Surgical site infections (SSI) are defined as infections that develop at the surgical site within 30 days of surgery (or within 90 days for some surgeries such as breast, cardiac and joint surgeries including implants).

- SSIs can cause significant morbidity and mortality as well as economic burden if left untreated

- SSI affects up to one third of patients who have undergone a surgical procedure, incidence is higher following abdominal operations

- In India, several studies reported SSI rate ranging from 4 to 11 per 100 surgeries.

Microbiology

 The type of etiological agents implicated in SSI depends upon the site of surgical procedure and the source of infection from which they are acquired.

- Endogenous source such as the patient’s own flora present on

* Skin: S. aureus (the most common organism causing SSI), coagulase negative staphylococci (CoNS)

* Mucosa (from opened viscus such as GIT, respiratory or genitourinary): Consists of predominantly aerobic gram-negative bacilli (E.coli, Klebsiella), gram positive cocci (Enterococcus) and anaerobes such Bacteroides, and others.

- Exogenous source from contact with the operative room personnel or instruments or environment: S. aureus and gram-negative bacilli including nonfermenters such as Pseudomonas and Acinetobacter.

The inoculum load and the virulence of the microorganism can determine the risk of SSI

- Inoculum of bacteria: Surgical procedures involving the sites (e.g. bowel, vagina) which are heavily colonized with bacteria have a higher risk of developing SSI as large inoculum of bacteria lodge into the wound during surgery

- Virulence of bacteria: Higher is the virulence of infecting organism, more is the risk of development of SSI.

Pathogenesis and risk factors

In most patients, infection does not develop at surgical site due to the presence of strong host innate immunity eliminating microbial contaminants at the surgical site. However, when host defense mechanisms fail to eliminate the microbial contamination, compounded by the greater load and higher virulence of the invading microbes, all together pave path to the development of SSI.

The risk factors leading to the development of SSIs can be classified into patient-related, procedure-related, organism-related and environmental-related risk factors (Table 1); out of which the type of wound class is the most important, discussed below.

Table1. risk factors for development of SSI.

Wound Class Type

 Depending up on the degree of microbial contamination, wounds are classified as clean, clean-contaminated, contaminated, or dirty/infected. The contaminated and dirty wound classes have a higher risk of developing SSI.

Class I, Clean

 Wound It is an uninfected operative wound in which no inflammation is encountered and the hollow viscus such as respiratory, alimentary, genital, or urinary tract is not entered. SSI rate is usually less than 2% in clean operated wounds.

Class II, Clean-contaminated Wound

It is an operative wound in which the hollow viscus such as respiratory, alimentary, genital, or urinary tracts are entered under controlled conditions and without unusual contamination.

- Surgeries included in this category involve the biliary tract, appendix, vagina, and oropharynx; provided that no evidence of infection or major breach in the technique is encountered

- SSI rate is about 3% to 11% in clean-contaminated wounds.

Class III, Contaminated Wound

Contaminated wound includes the following:

- Open, fresh, accidental wounds

- Operations with major breaks in the sterile technique (e.g. open cardiac massage) or

- Operations with gross spillage from the gastrointestinal tract (colonic surgeries)

- Entry into biliary or genitourinary tract in the presence of infected bile or urine

-Incisions in which acute, nonpurulent inflammation is encountered including necrotic tissue without evidence of purulent drainage (for example, dry gangrene).

SSI rate is >10% in contaminated wounds even with administration of surgical antimicrobial prophylaxis.

Class IV, Dirty/Infected Wound

Surgical procedures performed when active infection is already present are considered dirty wounds. Examples include:

- Abdominal exploration for acute bacterial peritonitis or perforated viscera

- Intra-abdominal abscess

- Old traumatic wounds with retained devitalized tissue. In dirty wound, the SSI rate can exceed 20–40%.

classification and diagnosis of SSI

 SSIs are classified based on the level where infection developed into three types:

- Superficial SSI—develops at the level of superficial incisional site (skin and subcutaneous level) within 30 days regardless of the type of surgery

- Deep SSI—develops at the level of deep incisional site (muscle and facial level) within 30 days for all surgeries except for breast, cardiac and implant surgeries (90 days)

- Organ space SSI—develops at the level of organ space site within 30 days for all surgeries except breast, cardiac and implant surgeries (90 days).

 The criteria for diagnosis of above mentioned three types of SSIs have been discussed in detail under surveillance of SSI, subsequently in this chapter.

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