Trichophyton rubrum
Summary
Trichophyton rubrum is the most common cause of dermatophytosis in humans and is found worldwide. It is likely that 70% of humans will have clinical dermatophytosis in their lifetime. Hypersensitivity to dermatophyte molds has been implicated in the pathogenesis of intrinsic, late onset asthma, although the route of allergen exposure is yet to be defined. Trichophyton sensitization is associated with more severe allergic respiratory disease and improvement of clinical hypersensitivity has been achieved with antifungal treatment. Two T. rubrum allergens are described, with evidence of cross-reactivity with other molds.
Epidemiology
Worldwide distribution
Dermatophytosis is common worldwide and Trichophyton rubrum is the most common dermatophyte in developed countries. An estimated 70% of the world’s population are thought develop dermatophytosis at some point in their lifetime. Men are more frequently affected than women.
The prevalence of sensitization to Trichophyton spp. in patients with allergic skin disease and dermatophyte infection (atopic-chronic dermatophytosis syndrome) was 79% in one study. A similar phenomenon was described in patients with chronic urticaria and dermatophyte infection, where 75.5% of patients were sensitized to T. rubrum, a significantly greater prevalence than in the control patient groups.
Route Of Exposure
Main
The primary route exposure to the T. rubrum allergen is not yet defined. Trichophyton-specific IgE can be found in patients with dermatophytosis, whether they have atopic disease or not. However, inhaled exposure is still implied for sensitized patients with asthma or rhinoconjunctivitis but without demonstrable dermatophytosis. Host- fungal interactions outside the classical sensitization process may also be at play.
Secondary
Occupational exposure risk to airborne T. rubrum has been described for nail technicians and podiatrists through inhalation of nail dust. A high prevalence of allergen-specific IgE (31%) and sensitization to Trichophyton (16.5%) has been described in this at-risk demographic. However, the incidence of clinically apparent allergy in this group is not known.
Clinical Relevance
Trichophyton rubrum allergens can cause either immediate- (IH) or delayed-type hypersensitivity (DTH) reaction in sensitized patients. In some patients, both types of reactions are observed. DTH is associated with acute, severe inflammatory dermatophytosis with resolution of the infection. Conversely, an IH reaction is associated with chronic dermatophytosis with low-grade inflammation and is possibly a prerequisite for persistent infection. It has been proposed that dermatophyte induced IH reaction is a marker for immune dysregulation and is the mechanism for the association between chronic dermatophytosis and clinical atopy. Furthermore, monosensitivity to Trichophyton spp. has been described in patients presenting with “intrinsic” asthma, which can be severe.
This association has been supported by a placebo-controlled trial where an improvement in allergic-airway symptoms was observed in patients with late-onset asthma and tinea who were treated with oral fluconazole.
Cross-Reactivity
Zhang and colleagues have demonstrated cross-reactivity in sensitized patients of T. rubrum with two other dermatophytes (T. mentagrophytes and Epidermophyton floccosum). However, none was reported with the more phylogenetically distant Candida albicans. Furthermore, comparative genome analysis of T. rubrum with four other dermatophyte molds shows that most genes are conserved across the five species. Furthermore, the allergen Pen c 1 has significant amino acid sequence homology with Tr r 2, suggesting cross-reactivity amongst mold genera may be important in the pathophysiology of asthma where sensitization to molds is prevalent.
References
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