Susceptibility of rapidly growing mycobacteria isolated from Australian cats to ivermectin, moxidectin, ceftiofur and florfenicol.

OBJECTIVES: Rapidly growing mycobacteria (RGM) infections in cats typically manifest as a panniculitis, requiring long-term antimicrobial therapy for resolution. The search for novel antimicrobial therapies to reduce treatment duration and improve the rate of clinical resolution is imperative. Accordingly, RGM isolates underwent susceptibility testing to some avermectins and other antibacterial drugs currently available.
METHODS: Five Mycobacterium fortuitum and six Mycobacterium smegmatis isolates obtained from Australian cats underwent susceptibility testing by microbroth dilution to ivermectin, moxidectin, ceftiofur and florfenicol.
RESULTS: All isolates were resistant to the highest concentrations of ivermectin, moxidectin and ceftiofur tested (1024 µg/ml, 256 μg/ml and 32 μg/ml, respectively). All isolates of M fortuitum were resistant to the highest concentration of florfenicol tested (128 µg/ml). The minimum inhibitory concentration range of florfenicol that inhibited growth of M smegmatis isolates was 32-64 µg/ml. CONCLUSIONS AND RELEVANCE: All drugs appear to have no efficacy in vitro for the treatment of RGM infections. © ISFM and AAFP 2015.