Management of Terbinafine‑Resistant Dermatophyte Infections

1. Efficacy and Resistance Patterns

Terbinafine is highly effective against Trichophyton species (e.g., T. tonsurans, T. violaceum, T. soudanense) but shows significantly lower efficacy against Microsporum species (e.g., M. canis, M. audouinii)【2】【3】.
In Microsporum infections, the minimum inhibitory concentration (MIC) of terbinafine often exceeds the maximum tissue concentration achievable, leading to treatment failure【3】【4】.
Terbinafine is poorly excreted in sweat and sebum, so it does not reach surface arthroconidia of Microsporum organisms effectively【3】【4】.

Prior to altering therapy, obtain mycological testing (skin‑scraping microscopy and culture) from the active lesion border【1】【2】.
Collect specimens using a scalpel, brush, or swab from the lesion’s active margin【1】【5】.
Request both KOH microscopy and culture to identify the dermatophyte species【1】【2】.

Switch to griseofulvin 15–20 mg/kg/day (≈1 g/day for patients > 50 kg) for 6–8 weeks【1】【2】【3】.
Griseofulvin is significantly more effective than terbinafine for Microsporum infections【2】【3】.

Switch to itraconazole 200 mg daily (≈5 mg/kg/day) for 4 weeks【1】【5】.
Itraconazole has activity against both Trichophyton and Microsporum species【1】【5】.

It is reasonable to start itraconazole empirically in cases of terbinafine failure pending mycological confirmation【1】【5】.

Rifampicin markedly reduces terbinafine plasma levels; cimetidine can increase them【4】.

Non‑compliance with medication and suboptimal absorption are common contributors to treatment failure【1】【5】.
Reinfection from untreated contacts or contaminated fomites (clothing, bedding, towels) must be addressed【1】【5】.
Screen household and close contacts, especially when T. tonsurans is identified, and treat asymptomatic carriers with high spore loads【1】【5】.
Decontaminate shared items (clothing, bedding, towels) to prevent re‑exposure【1】.

Re‑evaluate clinically 2–4 weeks after switching to second‑line therapy【1】【5】.
If clinical improvement occurs but mycological tests remain positive, continue therapy for an additional 2–4 weeks【1】【5】.
The treatment endpoint is mycological cure, not merely clinical resolution【1】【5】; repeat mycological sampling until clearance is confirmed【1】【5】.

Refer to dermatology or infectious‑disease specialists if there is no response to second‑line therapy (itraconazole or griseofulvin)【1】【5】.

*All statements are supported by cited evidence from the British Journal of Dermatology (citations 1–5).