In vitro susceptibility of Pythium insidiosum and a Lagenidium sp to itraconazole, posaconazole, voriconazole, terbinafine, caspofungin, and mefenoxam

Objective-To evaluate the radial growth assay for use in in vitro susceptibility testing of Pythium insidiosum and a Lagenidium sp and to assess susceptibility of representative isolates to itraconazole, posaconazole, voriconazole, terbinafine, caspofungin, and mefenoxam. Sample Population-6 isolates each of P insidiosum and Lagenidium sp. Procedures-Isolates were plated in triplicate onto agar supplemented with antifungal compounds at concentrations of 0.025 to 8 mu g/mL. Isolates on dimethyl sulfoxide- and water-supplemented agar served as control samples. Effect of antifungal concentration on colony diameter was assessed with a mixed linear model. Assay variability was assessed with the coefficient of variation. Results-Colony growth was uniform (mean intra-assay and interassay coefficients of variation were < 5%). Minimal inhibition was evident with voriconazole and posaconazole at 8 mu g/mL. Terbinafine at 8 mu g/mL significantly reduced growth of P insidiosum and at >= 1 mu g/mL significantly reduced growth of the Lagenidium sp. Caspofungin and mefenoxam (concentrations >= 1 mu g/mL and >= 0.025 mu g/mL, respectively) significantly reduced growth of both pathogens. Mefenoxam at 0.1 mu g/mL caused > 50% growth inhibition in 11 of 12 isolates and at 1 mu g/mL caused > 90% inhibition in all isolates. Conclusions and Clinical Relevance-Results suggested that the radial growth assay was a simple, reproducible technique for susceptibility testing of P insidiosum and a Lagenidium sp. Azoles had limited activity, whereas terbinafine and caspofungin caused significant but minimal to moderate inhibition. Only mefenoxam had a profound effect on both pathogens at concentrations likely to be achievable in tissues. (Am J Vet Res 2008:69:1463-1468)