Surgical and Traumatic Wound Infections

Surgical and Traumatic Wound Infections

Craig E. Greene and J. Scott Weese

Surgical site infections (SSIs), which hinder successful healing of operative wounds, are recognized as a relatively uncommon but inherent risk of every surgical procedure.51 Although every surgical wound becomes contaminated with bacteria, only a few become infected. The use of aseptic methods and minimization of tissue trauma were major breakthroughs in reducing postoperative infections. Subsequent development and use of antibacterials in association with surgery further reduced numbers of SSIs. However, SSIs will continue to be an inherent risk, and the risks will likely increase over time as a result of more invasive and prolonged surgical procedures, wider use of synthetic implants, and continued emergence of and dissemination of multidrug-resistant bacteria.


The skin is a complex organ that serves as a barrier to contamination. It has a complex endogenous microflora that comprises two main groups, the resident and transient populations. The resident skin microflora persists on the skin at all times without causing pathologic changes. The transient microflora is found on the skin for temporary periods. Although the resident microflora is the most refractory to removal during skin preparation, it is the transient microflora that is more likely to be pathogenic and causes most clinical infections.

When the physical barrier of the skin is compromised, as from surgical incisions, the resulting wound must progress through the predictable stages of wound healing. As mentioned, bacterial contamination of the wound bed occurs with every surgical procedure, but most surgical wounds do not develop an infection. The success of wound healing is influenced by the overall health of the patient, the surgical procedure and type of wound created, and the type and number of contaminating organisms introduced into the wound. In a prospective study of postoperative surgical site infections in dogs and cats, the risk of infection was statistically associated with greater duration of surgery, increasing number of operating room participants, and decreased cleanliness of the surgical site.19 These and other factors regarding postoperative surgical site infections are reviewed next.

The major source of bacteria that contaminate surgical wounds is the patient’s endogenous microflora. Surgical incisions can also be infected from the hands of veterinary personnel or owners and the environment, but these are typically of lesser concern. Infections with methicillin-resistant staphylococcal organisms have been of some concern because of their zoonotic implications (see Chapters 34 and 99).61a Skin-associated bacteria can be reduced but not eliminated by antisepsis. Bacteria residing in the deeper parts of the skin such as the hair follicles and sebaceous glands are not removed or killed by preparative scrubbing, and they may enter deeper tissues during the initial incision. Development of SSIs, however, is not a simple matter of bacterial contamination. Whether or not an infection develops involves the interaction of bacterial (organism, virulence, dose, antimicrobial resistance), patient (immune status, comorbidities), and procedure (tissue injury, foreign materials) factors.

In humans, the National Research Council (NRC) has established a surgical wound classification system that is summarized in Web Table 53-1 and in Web Box 53-1. This system has been used in veterinary medicine, although it usefulness in veterinary patients has been questioned.10 In a study of 1574 dogs and cats with clean, clean contaminated, contaminated, or dirty surgical wounds, infection rates were 4.7%, 5%, 12%, and 10%, respectively.10 Anaerobic bacteria should always be assumed to be a component of mixed surgical infections,17 or numerous therapeutic failures could result.

WEB BOX 53-1   National Research Council Surgical Wound Classification

Clean surgical wounds involve surgical incision sites with no prior trauma or inflammation, no breaks in sterile technique during surgery, and no contact with mucosal surfaces such as the respiratory, genitourinary, or alimentary tracts. Infection rates in animals with clean surgical wounds are very low when experienced surgeons perform the procedures and in some reports have been as low as 0.9%.53

Clean contaminated surgical wounds involve minor breaks in surgical technique (such as a torn glove), contact with normal mucosae of the gastrointestinal (GI) tract without spillage of visceral contents, or contact with uninfected genitourinary, biliary, or respiratory tracts. They also include otherwise clean procedures involving drain placement. Infection rates in humans for these surgical procedures are less than 10% when aseptic techniques are used.52

