In onychomycosis worldwide, two dermatophyte species are
most frequently involved as causal agents, namely, T. rubrum
and T. interdigitale. The prevalence of these pathogens, however, varies according to geography, climate, and patterns of
population migration, and it may differ among regions with
similar environmental conditions. The few onychomycosis
studies that have been carried out for the Balkan region (27,
39, 41, 48), where the patient population of the present study
originated, have shown that the two dermatophyte species predominate, making up 55 to 70% of isolates from nails, while
molds and yeasts are less common, growing from 10 to 27% of
specimens (but not always rigorously implicated as causal).
Variation among countries is evident. For example, in Croatia
(48) the most common species was T. interdigitale (55%),
whereas in our sample (Bulgaria and Greece) T. rubrum was
most frequently isolated (76%) and T. interdigitale made up
only 2% of isolates. An earlier study from the Plovdiv region of
Bulgaria (where our own Bulgarian collection also originated)
detected 83% T. rubrum and 17% T. interdigitale isolates
among 130 cases (41). This shows that the incidence of T.
interdigitale has dropped since 1965. Our results are therefore
in agreement with the global ascendancy of T. rubrum as the
predominant etiological agent of onychomycosis. Even using
PCR in the present study, nondermatophytic molds (Scopulariopsis spp., Fusarium spp., and A. nidulans) as confirmed or
suggestive etiologic agents were detected in only 3% of infections; also, only 3% of infections yielded mixed isolations.
Those results are comparable with those of the study done by
Saulov in Plovdiv (41), in which 2.5% of onychomycoses were
ascribed to molds and 3.3% were classified as mixed infections.
Scopulariopsis spp. were not detected in that study. The predominance of the dermatophytes in onychomycosis is very
likely due to their broad spectrum of secreted proteases (24)
acting as virulence factors enabling the destruction of the nail
plate.