The most commonly used drugs to treat dermatophytosis are the azoles (itraconazole, fluconazole) and terbinafine. Studies in humans and limited studies in cats have shown that itraconazole has posttreatment residual activity. Terbinafine is fungicidal and requires a lower mean inhibitory concentration (MIC) against most dermatophytes; however, whether the MIC of the drug remains in cat hair after treatment is unknown. In this study, 10 healthy cats were treated once daily with oral terbinafine at a dose of 30 to 40 mg/kg for 14 days. Samples of hair were collected before treatment, 12 hours after the last dose, and then weekly for 8 weeks. High-pressure liquid chromatography was used to measure concentrations of terbinafine in the hairs. The highest concentrations of terbinafine were found immediately after therapy (mean 2.30 ng/mg), and the drug has a half-life of 1.84 weeks. MIC concentrations ≥ 0.03 ng/mg with a 99% confidence interval were found for approximately 5.4 weeks after cessation of therapy. The drug was generally well tolerated; however, vomiting was noted in 40% of cats, and mild anorexia and mild liver enzyme elevations were noted in 3 cats. The most dramatic adverse effect was intense facial pruritus and a maculopapular eruption approximately 7 to 14 days after discontinuation of the drug.

COMMENTARY: There are several published protocols for the use of itraconazole as pulse therapy (week on/week off) or as short-term therapy daily for 14 to 21 days in combination with topical antifungal therapy. The findings in this study suggest that terbinafine also may be suitable for pulse therapy. Terbinafine is expensive and needs to be used at 30 to 40 mg/kg to achieve suitable serum concentrations. This makes it a possible choice for pet cats but not for large groups of cats. Clients must be warned about the potential for adverse cutaneous reactions. Sudden development of facial pruritus and papules after therapy can be problematic if more widespread use of the drug shows that this is a common adverse effect. These lesions could be confused with a relapse of dermatophytosis-some cats with dermatophytosis experience pruritic lesions similar to those described in this case report.

Evaluation of persistence of terbinafine in the hair of normal cats after 14 days of daily therapy. Foust AL, Marsella R, Akucewich LH, et al. Vet Dermatol 18:246-251, 2007.