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What is Erythrasma?
Erythrasma is an uncommon bacterial skin infection that affects the skin folds, most commonly the inguinal (groin) area. It is characterized by the appearance of asymptomatic, reddish-brown skin patches with clearly demarcated borders.
The condition predominantly affects adult and elderly males, though it can occur at any age and in both sexes, particularly in individuals with certain predisposing conditions.
The prognosis is excellent: erythrasma responds readily to appropriate antibiotic treatment, and the skin lesions resolve without scarring. However, the condition has a tendency to recur, particularly when underlying predisposing factors are not addressed.
Causes
Erythrasma is caused by Corynebacterium minutissimum, a Gram-positive bacterium (diphtheroids group) that is part of the normal resident flora of human skin. It is therefore not a foreign pathogen acquired from outside, but a commensal organism that becomes pathogenic only under specific favorable conditions.
How Does Infection Develop?
The disease typically occurs in geographic regions with high humidity and in individuals where skin folds create a warm, moist, occluded microenvironment that promotes bacterial overgrowth. Specific conditions that facilitate this process include:
- Maceration of the skin: Persistent moisture in skin folds softens and weakens the stratum corneum (the outermost protective layer of the epidermis)
- Friction and microtrauma: Repetitive rubbing of adjacent skin surfaces causes small damage to the stratum corneum, creating portals of entry for bacteria that are normally confined to the skin surface
- Bacterial penetration: Corynebacterium minutissimum enters the superficial epidermis through these microscopic disruptions
Once established in the epidermis, the bacteria produce porphyrins, which are light-sensitive molecules responsible for the distinctive coral-red fluorescence observed under Wood’s lamp examination (the key diagnostic test for erythrasma).
Contagiousness
Erythrasma has very limited contagiousness. Corynebacterium minutissimum is a normal commensal organism found on the skin of both affected and healthy individuals. The infection develops due to local environmental conditions (moisture, friction, warmth) rather than through person-to-person transmission.
Standard hygiene precautions, such as hand washing and avoiding sharing personal items like towels and clothing, are sufficient for close contacts. No specific isolation measures are required.
Risk Factors
Several conditions significantly predispose individuals to developing erythrasma by creating the favorable microenvironment needed for bacterial overgrowth:
- Hyperhidrosis (excessive sweating): Increased perspiration in skin folds raises humidity, promotes maceration, and creates ideal conditions for bacterial proliferation
- Obesity: Excess body weight creates larger and more numerous skin folds with greater surface-to-surface contact, warmth, and moisture accumulation
- Diabetes mellitus: Impairs immune function (particularly neutrophil activity and local skin defenses) and alters skin microbiome composition, significantly increasing susceptibility to opportunistic skin infections including erythrasma
Additional predisposing factors include:
- Hot, humid climatic conditions
- Immunosuppressed states (including HIV, organ transplantation, systemic corticosteroid therapy)
- Poor hygiene
- Wearing tight-fitting, non-breathable clothing
Symptoms
Skin Lesions
The characteristic presentation of erythrasma consists of clearly demarcated, irregularly shaped skin macules (flat patches) with the following features:
- Color: Reddish-brown or brown, sometimes with a slightly pink hue in early lesions.
- Borders: Sharp and well-defined, the transition from affected to normal skin is abrupt.
- Size: Initially small, individual macules that gradually enlarge and coalesce into larger plaques over time.
- Surface: Smooth, with only occasional fine desquamation (scaling). The scale is typically very subtle and powdery.
- Peripheral satellite lesions may occasionally be present.
Location
Erythrasma characteristically affects skin folds and occluded body areas where warmth and moisture accumulate:
| Location | Notes |
|---|---|
| Inguinal folds (groin) | Most common site in adults; particularly in the area where the scrotum contacts the thighs (males) or the labia majora contacts the thighs (females) |
| Interdigital spaces of the feet (toe webs) | Most common site in temperate climates; may coexist with tinea pedis |
| Axillae (armpits) | Second most common fold location |
| Intergluteal fold (between the buttocks) | |
| Inframammary folds (under the breasts) | Particularly in women with large breasts |
| Umbilicus | |
| Other occluded skin areas | Any body region with opposing skin surfaces |
Symptoms
The lesions of erythrasma are characteristically asymptomatic in the majority of patients. This is a clinically important feature that helps distinguish it from other conditions:
- No significant itching or pain in most cases
- Occasional mild pruritus, particularly triggered by:
- Intense perspiration
- Prolonged sitting
- Physical activity causing friction
- Occasional excoriation (scratch marks) in intensely pruritic cases
- Localized inflammation around lesions in some patients
💡 Clinical pearl: The asymptomatic or minimally symptomatic nature of erythrasma is one of its most characteristic features. Patients often present only because they noticed the color change, rather than because of discomfort. Unlike tinea cruris (ringworm), which is almost always pruritic.