Contaminated surgical wounds have accidental GI tract spillage from penetration of an infected viscus or tissue, foreign bodies, devitalized tissue, or pus, or involve a break in sterile technique. Bacterial contamination is suspected, but purulent discharge is absent. Clean lacerations of the skin or subcutaneous tissues that are not already infected are often categorized as contaminated. Contaminated surgical wounds have a high risk for developing postoperative infections and are two times more likely to become infected than clean contaminated wounds.19,52 For example, after colonic spillage, isolated pathogens are often mixed, and up to five species may be present. Aerobic species usually include Escherichia coli and enterococci, and anaerobes include Bacteroides species, anaerobic cocci, and clostridia (see Chapter 88, Intra-Abdominal Infections).47

Infected (dirty) surgical or traumatic wound infections are surgical wounds or nonsurgical defects that are already infected or have breaks in the skin associated with blunt trauma. Devitalized tissues, foreign bodies, or purulent discharges are often observed. Examples of dirty wounds include previously perforated viscera, devitalized wounds, compound fractures, foreign bodies, pus pockets, and acute cellulitis. Traumatic wounds are assumed to be contaminated and have a high risk of infection because microbes bypass the anatomic and immunologic barriers of the host. Necrotizing soft tissue infections are often colonized by numerous anaerobic and aerobic bacteria.18 Bacteria from these wounds have often already spread systemically. Establishment of tissue infection depends on the type and depth of the injury, extent of damage to the vascular supply, extent of tissue devitalization, and length of delay in seeking treatment. With orthopedic injuries, posttraumatic osteomyelitis occurs when a broken bone has a contaminated open wound, an avascular fragment, and a milieu of damaged necrotic tissue or hemorrhage9 (see Musculoskeletal Infections, Chapter 85). Subsequent to trauma and hospitalization, urinary and intravenous catheter infections are the most common infections, followed by pneumonia, intra-abdominal infections, and wound infections. Responsible organisms are often staphylococci, E. coli, Enterobacter, Pseudomonas, and Klebsiella. (See Chapter 51 and reviews by Holt and Griffin28 and Underman66 for a discussion of bite wound infections; see Chapter 33 for a discussion of necrotizing fasciitis; see Chapter 93 for a thorough discussion of surgical preparations needed to prevent postoperative infections.53)

The National Research Council (NRC) wound classification system has been widely used since its development in 1964. However, the risk for development of infection depends on many factors other than the wound environment, and use of wound classification alone provides limited information. In an attempt to account for other such factors in estimating the risk of wound infection, the National Nosocomial Infections Surveillance System (NNISS) developed an index to predict risk of surgical infection.14 A risk index score of 0, 1, 2, or 3 is based on the sum of three risk factors: (1) an American Society of Anesthesiologists preoperative assessment score of 3, 4, or 5 (Web Box 53-2); (2) an operation with an NRC classification of contaminated or dirty; and (3) an operative time of X hours, where X represents the time in which 75% of the given surgical procedures are completed. When the index was developed, it was compared with the NRC classification of surgical wound infections from more than 80,000 surgical procedures. Surgical wound infection rates for the NRC classification system were 2.1%, 3.3%, 6.4%, and 7.1% for each respective category. The NNISS index surgical wound infection rates were 1.5% with zero risk factors present, 2.9% with one risk factor present, 6.8% with two risk factors present, and 13% with three risk factors present. Compared with the NRC classification, risk stratification was significantly increased with the NNISS index. Comparable assessment has not been performed for veterinary patients, but awareness of factors that result in increased risk in humans is useful as they are likely also relevant in veterinary medicine.

The process of wound healing should be viewed as a complex interaction involving the patient, the local wound environment, and the contaminating pathogen. Each facet of this interaction has many associated factors, the combinations of which are unique to each situation. A thorough assessment of these factors and the common sources of perioperative contamination (Box 53-1) should be considered when preparing for each surgical procedure so that the most appropriate steps can be taken to minimize the risk for the development of postoperative infection (Box 53-2). However, there have been few proper risk factor investigations regarding SSIs in small animals, and most current practices are based on extrapolation from human medicine along with subjective determination of “best practices.”

