Dermatophytes are a unique group of fungi that are capable of infecting nonviable keratinized cutaneous epithelium including stratum corneum, nails, and hair. Dermatophytic genera include Trichophyton, Microsporum, and Epidermophyton. The term dermatophytosis thus denotes a condition caused by dermatophytes. It can be further specified according to the tissue mainly involved: epidermomycosis (epidermal dermatophytosis), trichomycosis (dermatophytosis of hair and hair follicles), or onychomycosis (dermatophytosis of the nail apparatus). The term tinea should be reserved for dermatophytoses and is modified according to the anatomic site of infection, e.g., tinea pedis. “Tinea” versicolor is better called pityriasis versicolor in that it is caused by Pityrosporum yeast and not dermatophytes.
Causes of Dermatophytoses
Dermatophytes synthesize keratinases that digest keratin and sustain existence of fungi in keratinized structures. Cell-mediated immunity and antimicrobial activity of polymorphonuclear leukocytes restrict dermatophyte pathogenicity.
- Host factors that facilitate dermatophyte infections: atopy, topical and systemic glucocorticoids, ichthyosis, collagen vascular disease
- Local factors favoring dermatophyte infection: sweating, occlusion, occupational exposure, geographic location, high humidity (tropical or semitropical climates)
The clinical presentation of dermatophytoses depends on several factors: site of infection, immunologic response of the host, species of fungus. Dermatophytes (e.g., T. rubrum) that initiate little inflammatory response are better able to establish chronic infection. Organisms such as M.canis cause an acute infection associated with a brisk inflammatory response and spontaneous resolution. In some individuals, infection can involve the dermis, as in kerion and Majocchi’s granuloma.
Treatment
Topical antifungal preparations
These preparations may be effective for treatment of dermatophytoses of skin but not for those of hair or nails. Preparation is applied bid to involved area optimally for 4 weeks including at least 1 week after lesions have cleared. Apply at least 3 cm beyond advancing margin of lesion. These topical agents are comparable. Differentiated by cost, base, vehicle, and antifungal activity.
- Imidazoles Clotrimazole (Lotrimin, Mycelex), Miconazole (Micatin), Ketoconazole (Nizoral), Econazole (Spectazole), Oxiconizole (Oxistat), Sulconizole (Exelderm)
- Allylamines Naftifine (Naftin), Terbinafine (Lamisil)
Systemic antifungal agents
For infections of keratinized skin: use if lesions are extensive or if infection has failed to respond to topical preparations. Usually required for treatment of tinea capitis and tinea unguium. Also may be required for inflammatory tineas and hyperkeratotic moccasin-type tinea pedis.
- Terbinafine 250-mg tablet. Allylamine. Rarely, nausea; dyspepsia, abdominal pain, loss of sense of taste, aplastic anemia. Most effective oral antidermophyte antifungal; low efficacy against other fungi.
- Azole/imidazoles Itraconazole and ketoconazole have potential clinically important interactions when administered with astemizole, calcium channel antagonists, cisapride-coumadin, cyclosporin A, oral hypoglycemic agents, phenytoin, protease inhibitors, tacrolimus, terfenadine, theophylline, trimetrexate, and rifampin.
- Griseofulvin Micronized: 250- or 500-mg tablets; 125 mg/teaspoon suspension. Ultramicronized: 165- or 330-mg tablets. Active only against dermatophytes; less effective than triazoles. Adverse effects include headache, nausea/vomiting, photosensitivity; lowers effect of crystalline warfarin sodium. T. rubrum and T.tonsurans infection may respond poorly. Should be taken with fatty meal to maximize absorption. In children, CBC and LFTs recommended if risk factors for hepatitis exist or treatment lasts longer than 3 months. Not used in Europe.
Prevention
Apply powder containing miconazole or tolnaftate to areas prone to fungal infection after bathing.
References