Group A Streptococcal Flesh Eaters

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Streptococcus pyogenes was introduced in the chapters on skin and respiratory infections as the cause of strep throat, scarlet fever, and other conditions. It is also a common cause of wound infections, which have generally been easy to treat since the bacteria are consistently susceptible to penicillin. Occasionally, however, S. pyogenes infections can progress rapidly, even leading to death despite antimicrobial treatment. These more severe infections are called invasive and include pneumonia, meningitis, puerperal or childbirth fever, necrotizing fasciitis or "flesh-eating" disease, and worst of all, streptococcal toxic shock, which is similar to staphylococcal toxic shock. Deaths from invasive strep infections have caused widespread popular concern as S. pyogenes became the "flesh-eating" bacterium of the tabloid press. This section will focus on necrotizing fasciitis (figure 27.5), a rare but dramatic complication of S. pyogenes infection. ■ Streptococcus pyogenes, p. 565 ■ staphylococcal toxic shock, p. 641

Symptoms

At the site of a surgical wound or accidental trauma, sometimes even without an obvious break in the skin, severe pain develops acutely. Within a short time, swelling becomes apparent, and the injured person develops fever and confusion. The overlying skin

Strep Fasciitis
Figure 27.5 Individual with Streptococcus pyogenes "Flesh-Eating" Disease (Necrotizing Fasciitis)

becomes tense and discolored because of the swelling. Unless treatment is initiated promptly, shock and death usually follow in a short time.

Causative Agent

Streptococcus pyogenes is a ^-hemolytic, Gram-positive, chain-forming coccus with Lancefield group A cell wall polysaccharide. Current evidence indicates that the strains of S. pyogenes that cause invasive disease are more virulent than other strains by virtue of at least two extracellular products. The first of these is pyrogenic exotoxin A, which is a superantigen and causes strep-tococcal toxic shock. The second product, exotoxin B, is a protease that destroys tissue by breaking down protein.

Pathogenesis

Like Staphylococcus aureus, S. pyogenes has a fibronectin-binding protein that aids colonization of wounds. In necrotizing fasciitis, the subcutaneous fascia and fatty tissue are destroyed. Fascia are bands of fibrous tissue that underlie the skin and surround muscle and body organs. In some cases, the fascia surrounding muscle is penetrated, and muscle tissue is also destroyed. Intense swelling occurs as fluid is drawn into the area because of increased osmotic pressure from the breakdown of tissue into small molecules. The organisms continue to multiply and produce toxic products in the mass of dead tissue, using the breakdown products as nutrients. In most cases, toxic products and organisms enter the bloodstream. Superantigens and other streptococcal products cause shock by releasing cytokines, and probably by other as yet unproven mechanisms.

Epidemiology

"Flesh-eating" infections by S. pyogenes have probably occurred at least since the fifth century B.C. based on descriptions of necrotizing fasciitis by Hippocrates. More than 2,000 cases of this condition were reported among soldiers during the Civil War. Cases in this country are generally sporadic, although small epidemics have occurred such as a 1996 outbreak in San Francisco among injected-drug abusers using contaminated "black tar" heroin. The number of cases with invasive S. pyogenes in the United States was estimated at 10,000 for 1998, resulting in 1,300 deaths, 60 of which were due to necrotizing fasciitis. There is no firm evidence of a trend toward increasing incidence. Underlying conditions that increase the risk of necrotizing fasciitis and other invasive S. pyogenes infections include diabetes, cancer, alcoholism, AIDS, recent surgery, abortion, childbirth, chickenpox, and injected-drug abuse, but such infections have occurred in healthy individuals following minor injuries.

Prevention and Treatment

There are no proven preventive measures. Because of the rapidity with which the toxins spread, urgent surgery is mandatory to relieve the pressure of the swollen tissue and to remove dead tissue. Amputation is sometimes necessary, to promptly rid the patient of the source of toxins. Penicillin is the drug of choice for early infection, but it has little or no effect on streptococci in necrotic tissue and no effect on toxins, and therefore it cannot substitute for surgery.

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