Associated Infections
In the toe web spaces, erythrasma can be found in combination with other infections:
- Tinea pedis (athlete’s foot), a concurrent fungal infection
- Interdigital intertrigo from Pseudomonas aeruginosa, producing a greenish discoloration
Diagnosis
Clinical Diagnosis
In most patients, the diagnosis begins with visual inspection of the characteristic reddish-brown, well-demarcated patches in typical skin fold locations, in a patient with relevant risk factors.
Wood’s Lamp Examination: The Diagnostic Gold Standard
The key confirmatory test is Wood’s lamp examination, a simple, non-invasive bedside procedure.
- Wood’s lamp emits near-ultraviolet light (UVA, ~365 nm) in a darkened room.
- The porphyrins produced by Corynebacterium minutissimum absorb UVA radiation and emit a characteristic coral-red (brick-red) fluorescence, which is highly specific for erythrasma.
Important technical note: The coral-red fluorescence is only reliably visible if the patient has not recently bathed or washed the affected area. Porphyrins are water-soluble and can be washed away, producing a false-negative result. The examination should ideally be performed before washing.
Wood’s Lamp: Differential Fluorescence Patterns
| Organism / Condition | Wood’s Lamp Color |
|---|---|
| Corynebacterium minutissimum (erythrasma) | Coral-red |
| Microsporum canis (ringworm, tinea capitis) | Blue-green |
| Other dermatophytes (tinea cruris, tinea pedis) | No fluorescence |
| Pseudomonas aeruginosa | Yellow-green |
| Normal skin / no infection | No fluorescence or blue-white |
This differentiation is clinically invaluable, particularly for distinguishing erythrasma from dermatophyte infections in the groin (tinea cruris), which do not fluoresce under Wood’s lamp.
Microbiological Culture (Rarely Needed)
Corynebacterium minutissimum can be cultured from skin scrapings using specialized culture media (enriched fetal bovine serum agar), allowing identification and subclassification. However, this procedure is almost never necessary in clinical practice, as the Wood’s lamp examination and clinical picture are sufficient for diagnosis in the vast majority of cases.
Differential Diagnosis
Several conditions can present with reddish-brown patches or discoloration in skin folds and must be distinguished from erythrasma:
| Condition | Key Distinguishing Features |
|---|---|
| Tinea cruris (fungal, e.g., Epidermophyton floccosum, Trichophyton) | Always pruritic; raised, actively scaling border; central clearing; no fluorescence under Wood’s lamp; KOH preparation positive |
| Pityriasis versicolor (Malassezia furfur) | Hypo- or hyperpigmented patches mainly on trunk; yellow-green fluorescence |
| Candidal intertrigo | More pruritic than erythrasma; satellite pustules; no coral-red fluorescence; KOH prep shows yeasts and pseudohyphae |
| Psoriasis (inverse/flexural) | Family or personal history of psoriasis; typical plaques elsewhere on body; no fluorescence; nail changes |
| Contact dermatitis | History of contact with irritant or allergen; more pruritic; positive patch test |
| Acanthosis nigricans | Velvety, thickened hyperpigmentation; associated with insulin resistance/obesity/malignancy |
Treatment
Prognosis
The prognosis of erythrasma is good; it responds effectively to antibiotic treatment. However, the condition has a significant tendency to recur, especially when underlying predisposing factors (hyperhidrosis, obesity, diabetes mellitus) are not adequately managed.
Topical (Local) Antibiotic Therapy: First Choice for Localized Disease
Topical antibiotics are the preferred treatment for localized erythrasma:
- Erythromycin gel or solution (2%): Applied twice daily for 2 to 4 weeks. This is the most widely used topical treatment and is highly effective.
- Clindamycin gel, lotion, or solution (1%): Applied twice daily for 2 to 4 weeks. This treatment is an effective alternative to erythromycin and is particularly useful when erythromycin resistance is a concern.
