by most of the antibiotics commonly used in a wide range of conditions.
Bacteremia is usually seen only in cases of invasive or deep infection. Occasionally, doctors cannot work out the site of a strep infection in the body, but will try and investigate using blood tests and scans. This is important because a patient might need surgery in addition to antibiotics.