eMedicine Specialties > General Surgery > Wounds

Wound Infection

Author: Hemant Singhal, MD, MBBS, FRCSE, FRCS(C), Senior Lecturer, Department of Surgery, Imperial College School of Medicine, UK; Consultant Surgeon, Northwick Park and St Marks Hospitals, UK
Coauthor(s): Kanchan Kaur, MBBS, MS, MRCS (Ed), Clinical Fellow, Department of Surgery, Northwick Park Hospital, UK; Charles Zammit, MD, Senior Specialist Registrar, Department of Surgery, Breast Unit Charing Cross Hospital of London, England
Contributor Information and Disclosures

Updated: Sep 8, 2009

Introduction

Background

History

The ancient Egyptians were the first civilization to have trained clinicians to treat physical aliments. Medical papyri, such as the Edwin Smith papyrus (circa 1600 BC) and the Ebers papyrus (circa 1534 BC), provided detailed information of management of disease, including wound management with the application of various potions and grease to assist healing.^1,2 (See images below and Images 1-2.)

The scale of wound infections was most evident in times of war. During the American Civil War, erysipelas (necrotizing infection of soft tissue) and tetanus accounted for over 17,000 deaths, according to an anonymous source in 1883. Because compound fractures at the time almost invariably were associated with infection, amputation was the only option, despite a 25-90% risk of amputation stump infection.

Koch (Professor of Hygiene and Microbiology, Berlin, 1843-1910) first recognized the cause of infective foci as secondary to microbial growth in his 19th century postulates. Semmelweis (Austrian obstetrician, 1818-1865) demonstrated a 5-fold reduction in puerperal sepsis by hand washing between performing postmortem examinations and entering the delivery room. Joseph Lister (Professor of Surgery, London, 1827-1912) and Louis Pasteur (French bacteriologist, 1822-1895) revolutionized the entire concept of wound infection. Lister recognized that antisepsis could prevent infection.⁴ In 1867, Lister placed carbolic acid into open fractures to sterilize the wound and to prevent sepsis and hence the need for amputation. In 1871, Lister began to use carbolic spray in the operating room to reduce contamination. However, the concept of wound suppuration persevered even among eminent surgeons, such as John Hunter, 1728-1793.⁵

World War I (WWI) resulted in new types of wounds from high-velocity bullet and shrapnel injuries coupled with contamination by the mud from the trenches. Antoine Depage (Belgian military surgeon, 1862-1925) reintroduced wound debridement and delayed wound closure and relied on microbiological assessment of wound brushings as guidance for the timing of secondary wound closure.⁶ Alexander Fleming (microbiologist, London, 1881-1955) performed many of his bacteriological studies during WWI and is credited with the discovery of penicillin.

As late as the 19th century, aseptic surgery was not routine practice. Sterilization of instruments began in the 1880s as did the wearing of gowns, masks, and gloves. Halsted (Professor of Surgery, Johns Hopkins University, United States, 1852-1922) introduced rubber gloves to his scrub nurse (and future wife) because she was developing skin irritation from the chemicals used to disinfect instruments. The routine use of gloves was introduces by Halsted's student J. Bloodgood.

Penicillin first was used clinically in 1940 by Howard Florey. With the use of antibiotics, a new era in the management of wound infections commenced. Unfortunately, eradication of the infective plague affecting surgical wounds has not ended because of the insurgence of antibiotic-resistant bacterial strains and the nature of more adventurous surgical intervention in immunocompromised patients and in implant surgery.

Pathophysiology

Wound healing is a continuum of complex interrelated biological processes at the molecular level. Healing is divided into the following phases for descriptive purposes: inflammatory phase, proliferative phase, and maturation phase.

The inflammatory phase commences as soon as tissue integrity is disrupted by injury; this begins the coagulation cascade to limit bleeding. Platelets are the first of the cellular components that aggregate to the wound, and, as a result of their degranulation (platelet reaction), they release several cytokines (or paracrine growth factors). These cytokines include platelet derived growth factor (PDGF), insulinlike growth factor-1 (IGF-1), epidermal growth factor (EGF), and fibroblast growth factor (FGF). Serotonin is also released, which, together with histamine (released by mast cells), induces a reversible opening of the junctions between the endothelial cells, allowing the passage of neutrophils and monocytes (which become macrophages) to the site of injury.

This large cellular movement to the injury site is induced by cytokines secreted by the platelets (chemotaxis) and by further chemotactic cytokines secreted by the macrophages themselves once at the site of injury. These include transforming growth factor alpha (TGF-alpha) and transforming growth factor beta (TGF-beta). Consequently, an inflammatory exudate that contains red blood cells, neutrophils, macrophages, and plasma proteins, including coagulation cascade proteins and fibrin strands, fills the wound in a matter of hours. Macrophages not only scavenge but they also are central to the wound healing process because of their cytokine secretion.

