Onychomycosis is a broad term that encompasses all fungal infections of the nail, including those caused by dermatophytes, non-dermatophytes, and yeasts ^[1][2][3][1,2,3]. There are multiple subtypes, including distal/lateral subungual, superficial white, and proximal subungual onychomycosis, among others [4]. Clinical features may include nail discoloration, subungual hyperkeratosis, and onycholysis [1]. Left untreated, complications include local pain, paresthesias, spread of infection, as well as the functional and social impairments of nail dystrophy ^[1][5][1,5]. The incidence of onychomycosis is estimated at about 6.5% amongst Canadians, and onychomycosis is thought to be responsible for approximately 50% of nail disorders globally [6].

Dermatophytes account for 90% of onychomycosis cases, and tinea unguium is a term which refers specifically to a dermatophyte infection of the nail ^[4][7][4,7]. Common causative agents include Trichophyton rubrum, T. interdigitale, and Epidermophyton floccosum ^[1][4][1,4]. In children, infection with T. tonsurans is frequently seen [4]. Though all dermatophytes can cause onychomycosis, infection due to Microsporum spp. is rare [4].

The pathophysiology of dermatophyte infection involves adhesion to the stratum corneum, followed by invasion into the underlying sublayers [8]. Adhesion is facilitated by fibrils on fungal spore cell walls which anchor to host keratinocyte membranes, as well as carbohydrate-specific adhesins which recognize mannose and galactose on host cells ^[8][9][10][8,9,10]. Following adhesion, spores germinate, forming hyphae that grow in multiple directions, including deeper into the stratum corneum which results in the destruction of subungual structures [9]. The invasion process is facilitated by fungal proteases that hydrolyze extracellular matrix proteins including keratin and collagen, whose breakdown in turn provides nutrients for invading dermatophytes ^[8][9][11][8,9,11].

Predisposing factors for onychomycosis include older age, trauma, diabetes, immunosuppression, and previous history of nail psoriasis or tinea pedis (athlete’s foot) [1]. Exposure to humid environments, occlusive footwear, and occupations which involve frequent travel, handwashing, or communal bathing facilities increase the risk of developing onychomycosis ^[7][12][7,12]. Studies have found that certain human leukocyte antigen class II genes may influence susceptibility to developing onychomycosis ^[13][14][15][13,14,15]. Though many biological risk factors have been well characterized, the role of the environment has been less clear. Of particular interest are the factors that influence transmission of onychomycosis between household members. Previous studies have suggested the risk of onychomycosis transmission in households with one affected member to be between 44% and 47% ^[16][17][16,17]. Indeed, using molecular techniques, it was shown that individuals within the same household were infected by the same dermatophyte strain, suggesting likely household transmission ^[16][18][16,18]. Proposed mechanisms of transmission between household members include sharing of slippers or even walking on carpets or bathroom floors previously walked on by an affected individual harboring infectious fungal elements [19]. Indeed, studies have found that dermatophytes are able to survive in washed textiles such as socks and contaminated nail polish containers ^[20][21][20,21]. Despite this, studies directly implicating environmental risk factors to transmission of onychomycosis are lacking, and more evidence is required to understand social determinants associated with the development of onychomycosis.

Dermatophytes have been found to persist on a variety of surfaces, particularly wet surfaces on which individuals walk barefoot. Multiple species, including T. rubrum and T. mentagrophytes have been isolated from walkways, changing rooms, and foot washing stations in swimming pool facilities, despite regular disinfection with chlorine ^[22][25]. Dermatophytes were found to be in greatest concentrations along walkways where people converged, including entrances and exits ^[23][26]. Aside from swimming pools, studies have found T. rubrum, T. mentagrophytes, and T. tonsurans on the floors of mosques, wrestling mats, and nursing homes ^[24][25][26][27,28,29].

