Onychomycosis is estimated at a prevalence of 10% worldwide with the infecting organism most commonly Trichophyton rubrum (T. rubrum). Traditional culture identification of causative organisms has inherent risks of overestimating dermatophytes, like T. rubrum, by inhibiting the growth of possible nondermatophyte mould (NDM) environmental contaminants which could be causative agents. Recently, molecular methods have revealed that a proportion of onychomycosis cases in North America may be caused by mixed infections of T. rubrum as an agent co-infecting with one or more NDM. Determining the global burden of mixed infections is a necessary step to evaluating the best therapies for this difficult-to-treat disease. To determine the prevalence of mixed infections in a global population, nail samples from onychomycosis patients in Brazil, Canada, and Israel (n = 216) were analyzed by molecular methods for the presence of dermatophytes and five NDMs. If an NDM was detected, repeat sampling was performed to confirm the NDM. T. rubrum was detected in 98% (211/216) of infections with 39% mixed (84/216). The infection type was more likely to be mixed in samples from Brazil, but more likely to be a dermatophyte in samples from Canada and Israel (Χ 2 = 16.92, df = 2, P<0.001). The most common cause of onychomycosis was T. rubrum. In all countries (Brazil, Canada and Israel combined) the prevalence of dermatophyte (Χ 2 = 211.15, df = 3, P<0.001) and mixed (dermatophyte and NDM; Χ 2 = 166.38, df = 3, P<0.001) infection increased with patient age. Our data suggest that mixed infection onychomycosis is more prevalent than previously reported with the aging population being at increased risk for mixed infections.

Mixed onychomycosis infections may be more prevalent than previously reported.

Nine T. rubrum DNA strains were identified in a North American population: three novel and six predominant to a European population. Although DNA strain type switching in onychomycosis is a natural phenomenon, with presence in the placebo group, increases following the course of failed onychomycosis treatment suggest an antifungal-induced response.

Culture was three or four times more likely to report a false-negative result compared with PCR. The increased agreement between methods observed by excluding false-negative reports statistically clarifies and highlights the major discord caused by false-negative cultures. The increased agreement of NDM identification from poor to fair with repeated sampling along with their poor agreement in the single samples, with and without false-negatives, affirms the complications of NDM identification and supports the recommendation that serial samples help confirm the diagnosis of NDM infections.

BACKGROUND:Onychomycosis is estimated at approximately 10% of the global population with most cases caused by Trichophyton rubrum (T. rubrum). Some of the persistent cases of onychomycosis are caused by mixed infections of T. rubrum and one or more co-infecting nondermtophyte mould (NDM). T. rubrum strain types in onychomycosis may naturally switch and may also be triggered to switch as a response to antifungal therapy. T. rubrum strain types in mixed infections of onychomycosis have not been characterized. METHODS:T. rubrum DNA strains in mixed infections of onychomycosis containing co-infecting NDM(s) were compared to a baseline North American population through polymerase chain reaction amplification of ribosomal DNA tandemly repetitive subelement one and two (TRS1 and TRS2). The baseline strain types consisted of 102 clinical isolates of T. rubrum from onychomycosis patients and the patients with mixed infections of T. rubrum and an NDM(s) consisted of 63 repeat toenail samples from 15 onychomycosis patients with mixed infections of T. rubrum and an NDM(s). RESULTS:Two unique TRS2-types among the clinical isolates contributed to four unique TRS1;TRS2 strain types. Six TRS1;TRS2 strain types represented 92% of the clinical isolates of T. rubrum. Four TRS1;TRS2 strain types accounted for 100% of the T. rubrum within mixed infections.CONCLUSIONS:Four unique North American T. rubrum strains were identified. In support of a shared ancestry, the T. rubrum DNA strain types found in mixed infections with NDMs were among the most abundant types. A population of T. rubrum strains in mixed infections of onychomycosis has been characterized with more than one strain detected within some nails. The presence of a co-infecting NDM in mixed infections may contribute to failed therapy by stabilizing the T. rubrum strain type, possibly preventing antifungal therapy-induced strain type switching observed with infections caused by T. rubrum alone.