The proliferative phase begins as the cells that migrate to the site of injury, such as fibroblasts, epithelial cells, and vascular endothelial cells, start to proliferate and the cellularity of the wound increases. The cytokines involved in this phase include FGFs, particularly FGF-2 (previously known as basic FGF), which stimulates angiogenesis and epithelial cell and fibroblast proliferation. The marginal basal cells at the edge of the wound migrate across the wound, and, within 48 hours, the entire wound is epithelialized. In the depth of the wound, the number of inflammatory cells decreases with the increase in stromal cells, such as fibroblasts and endothelial cells, which, in turn, continue to secrete cytokines. Cellular proliferation continues with the formation of extracellular matrix proteins, including collagen and new capillaries (angiogenesis). This process is variable in length and may last several weeks.

In the maturation phase, the dominant feature is collagen. The dense bundle of fibers, characteristic of collagen, is the predominant constituent of the scar. Wound contraction occurs to some degree in primary closed wounds but is a pronounced feature in wounds left to close by secondary intention. The cells responsible for wound contraction are called myofibroblasts, which resemble fibroblasts but have cytoplasmic actin filaments responsible for contraction.

The wound continuously undergoes remodeling to try to achieve a state similar to that prior to injury. The wound has 70-80% of its original tensile strength at 3-4 months postoperative.

Frequency

United States

Surgical site infections (SSIs) are not an extinct entity; they account for 14-16% of the estimated 2 million nosocomial infections affecting hospitalized patients in the United States.⁷

International

Internationally, the frequency of SSI is difficult to monitor because criteria for diagnosis might not be standardized. A survey sponsored by the World Health Organization demonstrated a prevalence of nosocomial infections varying from 3-21%, with wound infections accounting for 5-34% of the total.⁸ The 2002 survey report by the Nosocomial Infection National Surveillance Service (NINSS), which covers the period between October 1997 and September 2001, indicates that the incidence of hospital acquired infection related to surgical wounds in the United Kingdom is as high as 10% and costs the National Health Service in the United Kingdom approximately 1 billion pounds (1.8 billion dollars) annually.

Collated data on the incidence of wound infections probably underestimate true incidence because most wound infections occur when the patient is discharged, and these infections may be treated in the community without hospital notification.

Mortality/Morbidity

SSIs are associated not only with increased morbidity but also with mortality. Seventy-seven percent of the deaths of surgical patients were related to surgical wound infection.⁹ Kirkland et al calculated a relative risk of death of 2.2 attributable to SSIs, compared to matched surgical patients without infection.¹⁰

Clinical

History

Surgical site infection is a difficult term to define accurately because it has a wide spectrum of possible clinical features.

The Centers for Disease Control and Prevention (CDC) have defined SSI to standardize data collection for the National Nosocomial Infections Surveillance (NNIS) program.^11,12 SSIs are classified into incisional SSIs, which can be superficial or deep, or organ/space SSIs, which affect the rest of the body other than the body wall layers.

• Definitions of surgical site infection (see image below and Image 3) 
  • Superficial incisional SSI: Infection involves only skin and subcutaneous tissue of incision.
  • Deep incisional SSI: Infection involves deep tissues, such as fascial and muscle layers. This also includes infection involving both superficial and deep incision sites and organ/space SSI draining through incision.
  • Organ/space SSI: Infection involves any part of the anatomy in organs and spaces other than the incision, which was opened or manipulated during operation.

• Superficial incisional SSI is more common than deep incisional SSI and organ/space SSI. Superficial incisional SSI accounts for more than half of all SSIs for all categories of surgery. The postoperative length of stay is longer for patients with SSI, and when adjusted for other factors influencing length of stay.

Physical

According to a report by the NNIS program,¹³ surgical site infections are defined as follows:

• Superficial incisional SSI
  • Occurs within 30 days after the operation
  • Involves only the skin or subcutaneous tissue
  • At least 1 of the following:
    • Purulent drainage is present (culture documentation not required).
    • Organisms are isolated from fluid/tissue of the superficial incision.
    • At least 1 sign of inflammation (eg, pain or tenderness, induration, erythema, local warmth of the wound) is present.
    • The wound is deliberately opened by the surgeon.
    • The surgeon or clinician declares the wound infected.
  • Note: A wound is not considered a superficial incisional SSI if a stitch abscess is present; if the infection is at an episiotomy, a circumcision site, or a burn wound; or if the SSI extends into fascia or muscle.
• Deep incisional SSI
  • Occurs within 30 days of the operation or within 1 year if an implant is present
  • Involves deep soft tissues (eg, fascia and/or muscle) of the incision
  • At least 1 of the following:
    • Purulent drainage is present from the deep incision but without organ/space involvement.
    • Fascial dehiscence or fascia is deliberately separated by the surgeon because of signs of inflammation.
    • A deep abscess is identified by direct examination or during reoperation, by histopathology, or by radiologic examination.
    • The surgeon or clinician declares that a deep incisional infection is present.
• Organ/space SSI
  • Occurs within 30 days of the operation or within 1 year if an implant is present
  • Involves anatomical structures not opened or manipulated during the operation
  • At least 1 of the following:
    • Purulent drainage is present from a drain placed by a stab wound into the organ/space.
    • Organisms are isolated from the organ/space by aseptic culturing technique.
    • An abscess in the organ/space is identified by direct examination, during reoperation, or by histopathologic or radiologic examination.
    • A diagnosis of organ/space SSI is made by the surgeon or clinician.

