Other fungi causing onychomycosis

Abstract

Nondermatophyte onychomycosis account for 2% to 12% of all nail fungal infections and can be caused by a wide range of fungi, mainly Scopulariopsis brevicaulis, Aspergillus versicolor, A. flavus, A. niger, A. fumigatus, Fusarium solani, F. oxysporum and Scytalidium spp. Among the predisposing factors are footwear, hyperhidrosis, local trauma, peripheral circulatory disease, and immunosuppression. These nondermatophyte fungi lack the keratinolytic capacity of dermatophytes, but they still can infect alone or in combination with the latter. Because most are considered laboratory contaminants, special criteria have been created for the correct diagnosis of nondermatophyte onychomycosis. The etiologic agent does not determine the clinical pattern of nail invasion, but superficial onychomycosis is frequently observed; leukonychia and melanonychia can also be clinical manifestations.

Introduction

According to the International Society for Human and Animal Mycology, onychomycosis is an invasive fungal infection of the nails, disregarding the causative agent. The term tinea unguium is reserved for nail disease caused by dermatophytes, which accounts for almost all cases. For this reason, both terms—onychomycosis and tinea unguium—are sometimes considered synonymous, although the former is also the correct term used for nondermatophyte molds (NDM) and yeast-originated onychomycosis¹; thus, onychomycosis will be the term used in this chapter.

Onychomycosis represents 30% of superficial mycosis and 50% of all nail disorders, with increasing incidence as people age. The estimated prevalence is more than 10% in the general population and 40% in elderly individuals, probably because of suboptimal immune function, inactivity, and the inability to maintain good foot care.²

NDM are filamentous fungi commonly found as soil saprophytes and plant pathogens. Unlike dermatophytes, NDM are not keratinolytic. They live on unkeratinized intercellular cement or must take advantage of previous keratin destruction by a dermatophyte, trauma, or another nail disease. For this reason, they are usually secondary invaders but are not primary parasites of the nail plate.³

NDM account for 2% to 12% of onychomycosis, with variations depending on the population and geographic area studied. Frequencies as high as 22% have been reported.⁴Scopulariopsis brevicaulis, mostly geophilic with a wide distribution, and Aspergillus versicolor are the most frequent organisms found in the feet (Fig. 1, Fig. 2, Fig. 3). The participation of A flavus, A niger, A fumigatus, A sydowii, A unguis, Fusarium solani, and F oxysporum as pathogens must be carefully considered because they have a universal distribution and are common laboratory contaminants.1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18Scytalidium spp (previously Hendersonula toruloidea) cause infection of the toenails and fingernails and can also affect the skin of the hands and feet, alone or combined with dermatophytes.

Significant predisposing factors are a family history of onychomycosis, footwear (61%), hyperhidrosis (43%), local trauma (26%), psoriasis, peripheral circulatory alterations, systemic immunodepression, and local factors, such as climate. Diabetic patients and those who require hemodialysis have a higher prevalence of this disease of 6.2% to 52%. It is the second most common nail disorder in dialysis patients. Studies report that dialysis duration and diabetes mellitus are independent risk factors.^5,19, 20, 21