The antimicrobial activity of bupivacaine, lidocaine and mepivacaine against equine pathogens: An investigation of 40 bacterial isolates.

Lameness is the most commonly reported health problem in horses, and lameness investigations which include local anaesthetic injections are routinely performed by equine practitioners. Through this process, bacteria can enter the tissues perforated by the needle and may cause local infections at the injection site. The objective of this in vitro study was to investigate if local anaesthetics at concentrations available in commercially available solutions could inhibit growth and/or kill bacteria that could be inoculated into the synovial space or soft tissues during injection. This study evaluated the antimicrobial activity of the local anaesthetics bupivacaine, lidocaine and mepivacaine against 40 equine clinical bacterial isolates of the Actinobacillus, Corynebacterium, Enterobacter, Escherichia, Pseudomonas, Rhodococcus, Staphylococcus and Streptococcus genera. Minimum inhibitory and minimum bactericidal concentrations (MICs and MBCs) were determined by the broth microdilution method. Clinically applied concentrations of bupivacaine, lidocaine, and mepivacaine inhibited visual growth of 93%, 93%, and 80% of isolates tested, respectively. For the majority (80%) of the inhibited isolates, the concentrations were also bactericidal. The tested local anaesthetics possessed antimicrobial activity against equine pathogens at concentrations that are routinely applied in clinical cases. However, this antimicrobial activity should not discourage antiseptic preparation prior to local anaesthetic injections.

The stability and microbial contamination of bupivacaine, lidocaine and mepivacaine used for lameness diagnostics in horses.

Local anaesthetics (LAs) are frequently used for diagnostic procedures in equine veterinary practice. The objective of this study was to investigate the physico-chemical stability and bacterial contamination of bupivacaine, lidocaine and mepivacaine used for lameness examinations in horses. The LAs were stored in 12 different groups at different temperatures (-18 °C to 70 °C), light intensities and in common veterinary field conditions for up to 16 months. The pH, presence of bacterial contamination and concentrations of LAs and methylparaben (a preservative present in lidocaine) were determined serially in both new and repeatedly punctured (RP) vials. Mepivacaine remained chemically stable. A 1.9% increase in bupivacaine concentration was evident in one group, whereas a 1.9-3.7% decrease was noted in six groups. Risk factors associated with a change in concentration were light and RP vials. Lidocaine concentration decreased 6.3% in one group and increased 5.3-7.2% in two groups. Risk factors for degradation were heat and RP vials whereas storage in practice vehicles was a risk factor for increased concentrations. Methylparaben decreased 8.3-75.0% in seven groups, and RP vials, heat and storage in practice vehicles were risk factors for degradation. No contamination was present in any of the LAs and pH remained stable. Commercially available solutions of lidocaine, mepivacaine and bupivacaine stored under common veterinary field conditions are extremely stable and sterile for extended periods. The minor changes in concentration documented in this study are unlikely to affect anaesthetic efficacy during equine lameness examinations. When using products containing methylparaben, degradation of the preservative over time is to be expected.