Residency · Residency · Family Medicine

Common Fungal and Viral Skin Infections

Introduction

Superficial fungal and viral skin infections are among the most frequent dermatologic complaints in primary care. Accurate diagnosis, often achievable with simple bedside tools like KOH preparation and clinical inspection, prevents unnecessary antibiotic use and guides targeted therapy. This lecture covers the most common fungal dermatophyte infections, yeast infections, and viral exanthems encountered by family physicians.

Dermatophyte Infections (Tinea)

General Principles

Dermatophyte infections are caused by organisms of the genera Trichophyton, Microsporum, and Epidermophyton that invade keratinized tissue. They are named by anatomic location: tinea corporis (body), tinea pedis (feet), tinea cruris (groin), and tinea capitis (scalp). KOH preparation is the most practical bedside diagnostic tool: scales are scraped from the active, scaling border onto a slide, 10 to 20% KOH is added, and the specimen is examined for branching septate hyphae.

Tinea Corporis

Tinea corporis presents as annular, erythematous plaques with raised, scaling borders and central clearing. The differential diagnosis includes nummular eczema, granuloma annulare, and pityriasis rosea. Treatment consists of topical antifungals such as terbinafine 1% cream or clotrimazole 1% for 2 to 4 weeks. Tinea incognito is an altered presentation that results from prior topical steroid use and should be suspected when a rash worsens with steroids.

Tinea Pedis

Tinea pedis is the most common dermatophyte infection and presents in three patterns. The interdigital pattern shows maceration and scaling between toes, especially the fourth to fifth web space. The moccasin pattern involves diffuse hyperkeratosis of the soles and is often chronic, potentially requiring oral therapy. The vesiculobullous pattern is the least common. Treatment involves topical antifungals for most cases, with oral terbinafine (250 mg daily for 2 weeks) reserved for the moccasin type.

Tinea Cruris

Tinea cruris presents as erythematous, scaling plaques on the inner thighs and typically spares the scrotum, which distinguishes it from candidal intertrigo. It is more common in males and is frequently associated with concurrent tinea pedis, which should be treated simultaneously. Topical antifungals for 2 to 4 weeks combined with keeping the area dry are the standard approach.

Tinea Capitis

Tinea capitis primarily affects prepubertal children and presents as scaling, alopecia, and sometimes a kerion, which is a boggy, inflammatory mass. Systemic therapy is required because topical therapy alone is inadequate, as the fungus invades the hair shaft. Oral griseofulvin remains the gold standard in children, with terbinafine as an alternative. Household contacts should use ketoconazole or selenium sulfide shampoo to reduce carriage.

Onychomycosis

Onychomycosis is a fungal nail infection affecting 10% of the general population, with higher rates in the elderly. Distal subungual onychomycosis is the most common pattern, presenting with nail thickening, discoloration, and subungual debris. The diagnosis should be confirmed before treatment with KOH preparation, fungal culture, or PAS stain of a nail clipping. Oral terbinafine (250 mg daily for 6 weeks for fingernails and 12 weeks for toenails) is the most effective therapy. Topical options such as efinaconazole and tavaborole have lower cure rates of approximately 15 to 18%.

Yeast Infections

Candidal Intertrigo

Candidal intertrigo presents as bright red, moist plaques with satellite papules and pustules in skin folds. It is common in patients with obesity, diabetes, and incontinence. Treatment consists of topical nystatin or clotrimazole, combined with addressing underlying moisture and friction.

Tinea (Pityriasis) Versicolor

Tinea versicolor is caused by Malassezia furfur, a yeast rather than a dermatophyte. It presents as hypopigmented or hyperpigmented macules on the trunk and proximal extremities. KOH preparation shows the characteristic "spaghetti and meatballs" pattern of short hyphae and round spores. Treatment includes topical ketoconazole 2% or selenium sulfide, with oral fluconazole for widespread disease. Recurrence is common, and pigment changes may persist for months after successful treatment.

Viral Skin Infections

Verrucae (Warts)

Warts are caused by human papillomavirus (HPV), with common warts associated with HPV types 2 and 4, plantar warts with HPV 1, and flat warts with HPV types 3 and 10. Many resolve spontaneously within 2 years. Treatment options include cryotherapy with liquid nitrogen (the most common in-office treatment), salicylic acid at 17 to 40% as first-line home therapy applied daily after peeling, cantharidin (blister beetle extract) as a painless application particularly effective for pediatric patients, and immunotherapy with intralesional candida antigen or squaric acid for recalcitrant warts.

Molluscum Contagiosum

Molluscum contagiosum is caused by a poxvirus and presents as firm, dome-shaped, umbilicated papules. It is common in children and immunocompromised adults. The condition is self-limited, typically resolving in 6 to 12 months, though it may persist for years. Treatment options when desired include cryotherapy, curettage, cantharidin, and topical retinoids. In adults, widespread molluscum warrants HIV testing.

Herpes Simplex Virus (HSV)

HSV-1 is primarily orolabial and HSV-2 primarily genital, though overlap exists. The classic presentation is grouped vesicles on an erythematous base with prodromal tingling or burning. Diagnosis is made by Tzanck smear showing multinucleated giant cells, viral PCR (the preferred method), or viral culture. Treatment for primary infection is valacyclovir 1 g twice daily for 7 to 10 days; for recurrent episodes, 500 mg twice daily for 3 days. Suppressive therapy with valacyclovir 500 mg to 1 g daily is appropriate for patients with frequent recurrences (more than 6 per year).

Herpes Zoster (Shingles)

Herpes zoster results from reactivation of varicella-zoster virus (VZV) from dorsal root ganglia and presents as a unilateral, dermatomal, painful vesicular eruption. Postherpetic neuralgia (PHN) is the most common complication, especially in elderly patients. Treatment should begin within 72 hours of rash onset with valacyclovir 1 g three times daily for 7 days. Prevention with Shingrix (recombinant zoster vaccine), a two-dose series, is recommended for adults age 50 and older.

Key Clinical Pearls

Always confirm onychomycosis with KOH preparation or culture before committing to prolonged systemic antifungal therapy. Tinea capitis requires oral antifungal treatment because topical therapy alone does not penetrate the hair shaft. A rash that worsens with topical steroids should raise suspicion for tinea incognito, and a KOH preparation should be performed. Widespread molluscum contagiosum in an adult is a red flag for immunosuppression and warrants HIV testing. Shingrix is over 90% effective at preventing herpes zoster and should be offered to all eligible patients age 50 and older.

References

1. Gupta, A. K., et al. (2017). Onychomycosis: A review of management strategies. Journal of Cutaneous Medicine and Surgery, 21(4), 310-321.
2. Andrews, M. D. (2004). Cryosurgery for common skin conditions. American Family Physician, 69(10), 2365-2372.
3. Ely, J. W., Rosenfeld, S., & Seabury Stone, M. (2014). Diagnosis and management of tinea infections. American Family Physician, 90(10), 702-710.
4. Dooling, K. L., et al. (2018). Recommendations of the Advisory Committee on Immunization Practices for use of herpes zoster vaccines. MMWR, 67(3), 103-108.

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