BOX 53-2   Methods for Reducing Risk of Surgical Wound Infection


Use prophylactic antimicrobials (1) when manipulating contaminated tissues or intestine, (2) for prolonged surgical procedures (longer than 3 hours), and (3) with synthetic implants.

Keep surgical room free of dust and insects.

Keep surgical field clean, and use routine antiseptic technique.

Ensure that incision and dissection are accurate and sharp.

Avoid excessive use of electrocautery.

Prevent normal skin or mucosal flora from contacting body cavities or internal tissues.

Minimize drying and exposure of handled tissues.

Surgically debride all tissues to healthy vascular areas.

Remove all foreign bodies, avascular tissue, and dead space.

Irrigate contaminated areas with antimicrobial or disinfectant solutions.

Avoid circulatory compromise.

Use meticulous hemostasis to decrease risk of tissue hemorrhage and blood clots (hematoma formation).

Place stab wound drains at other than incision sites.

Delay surgical closure with dead space.

Handle soft tissues and abdominal viscera gently.

Change suction tips during prolonged surgical procedures.

Reduce number of people in the operating room.

Preoperative preparation of the surgical site is a critical factor in preventing wound infection (see Chapter 93). Skin trauma produced during surgical preparation greatly increases the local bacterial population. Preoperative clipping time is an important factor in the development of postoperative wound infections.10 Animals with surgical sites that were clipped before anesthesia induction rather than immediately before surgery were three times more likely to develop surgical wound infections. Animals clipped hours or days before the surgery because ultrasonographic studies were performed had a postoperative infection rate three times greater.10 This presumably relates to mild skin trauma or abrasion that predisposes to proliferation of bacteria and inhibition of the efficacy of surgical skin preparation. Animals with endocrinopathies such as hypothyroidism or hyperadrenocorticism are much more likely to develop postoperative wound infections.46 Intact males were also found to have a higher risk of wound infection, which parallels findings in people and rodents and is presumably a result of the inhibitory effect of testosterone on some inflammatory cytokines.59 Many additional risk factors have been identified in humans, and some could also apply to veterinary patients, such as diabetes mellitus, obesity, malnutrition, immunosuppressive therapy, and colonization with specific pathogens such as methicillin-resistant Staphylococcus aureus.4,38

Use of proper surgical technique is perhaps the single most important factor in preventing postoperative infections. The risk of tissue infection is directly proportional to the increased amount of tissue handling and trauma. Vascular compromise to tissue, excessive electrocautery, and bleeding into tissue spaces are the major contributory factors. Foreign material and blood clots allow for adherence and replication of microorganisms and facilitate formation of biofilms. Experimentally, use of fibrinolytic agents prevents infections, abscesses, and adhesions after surgery. Bacteria that invade surgical sites or implants can remain dormant for months to years, and although less common, the sites can be entered during the healing and recovery phases. In some animals, occult orthopedic infections develop. The bacteria remain at the site of the healed fracture but do not cause clinical or radiographic evidence of osteomyelitis.16 The infection may persist locally and be refractory to treatment until the orthopedic implants are removed. Surgical wounds of pets may be contaminated with the human commensal S. aureus, from contact with veterinary personnel or owners, which may lead to infections that are resistant to treatment if methicillin-resistant strains are involved (see Chapter 34).42,49,49