- Benzoyl peroxide gel (2.5–5%): An alternative option with broad antibacterial activity against Corynebacterium; also helpful when concurrent folliculitis is present.
All topical agents should be applied to clean, dry skin and continued for the full prescribed duration, even after visible improvement. Stopping treatment too soon increases the risk of relapse.
Systemic (Oral) Antibiotic Therapy: For Extensive or Resistant Disease
When the infection is particularly widespread, involves multiple body sites, or does not respond adequately to topical treatment:
- Erythromycin (macrolide): 250 mg orally four times daily for 7 days, which is the traditional standard systemic treatment.
- Clarithromycin: A single oral dose of 1 g, which is increasingly used due to its convenience (single-dose protocol) and good efficacy, may improve adherence.
- Azithromycin: Alternative macrolide with a favorable pharmacokinetic profile.
Note: None of these treatments, whether topical or systemic, guarantees prevention of recurrence. Addressing the underlying predisposing factors is equally important to long-term management.
Summary of Treatment Options
| Situation | Treatment |
|---|---|
| Localized erythrasma | Topical erythromycin or clindamycin gel (2–4 weeks) |
| Localized, alternative | Benzoyl peroxide gel (2–4 weeks) |
| Extensive / widespread | Oral erythromycin (250 mg × 4/day × 7 days) |
| Extensive, simplified regimen | Single oral dose of clarithromycin 1 g |
| Recurrent erythrasma | Treat recurrence + address predisposing factors |
Prevention
Prevention of erythrasma centers on eliminating or minimizing the predisposing environmental factors that allow Corynebacterium minutissimum to overgrow. Key preventive measures include:
Skin Hygiene
- Frequent washing with antibacterial cleansers: Regular cleansing of skin folds with products containing chlorhexidine or triclosan helps reduce the bacterial load on the skin surface
- Thorough drying after washing: Pay special attention to drying all skin folds meticulously. Moisture trapped in folds is the primary factor that enables erythrasma.
- Use a hair dryer on a cool setting to dry toe web spaces and groin folds if needed
Managing Predisposing Conditions
- Hyperhidrosis: Use antiperspirant products in affected areas (aluminum-based antiperspirants in skin folds); consider medical management of hyperhidrosis if severe.
- Obesity: Weight reduction reduces the number and depth of skin folds, significantly lowering the risk of erythrasma and other intertrigo conditions.
- Diabetes mellitus: Optimal glycemic control is essential, as poorly controlled diabetes impairs skin immunity and promotes recurrent skin infections of all types. Regular foot care and self-examination are particularly important for diabetic patients.
Clothing and Lifestyle
- Wear loose-fitting, breathable clothing, such as natural fibers like cotton or linen, or moisture-wicking technical fabrics.
- Change clothing regularly, particularly undergarments and socks.
- Avoid prolonged skin-to-skin contact in occluded areas. If persistent moisture is a problem, use absorbent powders, such as talcum powder or antifungal powder, in skin folds.
- Regular foot hygiene, which includes inspecting and cleaning the toe web spaces, is particularly important for preventing toe web erythrasma and associated infections.
Sources and Bibliography
- Braun-Falco O, Plewing G, Wolff HH, Burgdorf WHC. Dermatologia. Edizione italiana a cura di Carlo Gelmetti. Springer-Verlag Italia 2002.
- Wolff K, Johnson R, Saavedra A. Fitzpatrick Manuale ed Atlante di Dermatologia Clinica. Edizione italiana sulla settima di lingua inglese a cura di Mauro Alaibac. Piccin 2015.
- Holdiness MR. “Management of cutaneous erythrasma”. Drugs. 2002;62(8):1131–1141. doi:10.2165/00003495-200262080-00002
- Groves RW, Botham PA, Leatherbarrow B. “Coral red fluorescence of erythrasma under Wood’s light: a note of caution”. Clin Exp Dermatol. 1991;16(1):54–56.
- Darras-Vercambre S, Carpentier O, Vincent P, Bonnevalle A, Thomas P. “Photodynamic action of red light for treatment of erythrasma: preliminary results”. Photodermatol Photoimmunol Photomed. 2006;22(3):153–156.
- Wharton JR, Wilson PL, Kincannon JM. “Erythrasma treated with single-dose clarithromycin”. Arch Dermatol. 1998;134(6):671–672.
- DermNet NZ – Erythrasma
- American Osteopathic College of Dermatology (AOCD) – Erythrasma
