Determination of strain variability of Microsporum canis to disinfectants

The isolated infected hair model is a commonly used technique to test the fungicidal efficacy of topical therapies and/or disinfectants against Microsporum canis. Results from various laboratories have differed and it is unclear if they are due to methodology and/or strain variability. The objective of this study was to look for strain variability of M. canis to disinfectants. Ten strains of M. canis were used in this study. Infected hairs and crusts were soaked and processed in order to remove and isolate ectothrix spores. Eight dilutions of the following disinfectants were tested: Virkon® S, (Antec International Ltd, Suffolk, UK), chlorhexidine, lime sulfur, enilconazole and bleach. Sterile distilled water was used as a control. An equal volume of spore suspension and disinfectant solution were incubated for 5 min and then plated onto Mycosel fungal culture plates (Mycosel Agar, Becton Dickinson, Cockeysille, MD, USA). Fungal cultures were counted at days 7 to 10. Fungal culture data was evaluated using an end-point dilution at which there was 100% fungicidal activity, i.e. no growth on the plates. The 10 samples showed identical results. Chlorhexidine and Virkon® S were ineffective even when used at 4× the manufacturer’s recommended dilution. Lime sulfur (1:33), enilconazole (20 µL mL−1), and bleach (1:10) were consistently effective when used at the recommended dilution. In addition, lime sulfur and enilconazole were 100% fungicidal even when the recommended concentration was diluted 1:4 or 4× as dilute as recommended. In conclusion, using isolated infective spores of M. canis we were unable to detect strain variability to commonly used disinfectants.  This study was funded by the Winn Foundation and the SVM Companion Animal Research Fund.