Discussion

Introduction:
Antimicrobial use in equine orthopaedics can be divided into prophylactic and therapeutic indications. A rational approach to antimicrobial use requires that there be an element of reasoning, logic or scientific basis behind the decision-making process of choosing an antimicrobial drug for a particular use. In addition, the questions of timing, dosing and delivery should also have a rational basis. Each of these aspects of antimicrobial use, as they pertain to equine orthopaedics will be addressed, with particular emphasis on using a scientific basis for decision making. Unfortunately, the science is incomplete in this field, and so logic, educated reasoning and extrapolation from other species is often required to complete the argument for specific antimicrobial uses in equine orthopaedics.

Prophylactic use of antimicrobials in orthopaedics Prophylactic use of antimicrobials has clearly been shown to reduce the rate of surgical site infection for clean and clean-contaminated orthopaedic procedures in humans and is typically recommended by most national healthcare agencies to minimise patient morbidity (Mangram et al. 1999; Miliani et al. 2009; Weese and Cruz 2009). Equine orthopaedic surgeons have generally adopted a similar approach to prophylactic antimicrobial use. Recently, concerns regarding hospital acquired enteric infections (such as Clostridium difficile) have added to the longer standing concern of promoting bacterial resistance as a counterbalancing argument against antimicrobial prophylaxis. Similar concerns regarding the development of colitis following elective orthopaedic procedures exist in equine surgery. Prophylactic antimicrobial use should follow these principles regarding drug choice, timing and duration of administration:
1. Drug - use a safe, bacteriocidal and relatively inexpensive drug that is effective against the spectrum of expected surgical contaminants or potential pathogens.
2. Timing - give an appropriate intravenous dose within 1 - 2 h of skin incision to ensure effective tissue and serum concentration of drug during the procedure.
3. Duration - continue administration to maintain effective tissue drug concentrations for up to 24 h following the procedure, including intraoperative dosing as necessary (Miliani et al. 2009).

A recent retrospective review of prophylactic antimicrobial use for equine arthroscopic surgery found that these guidelines for both timing and duration of administration were often not followed. Only 6% of horses received an intravenous dose of antimicrobial within 1 h of skin incision, none of the horses received intraoperative dosing despite prolonged duration from the initial dose, and 34% of horses were treated for greater than
24 h after skin closure (Weese and Cruz 2009).

Therapeutic use of systemic antimicrobials in orthopaedics:
Systemic antimicrobials remain central to the treatment of orthopaedic infections in the horse, despite recent emphasis on methods of local drug delivery. It is well known that therapeutic antimicrobial use should, whenever possible, be based upon microbial culture and sensitivity data. However, none of our patients presenting for treatment initially arrive with those necessary laboratory results!! Therefore, an empirical approach to initial therapy is necessary, typically using a broad-spectrum combination of drugs. The widely utilised combination of an aminoglycoside (amikacin or gentamicin) and a beta-lactam antimicrobial such as penicillin in horses fits many of the criteria for an initial choice; including spectrum of activity, safety and cost. While culture and sensitivity data are often utilised, the clinician must be reminded that they are not always reflective of the in vivo efficacy against a particular organism and such factors as drug penetration, immune avoidance, bacterial sequestration within tissues (including biofilm formation) and development of specific bacterial resistance all add to the complexity of choosing an appropriate systemic antimicrobial.

Therapeutic use of local antimicrobials in orthopaedics Local antimicrobial delivery methods in horses have evolved considerably over the past 20 years. Several specific methods of drug delivery are available with each having a particular set of advantages and disadvantages. The premise for using local drug delivery in horses for orthopaedic infections is 3-fold:
1. High drug concentrations (relative to systemic dosing) may be achieved at the site of infection.
2. Systemic toxicity may be reduced.
3. Otherwise cost prohibitive drugs may be considered for use when indicated.

Direct synovial injection, continuous synovial infusion, regional limb perfusion (intravenous or intra-osseous), implantable antimicrobial impregnated beads (absorbable or nonabsorbable) or collagen sponges have all been investigated in the horse. While a strong trend for their use clinically has persisted, conclusive evidence for efficacy beyond systemic antimicrobials in the treatment of orthopaedic infections is still lacking.

Conclusion:
Guidelines for either prophylactic or therapeutic antimicrobial use in equine orthopaedics lack the scientific rigour that is available from large randomised clinical trials and meta-analyses in human medicine. A rational approach to antimicrobial use continues to rely on a combination of data available in horses, as well as extrapolation from other species, and logical arguments applied directly to the clinical setting. The clinician often also faces the added pressures of client economics and patient soundness for athletic use when treating equine orthopaedic infections.