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Minimum Inhibitory Concentrations of Tulathromycin and Other Select Antimicrobials for Isolates of Corynebacterium pseudotuberculosis Obtained from Clinical Samples

KE Washburn, JA Miga*, MC Libal

Objective – To determine the in vitro susceptibility of Corynebacterium pseudotuberculosis to tulathromycin, in addition to certain other antimicrobials. C. pseudotuberculosis is the causative agent of caseous lymphadenitis (CL), a significant disease of small ruminants. There is no consistently rewarding antibacterial used in the treatment of CL, possibly due to poor penetration of the drug into characteristic abscesses. Therefore, an antimicrobial that can reach and maintain efficacious inhibitory concentrations in these abscesses would be beneficial in the treatment of this disease. Tulathromycin, a triamilide semisynthetic macrolide, is currently labeled for respiratory disease of cattle and swine; however it may be a good candidate for the treatment of CL because of its pharmacokinetic properties.

Sample population – Thirty-five confirmed isolates of Corynebacterium pseudotuberculosis were obtained from clinical cases at the Texas A&M Univeristy College of Veterinary Medicine Large Animal Hospital.

Procedure –  Isolates were tested by disk diffusion and microbroth dilution for susceptibility to tulathromycin. MIC data was concurrently gathered for other antimicrobials present on the commercial susceptibility testing plate.

Results –  The range of MICs of tulathromycin for C. pseudotuberculosis was ≤ 1 – 2 µg/ml, with 25 of the 35 isolates yielding an MIC of ≤ 1µg/ml. The MIC data for the other antimicrobials is also reported. Disk diffusion zones of inhibition for tulathromycin ranged from 23 – 31 mm in diameter.

Clinical Relevance – Further studies may show that obtaining and  maintaining an intralesional tulathromycin concentration of 2 µg/mL in CL patients may show efficacy in treating this disease. Additional studies are needed to correlate the disk diffusion data with MIC data.