Definitions

 Prophylactic antibiotic - the administration of antibiotic in the absence of infection.

 Therapeutic antibiotic - the use of antibiotics to treat an established bacterial infection.

 Perioperative antibiosis refers to the administration of prophylactic antibiotics in the period immediately before, during and potentially for a limited time following a surgical procedure (usually not more than 24 hours).

Introduction

The aim of prophylactic antibiosis is to achieve effective concentrations of antibiotic in tissues before bacterial contamination occurs and therefore reduce the degree of contamination below the critical level required to induce an infection. Prophylactic antibiosis is only intended to encompass the time period of the procedure itself including approximately 4 hours following closure of the surgical wound (until a fibrin seal is formed) and is not intended to prevent postoperative contamination.

Therapeutic antibiosis is used in the treatment of an infection and requires the prescription of a course that extends beyond clinical cure. Surgical site infections (SSIs) are defined as any infection occurring at the surgical site within 30 days, or 1 year if implants remain in situ. Therapy is based on identification and sensitivity testing of the causative organism.

Wound Infection

Bacterial contamination during a surgical procedure can originate from the flora of the animal itself (endogenous bacteria) or the environment or temporary skin contaminants (exogenous bacteria). All surgical wounds become contaminated with bacteria, but not all become infected. It follows then that a critical level of contamination is required before surgical site infection (SSI) occurs but this level is influenced by a number of key factors.

Classification of Surgical Wounds

Surgical wounds can be classified according to the level of bacterial contamination present at the time of the procedure (Figure 1). Veterinary clean and clean-contaminated procedures are associated with an approximately 2–5% risk of postoperative infection while contaminated and dirty procedures are reported as having approximately 10–12% risk.

Figure 1. The four basic classifications of surgical wounds.

Factors Affecting Surgical Wound Infection

Host Risk Factors

Animals suffering from systemic conditions, endocrinopathies (e.g., diabetes mellitus) or receiving medication that reduce the competence of their immune system are usually considered more susceptible to infection.

Additional factors that affect the animal as a whole and should be considered in relation to increased susceptibility to infection include geriatric status, severe malnutrition, active infection elsewhere in the body, obesity, American Society of Anesthesiologists' preoperative assessment score (ASA score) ≥ 3 and being an intact male.

Surgical Wound Factors

Local factors that are associated with increased risk of infection following surgery include:

 Clipping the patient prior to induction - the skin reaction associated with clipping seems to allow increased numbers of bacteria to grow on the surface.

 Inadequate skin preparation. Endogenous bacteria represent the most common surgical wound contaminants.

 Length of general anaesthesia. Increased length of anaesthesia has been associated with increased risk of postoperative infection in veterinary patients undergoing clean or clean-contaminated procedures.

 Surgical time. A procedure lasting 90 minutes has been associated with a two-fold increase in risk compared to one lasting 60 minutes with each additional hour of surgery doubling the risk.

 Areas of devitalised tissue including excessive use of diathermy. Compromised tissue represents an ideal growth substrate for microorganisms.

 Seroma/haematoma formation. Again these environments provide excellent microorganism growth media (particularly for Staphylococcus species) exacerbated by the poor activity of the host immune response in this fluid environment.

 Foreign material. Any foreign material (e.g., orthopaedic implants, suture material or contamination by road debris) will provide a focus for microorganisms to 'hide' from the host immune system and increase in numbers.

 Classification of the wound as 'dirty' before the procedure.

 Previous local radiotherapy. Surgery carried out on areas previously treated by radiotherapy have been associated with significant risk of postoperative infection.

Pathogen Factors

In addition to the host and wound risk factors, the ability of bacteria to set up an infection and the level of damage they cause (virulence and pathogenicity) vary between bacteria species; for example Staphylococcus aureus has a thick capsule that helps resist phagocytosis by white blood cells. The cell wall surrounding gram-negative bacteria offers some protection from the immune system, resists penetration by antibiotics and also forms toxic breakdown products when the cells die (endotoxin), all of which increase the virulence and pathogenicity of gram-negative bacteria compared to gram-positive bacteria.

The bacteria most frequently associated with small animal surgical wound contamination are endogenous skin flora, particularly Staphylococcus pseudintermedius and Staphylococcus aureus, although in cats Pasteurella multocida is also a common endogenous organism and SSI isolate. In addition, Escherichia coli is also relatively frequently cultured due to its association with the urogenital and gastrointestinal systems in small animals.

