Local anesthetics and antibiotics display synergistic and antagonistic drug interactions against pathogens causing septic arthritis in horses.

This research investigates the effect of combining local anaesthetics with antibiotics to treat septic arthritis in horses, finding that in some cases the combination enhances antibiotic action, while in others it diminishes it.

Research Methodology

• The study explored the impact local anesthetics (LAs) such as bupivacaine hydrochloride, lidocaine hydrochloride, and mepivacaine hydrochloride, when paired with certain antibiotics namely amikacin, ceftiofur, and gentamicin.
• The antibiotic-LA pairs were often used for intrarticular (IA) injections and regional limb perfusion (RLP) in veterinary equine medicine. The researchers aimed to understand what effect these LAs have on antibiotic efficiency.
• To do this, 17 clinical isolates from equine patients were subjected to the checkerboard method, a standard tool used to assess the minimum inhibitory concentration (MIC) needed to inhibit bacterial growth of various antibiotic-LA combinations. This study was conducted between August 2020 and December 2023.

Significant Findings

• The study found that for most combinations, the effectiveness of the antibiotic was not impacted by the addition of the LA.
• However, they found a number of combinations (n = 70) where the LA had either a positive (synergistic; n = 14) or negative (antagonistic; n = 56) effect on the antibiotic’s efficacy.
• In the case of antagonism, increased MICs were seen, but these usually remained within the clinically relevant range, meaning they likely won’t have a significant impact on treatment outcomes.
• One exception was with aminoglycoside-LA combinations, where antagonism resulted in MICs higher than those achievable by RLP, suggesting potential negative impacts on treatment outcomes.
• In synergistic combinations, the MIC often reduced, sometimes from levels way above the clinically relevant range to within or below that range, marking an increased efficacy of the antibiotic due to the LA presence.

Conclusion and Recommendations

• The study concluded that adding LAs to antibiotic solutions for IA injections does not usually compromise antibiotic effectiveness in the treatment of common equine bacterial infections.
• On the contrary, for RLP, adding LAs may decrease antibiotic efficacy at clinically relevant concentrations.
• The researchers therefore caution against carelessly combining LAs with certain aminoglycosides (amikacin and gentamicin) in RLP without considering the specific antimicrobial susceptibility as tested via MICs.