Causes

All surgical wounds are contaminated by microbes, but in most cases, infection does not develop because innate host defenses are quite efficient in the elimination of contaminants. A complex interplay between host, microbial, and surgical factors ultimately determines the prevention or establishment of a wound infection. Factors that affect surgical wound healing are classified in the chart below and in Image 4.

• Microbiology: Microbial factors that influence the establishment of a wound infection are the bacterial inoculum, virulence, and the effect of the microenvironment. When these microbial factors are conducive, impaired host defenses set the stage for enacting the chain of events that produce wound infection.
  • Most SSIs are contaminated by the patient's own endogenous flora, which are present on the skin, mucous membranes, or hollow viscera. The traditional microbial concentration quoted as being highly associated with SSIs is that of bacterial counts higher than 10,000 organisms per gram of tissue (or in the case of burned sites, organisms per cm² of wound).¹⁴
  • The usual pathogens on skin and mucosal surfaces are gram-positive cocci (notably staphylococci); however, gram-negative aerobes and anaerobic bacteria contaminate skin in the groin/perineal areas. The contaminating pathogens in gastrointestinal surgery are the multitude of intrinsic bowel flora, which include gram-negative bacilli (eg, Escherichia coli) and gram-positive microbes, including enterococci and anaerobic organisms. See Table 1 for pathogens and their frequencies. Gram-positive organisms, particularly staphylococci and streptococci, account for most exogenous flora involved in SSIs. Sources of such pathogens include surgical/hospital personnel and intraoperative circumstances, including surgical instruments, articles brought into the operative field, and the operating room air.
  • The most common group of bacteria responsible for SSIs are Staphylococcus aureus. The emergence of resistant strains has considerably increased the burden of morbidity and mortality associated with wound infections.
  • Methicillin resistant Staphylococcus aureus (MRSA) is proving to be the scourge of modern day surgery. Like other strains of S aureus, MRSA can colonize the skin and body of an individual without causing sickness, and, in this way, it can be passed on to other individuals unknowingly. Problems arise in the treatment of overt infections with MRSA because antibiotic choice is very limited. MRSA infections appear to be increasing in frequency and are displaying resistance to a wider range of antibiotics.
  • Of particular concern are the vancomycin intermediate Staphylococcus aureus (VISA) strains of MRSA. These strains are beginning to develop resistance to vancomycin, which is currently the most effective antibiotic against MRSA. This new resistance has arisen because another species of bacteria, called enterococci, relatively commonly express vancomycin resistance.

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Table

Pathogen	Frequency (%)
Staphylococcus aureus	20
Coagulase-negative staphylococci	14
Enterococci	12
Escherichia coli	8
Pseudomonas aeruginosa	8
Enterobacter species	7
Proteus mirabilis	3
Klebsiella pneumoniae	3
Other streptococci	3
Candida albicans	3
Group D streptococci	2
Other gram-positive aerobes	2
Bacteroides fragilis	2

Pathogen	Frequency (%)
Staphylococcus aureus	20
Coagulase-negative staphylococci	14
Enterococci	12
Escherichia coli	8
Pseudomonas aeruginosa	8
Enterobacter species	7
Proteus mirabilis	3
Klebsiella pneumoniae	3
Other streptococci	3
Candida albicans	3
Group D streptococci	2
Other gram-positive aerobes	2
Bacteroides fragilis	2

• Risk factors (other than microbiology)
  • Decreased host resistance can be due to systemic factors affecting the patient's healing response, local wound characteristics, or operative characteristics.
    • Systemic factors include age, malnutrition, hypovolemia, poor tissue perfusion, obesity, diabetes, steroids, and other immunosuppressants.
    • Wound characteristics include nonviable tissue in wound; hematoma; foreign material, including drains and sutures; dead space; poor skin preparation, including shaving; and preexistent sepsis (local or distant).
    • Operative characteristics include poor surgical technique; lengthy operation (>2 h); intraoperative contamination, including infected theater staff and instruments and inadequate theater ventilation; prolonged preoperative stay in the hospital; and hypothermia.
  • The type of procedure is a risk factor. Certain procedures are associated with a higher risk of wound contamination than others. Surgical wounds have been classified as clean, clean-contaminated, contaminated, and dirty-infected (see Table 2).

Table 2: Surgical Wound Classification and Subsequent Risk of Infection (If No Antibiotics Used)^11,15

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