Ultraviolet C inactivation of dermatophytes: Implications for treatment of onychomycosis

Background: Onychomycosis responds to systemic antifungals and sometimes to topical lacquers, but alternative treatments are desirable. Topical application of germicidal ultraviolet (UV) C radiation may be an acceptable and effective therapy for infected nails. Objectives: To test the ability of UVC to inactivate dermatophyte suspensions in vitro and to sterilize a novel ex vivo model of nail infection. Methods: Trichophyton rubrum, T. mentagrophytes, Epidermophyton floccosum and Microsporum canis suspensions were irradiated with UVC (254 nm) at a radiant exposure of 120 mJ cm^-2 and surviving colony-forming units quantified. T. rubrum infecting porcine hoof slices and human toenail clippings was irradiated with UVC at radiant exposures of 36-864 J cm^-2. Results: In vitro studies showed that 3-5 logs of cell inactivation in dermatophyte suspensions were produced with 120 mJ cm^-2 UVC irradiation. Depending on factors such as the thickness and infectious burden of the ex vivo cultures, the radiant exposure of UVC needed for complete sterilization was usually in the order of tens to hundreds of J cm^-2. Resistance of T. rubrum to UVC irradiation did not increase after five cycles of subtotal inactivation in vitro. Conclusions: UVC irradiation may be a less invasive treatment option for onychomycosis, when the appropriate consideration is given to safety.