Evaluation of incubation time for Microsporum canis dermatophyte cultures.

Objectives The goal of this study was to determine how frequently Microsporum canis was isolated after 1, 2 and 3 weeks of incubation on dermatophyte culture medium either from untreated cats or cats during treatment. Methods This was an observational retrospective study. Toothbrush fungal culture results were examined from two data pools: untreated cats with suspect skin lesions and weekly fungal cultures from cats being treated for dermatophytosis. Results Results from 13,772 fungal cultures were reviewed and 2876 (20.9%) were positive for M canis. Of these, 2800 were confirmed as positive within 14 days of incubation and only 76 (2.6%) required >14 days for confirmation of M canis. In pretreatment specimens, 98.2% (1057/1076) of M canis isolates were recovered within 14 days of incubation in specimens from cats not known to have received prior antifungal treatment. For cats receiving treatment, 96.8% (1743/1800) of M canis isolates were recovered within 14 days of incubation. Of the 57 cultures that required >14 days for finalization, 21 required extra incubation time because cultures were grossly abnormal, 12 had concurrent contaminant growth delaying microscopic confirmation and 24 had no growth in the first 14 days. Of these 24, 19 had 1-2 colony-forming units (cfu)/plate and the remaining five plates had 5 to >10 cfu/plate, all with abnormal morphology. Conclusions and relevance The findings of this study show that it is not necessary to hold pretreatment or post-treatment fungal cultures for 21 days before finalizing cultures for no growth. Growth requiring >14 days had grossly abnormal morphology.