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Chapter: Medical Microbiology: An Introduction to Infectious Diseases: Skin and Wound Infections

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Wound Infections

Wounds subject to infection can be surgical, traumatic, or physiologic.

WOUND INFECTIONS

Wounds subject to infection can be surgical, traumatic, or physiologic. The latter include the endometrial surface, after separation of the placenta, and the umbilical stump in the neonate. Traumatic wounds comprise such diverse damage as deep cuts, compound frac-tures, frostbite necrosis, and thermal burns. Sources of infection include (1) the patient’s own normal flora; (2) material from infected individuals or carriers that may reach the wound on fomites, hands, or through the air; and (3) pathogens from the environment that can contaminate the wound through soil, clothing, and other foreign material. Examples of such infections include contamination of a penetrating stab wound to the abdomen by colonic flora, contamination of a clean surgical wound in the operating room with S. aureus spread from the flora of a perineal carrier, and introduction of spores of Clostridium tetani into the tissues on a splinter.

Classification of Wounds

Surgical and traumatic wounds are classified according to the extent of potential contami-nation and thus, the risk of infection. These criteria carry important implications regard-ing surgical treatment and chemoprophylaxis. Clean wounds are surgical wounds made under aseptic conditions that do not traverse infected tissues or extend into sites with a normal flora. Clean contaminated wounds are operative wounds that extend into sites with a normal flora (except the colon) without known contamination.Contaminatedwounds include fresh surgical and traumatic wounds with a major risk of contamination,such as incisions entering nonpurulent infected tissues. Dirty and infected wounds in-clude old, infected traumatic wounds; wounds substantially contaminated with foreign material; and wounds contaminated with spillage from perforated viscera.

Infection rates in clean surgical wounds should be less than 1%, whereas untreated dirty wounds have a higher probability of infection. Similar considerations apply to the chance of infection developing in a placental site or on the umbilicus. A normal delivery without retained products will rarely be followed by endometrial infection. A prolonged delivery after rupture of the membranes with retained placental fragments poses an in-creased risk. In some rural cultures in Africa, soil is applied to the umbilical stump, and neonatal tetanus is common, whereas it is almost unknown in other cultures.

Factors Contributing to Wound Infection

Various factors, in addition to those indicated previously, contribute to the probability of a wound becoming infected. The contaminating dose of microorganisms and their virulence can be critical and, other things being equal, the chance of infection developing increases progressively with the contaminating dose. The physical and physiologic condition of the wound also influences the probability of infection. Areas of necrosis, vascular strangula-tion from excessively tight sutures, hematomas, excessive edema, poor blood supply, and poor oxygenation all compromise normal defense mechanisms and substantially reduce the dose of organisms needed to initiate infection. Thus, removal of necrotic tissue and the surgeon’s skill, gentleness, and attention to detail are major factors in preventing the development of infection.

The general health, nutritional status, and ability of patients to mount an inflammatory response are also major determinants of whether a wound infection develops. Infection rates are higher in the elderly, the obese, individuals with uncontrolled diabetes, and those on immunosuppressive or corticosteroid therapy. Nutritional deficiencies enhance the risk of infection, and new approaches to avoid protein – calorie malnutrition in patients with severe burns, for example, have led to substantial reductions in serious clinical infections.

There is strong evidence that the critical period determining whether contamination of surgical wounds proceeds to infection lies within the first 3 hours after contamination.

For this reason, prophylactic chemotherapy of some surgical wounds and procedures can be restricted to the operative and immediate perioperative period. There is general agree-ment that extending such prophylaxis beyond 24 hours increases the chance of complica-tions without reducing the risk of infection.

Treatment and Prevention

Severe wound infections are almost always treated with a combination of surgical and chemotherapeutic approaches. Necrotic tissue and contaminated foreign bodies, such as sutures, must be removed, pockets of pus opened, and drainage established. This ap-proach permits access of the appropriate antibiotics to viable tissues in which they can act. Epidemiologic approaches to the prevention of wound infection and the appropriate uses of chemoprophylaxis are considered. There has been increasing inter-est in the possibilities of active or passive immunization against common Gram-negative antigens. Despite some encouraging experimental results, the clinical application of these findings to burns and severe trauma seems distant.

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