Surgery carried out on different systems and areas within the animal will also risk contamination by organisms present as endogenous flora in that particular area.

Prevention of Surgical Site Infection (SSI)

Complete elimination of SSIs is an unrealistic goal and there will be a minimum level of infection that will occur regardless of the surgeon, facilities and level of care provided. Therefore, the aim must be to reduce the incidence of SSI to this minimal level. The reduction of SSI occurrence to this base level can only be achieved by a multilayered approach to case and hospital management:

 Thorough patient assessment. Identification of the host-associated risk factors and thorough planning of the procedure to minimise anaesthetic and surgical time.

 Strict aseptic technique. Theatre design, personnel clothing, patient preparation, instrument sterilisation, etc.

 Theatre personnel. The risk of SSI is directly influenced by the number of people in theatre.

 Perioperative antibiotic protocol. Strict adherence by all staff to a defined protocol will reduce SSI occurrence but only in conjunction with the other steps.

 Routine hand hygiene of an appropriate standard between handling animals in the hospital.

 Surveillance. One member of the hospital team should take responsibility for ensuring thorough and regular monitoring of sterilisation procedures, assessment and control of environmental contamination within the hospital and monitoring of the incidence of SSIs. The designated team member can then identify and address potential outbreaks of infection as they occur, and the information can be used to determine an appropriate perioperative antibiotic protocol.

 Effective pain relief. Animals receiving effective pain relief will eat, heal, fight infection, allow nursing care and be discharged more quickly and efficiently, which reduces risk of infection.

Antibiotic Prophylaxis in the Surgical Patient

Haphazard administration of antibiotics leaves individual animals open to infection and provides the ideal environment for the emergence of resistant strains of bacteria.

Every veterinary practice should design a perioperative antibiotic protocol that outlines use of an appropriate standard choice of antibiotic at effective dosages. The protocol should also provide clear guidelines for the appropriate assessment of each surgical case including the risk factors discussed above. This assessment can then be used in order to decide on whether antibiotic prophylaxis is indicated and whether continuation as postoperative therapy is also warranted.

Many procedures designated as 'clean' and lasting less than 90 minutes do not require prophylactic antibiotic.

The following considerations should be given when designing the protocol for use of prophylactic antibiotic in appropriate cases:

 Antimicrobial selection. Broad-spectrum bactericidal activity against likely pathogens including efficacy against Staphylococcus and Pasteurella species. Potentiated amoxicillin, first-generation cephalosporins (e.g., cephazolin) and second-generation cephalosporins (e.g., cefuroxime) are common choices.

 Intravenous administration. The antimicrobial should be safe, cost effective and rapidly distributed within the tissues. Intramuscular or subcutaneous administration routes are usually far slower to reach maximum levels in the tissue, may not reach high enough tissue concentrations and may require the dose to be given several hours before induction of anaesthesia.

 Dose (Figure 2). The protocol should ensure that the dose administered is in the upper half of the therapeutic range to ensure that the appropriate concentration is reached at the operative site and to maximise efficacy.

 Additional dosage. Evidence suggests that re-administration should occur within two half-lives of the antimicrobial agent. Generally if the procedure is likely to last longer than 3 hours, additional dosages of the prophylactic antibiotic are administered following the initial dose (e.g., potentiated amoxicillin every 2 hours or up to 3 or 4 hours for cefazolin). The drug, route and quantity administered should be identical to that given at induction. Some surgeons will request antibiotic every 90 minutes although the evidence for this requirement is lacking

 Prophylactic administration. The drug should remain at effective levels until the fibrin seal has formed postoperatively (approximately 4–6 hours). Continuation of antibiotic use for up to 24 hours may be performed when one or more risk factors for infection are present, although the evidence in favour of this is lacking. There is no evidence in the human or veterinary literature to support administration of perioperative antibiotic beyond 24 hours postoperatively.

 Extension of use. Rates of SSI in humans who receive antimicrobials for 24 hours postoperatively are the same as in those who receive a 5-day course and there is therefore currently no evidence to support the extension of prophylactic antimicrobial administration beyond the operative period. This includes those patients considered at increased risk of SSI during the preoperative assessment. In fact, in addition to needlessly adding to the cost of the procedure, inappropriate administration beyond 24 hours to this group of animals will significantly add to the selection pressure for resistant organisms within the hospital environment.

Figure 2. Prophylactic antibiotic dosages.