Orthopedic implants have greatly improved treatment of bone fractures and noninfectious arthritis, but they are associated with an increased risk for orthopedic device–related infection.20b,75 Similar risks have been found for cardiovascular implants in dogs, where Pseudomonas aeruginosa and Staphylococcus spp. were most commonly isolated.20 Vascular implants have a high risk of infection in dogs, which can be partially controlled by the use of antimicrobial-impregnated materials.58 Immediately after implantation, all synthetic materials undergo a race in colonization of their surface by tissue and bacterial cells in the local area. In addition to body fluids containing serum proteins (albumin) and platelets, bacteria such as staphylococci have adhesins that attach bacteria to the biomaterial. Adherence leads to colonization of the foreign body surface, which can result in overt infection or bacteremia. Under these conditions, bacteria become sessile and develop antimicrobial resistance in this quiescent phase. Standard antimicrobial therapy at this time may eliminate the clinical illness, but the bacteria may persist in the biofilm. Various organisms such as coagulase-negative staphylococci and P. aeruginosa are able to produce a pathogenic biofilm composed of polysaccharide glycocalyx (slime). With strict anaerobic conditions or with molecular identification techniques, anaerobes such as Propionibacterium species are often recognized.65 They form an additional layer on the surface of prosthetic implants that can originate during the surgical procedure.3 The glycocalyx slime promotes intercellular adhesion, captures nutrients, and protects microorganisms from antibacterial therapy.

The use and timing of prophylactic antimicrobial therapy affects infection rates in animals with clean surgical wounds. Guidelines in human surgery have been established to justify their use and ensure maximal effects with minimal consequences; however, implementation of standardized protocols has been inconsistent in veterinary practice.71 In a study at a teaching hospital, 72.5% of 1100 animals with clean surgical wounds received perioperative antibacterials, which were associated with a lower infection rate.68 However, in another comparable study in which 41% of 1146 animals with clean surgical wounds received antibacterials at varying times, the infection rate varied according to the timing of the antibacterial administration.10 Animals with clean wounds receiving perioperative antibacterials, no antibacterials, or postoperative antibacterials had infection rates of 2.2%, 4.4%, and 8.2%, respectively. In another study at a teaching hospital, dogs undergoing elective orthopedic surgery were divided into three groups and given either no antimicrobials, penicillin G, or cefazolin 30 minutes before surgery, and again if the surgery lasted longer than 90 minutes.74 Dogs in both antibacterial treatment groups had lower infection rates; therefore the control group was abandoned. Results of these studies emphasize the importance of having maximal antibacterial activity at the time of the surgical procedure. (See Tables 53-1 and 53-2 and the Drug Formulary in the Appendix for a list of dosages.)

TABLE 53-1

Indications and Drugs for Antimicrobial Prophylaxis or Treatment in Surgery

Surgical Class Examples Associated Bacteria Recommended Therapy
First Choice Alternatives
Cleana Routine surgery None None None
Clean contaminated Genital surgery Aerobes: gram-negative
Cefazolinb Fluoroquinolone
Prolonged (>3 hr) surgery, orthopedic prosthesis, amputation, open fracture reduction Escherichia coli, staphylococci, streptococci Cefazolinb β-Lactamase-resistant penicillin
Intra-abdominal Aerobes: gram-negative
Cefoxitinc Gentamicin, metronidazole
Dentistry Aerobes: gram-positives and anaerobes Cefazolinb Ampicillin, amoxicillin, chloramphenicol
Contaminated Bite wounds Aerobes and anaerobes Ampicillin or amoxicillin-clavulanate Clindamycin
Enterotomy with leakage, abdominal trauma Aerobes: streptococci, enterococci
Anaerobes: bifidobacteria, clostridia, fusobacteria, Bacteroides
Cefoxitinc or, for enterococci, use ampicillin or amoxicillin Aminoglycoside, metronidazole
Biliary infection cholecystectomy Enterobacteriaceae (E. coli, Klebsiella, Proteus), Bacteroides, Clostridium Cefoxitinc, cefotaximed Gentamicin
Colonic resectione E. coli, Bacteroides Neomycin and metronidazole preanesthesia, and enemas Cefoxitin, gentamicin, clindamycin
Infected Abscesses Aerobes Ampicillin or amoxicillin-clavulanate Aminoglycoside, clindamycin
Ruptured bowel, colonic leakage Anaerobes Cefotaximed Metronidazole
Pyometra Aerobic and anaerobes Cefazolinb Fluoroquinolone

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Aug 6, 2016 | Posted by in INTERNAL MEDICINE | Comments Off on Surgical and Traumatic Wound Infections

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