Multiple studies have shown that these surfaces may be not only harboring dermatophytes, but also driving transmission of onychomycosis. In an investigation of an outbreak in a long-term care facility in which patients were mostly bedridden with minimal direct interpersonal contact, transmission was attributed to a shared bathtub, in which 9 strains of T. interdigitale were found ^[26][29].

The use of slippers or other footwear not shared with other household members may limit dermatophyte spread from shared surfaces. In one study conducted in an Italian military school where individuals wore sandals while showering, only 0.2% of those tested were found to have onychomycosis ^[27][34].

In summary, shared surfaces may harbor dermatophytes and provide sources of infection. Applying these findings to the household environment, dermatophytes may likely be found on wet surfaces, such as patios, balconies, washrooms, showers, and bathtubs. There may also be a high density of dermatophytes in areas of convergence, such as entrances and narrow hallways. Care should be taken to properly disinfect and avoid walking barefoot on these surfaces.

2.2. Transfer through Shared Equipment

Aside from shared surfaces, shared equipment may also play a role in the transmission of onychomycosis among family members. Multiple studies have implicated use of shared hair equipment, including combs, hairbrushes, and headrests with the transmission of T. rubrum and T. mentagrophytes, causing tinea capitis ^[28][29][30][30,35,36]. One study conducted in the context of a barber shop found dermatophytes and non-dermatophytes, primarily Aspergillus spp., on 24.4% of combs, hairbrushes, shaving brushes, and chair headrests tested ^[29][35].

Other studies investigating dermatophytosis in general have suggested that sharing soaps, towels, bedding, and general articles of clothing may be implicated in dermatophyte transmission, and individuals are at greatest risk when there is any skin trauma or breaks in skin barrier ^[31][32][37,38]. Though this data relates to dermatophyte spread in general, it is clinically relevant for development of onychomycosis because studies have shown that 30% of cutaneous dermatophyte infections have nail involvement ^[33][39]. Development of these infections, particularly tinea pedis, has been established as risk factors for developing onychomycosis ^[33][34][39,40].

2.3. Persistence of Fungi within the Household Environment

Multiple studies have demonstrated the ability of fungi to persist on household cleaning supplies. Ekowati et al. found 15 clinically relevant fungi species on 21 of 24 (88%) samples of cleaning supplies, including mops, scrubbers and wipes ^[23][26]. In many cases, the same species on the cleaning supplies were also isolated from floors, and the authors argue that improperly disinfected cleaning tools may promote fungal spread ^[23][26]. Other studies have found M. gypseum and T. mentagrophytes, common agents of onychomycosis, in up to 48.4% of household vacuum cleaners tested ^[35][36][47,48], and it has been argued that vacuums without proper filters may result in further dissemination of fungi and spores ^[37][49].

Textiles, particularly clothing and bedding, may also harbor fungal pathogens, and studies have previously implicated textiles with human infection ^[32][38][38,50]. The warm environment and presence of desquamated keratinocytes provides for ideal growing conditions for fungi ^[39][51]. Multiple studies have shown persistence of dermatophytes on clothing, particularly socks, despite regular laundering ^[20][40][20,44]. Indeed, yeast and fungi have been shown to withstand washing temperatures below 40 °C and 60 °C, respectively ^[41][42][52,53]. This has implications for cold-water laundering, and studies have also shown that inadequately cleaned washing machines spread fungi to previously sterile textiles ^[43][44][54,55].

Pets may also play a role by harboring fungi, and pet ownership has been cited as a risk factor for developing onychomycosis ^[45][56]. In one study, contact with cats or dogs was reported in 39.5% of those with dermatophytosis ^[46][57]. M. canis has most commonly been reported in cats and dogs, and human dermatophyte infections have been directly linked to these animals ^[47][48][58,59]. Small rodents such as guinea pigs have been known to harbor dermatophytes ^[49][60]. In summary, dermatophytes are able to persist within the household environment, namely on cleaning supplies, textiles, and pets.