Erythrasma is not a sexually transmitted disease (STD). It is a common, superficial infection affecting the top layers of the skin. Unlike infections spread through sexual contact, Erythrasma is caused by the overgrowth of bacteria that are already a normal part of the human skin’s natural microbiome. This bacterial skin disorder arises from specific environmental and physiological conditions that allow resident bacteria to multiply excessively. The rash is often mistaken for a fungal infection, which can lead to misdiagnosis.
The Bacterial Cause and Appearance
The bacterium responsible for this infection is Corynebacterium minutissimum, a Gram-positive organism that typically resides harmlessly on the skin’s surface. The infection manifests as well-defined, thin patches that are usually pink or reddish-brown. Older lesions often develop a distinct brown color and may feature a fine, wrinkled, or scaly appearance, sometimes described as having a “cigarette paper” texture.
The infection is highly preferential for intertriginous areas, where skin surfaces rub together. Common sites include the groin, inner thighs, armpits (axillae), and the skin folds beneath the breasts. The spaces between the toes, especially the fourth and fifth toe webs, are also frequently affected. In these locations, the bacteria invade the upper third of the stratum corneum, the outermost layer of the skin.
Risk Factors and How Erythrasma Spreads
The proliferation of Corynebacterium minutissimum depends on a warm, moist, and occlusive environment. Excessive sweating (hyperhidrosis) is a major contributing factor because it supplies the moisture needed for bacterial growth. Living in hot and humid climates and poor hygiene, which allows sweat and debris to accumulate, further encourages bacterial overgrowth.
Several host factors increase susceptibility to developing Erythrasma. People with diabetes mellitus are at a higher risk, and the infection may be an early sign of type 2 diabetes. Obesity also predisposes individuals to the condition, as it creates larger skin folds where friction and moisture are trapped. Advanced age and wearing tight or occlusive clothing that restricts air circulation are additional risk factors.
Transmission between people is considered minimal and not highly contagious through casual contact. The infection primarily arises from the overgrowth of resident bacteria. Spreading often occurs through self-inoculation, where the bacteria are transferred from one affected body area to another, such as from the toe webs to the groin.
Diagnosis and Recommended Treatment Protocols
Diagnosis of Erythrasma is typically confirmed using a Wood’s Lamp, which emits long-wave ultraviolet (UV) radiation. When the UV light shines on the affected skin, the Corynebacterium minutissimum bacteria produce porphyrin compounds. These porphyrins fluoresce a characteristic bright coral-red or orange color, which is a definitive diagnostic marker. This fluorescence helps medical professionals differentiate Erythrasma from fungal infections like tinea.
Treatment protocols vary depending on the extent and location of the infection. For localized patches, first-line therapy involves topical antibiotics applied twice daily for several weeks. Other topical options include:
- Erythromycin solutions or gels
- Clindamycin solutions or gels
- Fusidic acid cream
- Benzoyl peroxide
Systemic oral antibiotics are reserved for cases that are widespread, resistant to topical treatment, or extensively affect the axilla and groin. Oral options include a single-dose regimen of clarithromycin or a course of oral erythromycin, typically administered in divided doses over a period of up to two weeks. Even after successful treatment, the infection may recur if underlying risk factors like diabetes or excessive moisture are not managed. Relapse rates can be high, making sustained attention to skin hygiene and dryness important for prevention.