Onychomycosis, or fungal infection of the nail unit, accounts for 50% of nail disorders seen in clinical practice. Population-based studies indicate that its prevalence varies depending on the geographic region, with around 4% in Europe and almost 19% in South America.  In most geographic areas, dermatophytes such as T. rubrum and T. interdigitale are responsible for the majority of cases of onychomycosis, although in some areas such as Africa and the Middle East yeasts like C. albicans and C. parapsilosis may be the causative agent. Non-D moulds such as Neoscytalidium, Aspergillus and Fusarium are responsible for only a minority of cases.
Treatment outcomes are related to a number of factors including the causative pathogen. 
Moreover, drug resistance, both primary (intrinsic) and secondary (acquired), after exposure to an antifungal agent is a problem of increasing concern.
Onychomycosis is a fungal infection of nails primarily caused by dermatophyte fungi, which are traditionally understood as existing in the environment as planktonic organisms; however, recent advancements in microbiology suggest that fungi can also form biofilms, which are complex sessile microbial communities irreversibly attached to epithelial surfaces by means of an extracellular matrix.
The extracellular matrix also acts as a protective barrier to the organisms within the biofilm.
With regards to treatment, a biofilm is surprisingly resistant to injury and may act as a persistent source of infection possibly accounting for antifungal resistance in onychomycosis.
The clinical factors associated with onychomycosis comprise percentage of nail involvement, lunula involvement, presence of yellow spikes, causative dermatophytoma and the presence of an air gap between the nail plate and nail bed.
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Watanabe S, Tsubouchi I, Okubo A, et al. Efficacy and safety of fosravuconazole L-lysine ethanolate, a novel oral triazole antifungal agent, for the treatment of onychomycosis: A multicenter, double-blind, randomized phase III study. J Dermatol. 2018 Oct;45(10):1151-9.
Faergemann J, et al. An open, single-center pilot study of efficacy and safety of topical MOBO15B in the treatment of distal subungual onychomycosis (DSO). AAD 2015, eP 963.
Yeung K, Ortner VK, Martinussen T, et al. Efficacy of laser treatment for onychomycotic nails: a systematic review and meta-analysis of prospective clinical trials. Lasers Med Sci. 2019 Oct;34(8):1513-25.
Presented by: Prof. Kristian Reich, Translational Research in Inflammatory Skin Diseases, Institute for Health Services Research in Dermatology and Nursing, University Medical Center Hamburg-Eppendorf, and Skinflammation® Center, Hamburg, Germany