Relationship between in vivo and in vitro activity of terbinafine against Microsporum canis infection in cats

Purpose. The in vitro antimicrobial activity of an agent can be expressed in terms of minimum inhibitory concentration (MIC). MIC data obtained by various investigations can differ considerably and the results depend on many factors such as the type of evaluation, the type of inoculum, the size of the inoculum, the medium, and the incubation temperature. A correlation between in vitro antifungal susceptibility and in vivo clinical results has been established only in a few cases. Due to the lack of correlation, many factors inherent to the fungi, to the host and to the drug must be considered. Moreover, MIC values are unlikely to be obtained upon systemic application in the infection sites when testing the most important antifungal agents like griseofulvin or azoles, characterized by a fungistatic mode of action. In this study we evaluated the activity of terbinafine, a fungicidal drug, in vivo in naturally occurring feline dermatophytoses and compared it with its in vitro activity against Microsporum canis strains isolated from these cats before treatment. Materials and methods. In vivo activity, 30 mg/kg day of oral terbinafine was administered for 14 consecutive days to 11 cats showing symptomatic ringworm. The cats were checked regularly for dermatophytes by hairbrush diagnosis at the end of the treatment and 1, 2 and 3 months later. In vitro activity. The activity of terbinafine was evaluated versus M. canis strains isolated from the same cats before treatment. The MIC values were obtained with a microdilution test. Results. In spite of the fact that nd, canis showed an impressive biological variability in terms of in vitro and in vivo susceptibility to the drug, there was a correspondence between the in vitro sensitivity and the in vivo clinical response. The cats whose isolates showed the highest susceptibility to the drug were mycologically cleaned one month after the end of the therapy. The higher the resistance to the drug, the longer was the mycological recovery. Discussion. This correspondence can be referred to a precise follow up of the cats, to a correct methodology in vitro, but mostly to the fungicidal action and the pharmacokinetics of terbinafine. These factors make it possible to reach a concentration of the drug far above the minimum fungicidal concentration in the target tissues.