Ecthyma is a bacterial skin infection that penetrates deeper than its more common cousin, impetigo. While impetigo stays in the outermost layer of skin, ecthyma extends into the dermis, the thicker layer underneath, producing painful ulcers that leave scars after healing. It’s caused by the same bacteria responsible for impetigo, primarily group A streptococcus and sometimes staphylococcus, but the infection digs deeper and takes longer to resolve.
How Ecthyma Develops
The infection typically starts in skin that’s already been compromised. A scratch, insect bite, rash, or minor wound gives bacteria a point of entry. In many cases, ecthyma begins as impetigo that was left untreated or didn’t respond well to initial care, allowing the bacteria to burrow past the skin’s surface into deeper tissue. It can also develop on its own in people whose skin barriers are weakened.
Certain conditions raise your risk. Diabetes, a weakened immune system, and malnutrition all make it easier for bacteria to establish a deeper infection. Living in warm, humid environments also plays a role, since moisture on the skin creates favorable conditions for bacterial growth. Poor hygiene and crowded living situations increase exposure to the responsible bacteria.
What Ecthyma Looks Like
Ecthyma lesions have a distinct appearance that sets them apart from superficial skin infections. They start as small fluid-filled blisters or pustules, but quickly develop a thick, hard crust that’s gray-yellow in color. This crust is noticeably thicker and tougher than the honey-colored, flaky crust you’d see with impetigo.
When that adherent crust is removed, it reveals a shallow, “punched-out” ulcer underneath, as if a small circle of skin has been scooped away. The edges of the ulcer are raised and firm to the touch, with redness and swelling around the border. Lesions most commonly appear on the legs, particularly the shins, but can show up anywhere on the body. They may be single or multiple, and they’re often tender or painful, unlike impetigo which is usually painless.
Diagnosis Is Usually Visual
Doctors can typically diagnose ecthyma based on its characteristic appearance alone. The combination of the thick gray-yellow crust, the punched-out ulcer beneath it, and the raised margins is distinctive enough that lab testing isn’t necessary in most cases.
Cultures of the lesion become important in one specific scenario: when the infection doesn’t respond to initial antibiotic treatment. Persistent infections should be cultured to check for MRSA (methicillin-resistant staphylococcus), which requires different antibiotics. A skin biopsy is rarely needed but may be considered if the diagnosis is uncertain or if the ulcer looks unusual.
Ecthyma vs. Ecthyma Gangrenosum
Despite the similar name, ecthyma gangrenosum is a different and far more serious condition. It’s traditionally associated with Pseudomonas bacteria and tends to occur in people who are severely immunocompromised, such as those with leukemia, multiple myeloma, or extensive burns. Interestingly, research has shown that the two conditions can look identical on physical examination, with no reliable difference in the appearance, location, or number of lesions. Some experts have argued that the distinction between Pseudomonas and non-Pseudomonas cases is artificial and that a broader, unified definition would be more useful clinically.
For the average person with a healthy immune system who develops a painful crusted ulcer, standard ecthyma is overwhelmingly more likely. Ecthyma gangrenosum is rare and almost always occurs in a hospital setting or in someone already being treated for a serious underlying illness.
How Ecthyma Is Treated
Because ecthyma reaches deeper skin layers, it generally requires more aggressive treatment than impetigo. Topical antibiotics can play a supporting role. Mupirocin cream, applied three times daily for about 10 days, is commonly used for infected skin wounds. However, most cases of ecthyma also need oral antibiotics to reach the deeper tissue where topical treatments can’t penetrate effectively.
Soaking the crusted areas to soften and gently remove the hard covering helps antibiotics reach the ulcer beneath. Keeping the wound clean and covered protects it from further contamination and speeds healing. Treatment typically lasts one to two weeks, though larger or more stubborn ulcers may take longer to fully close.
Scarring and Other Complications
Unlike impetigo, which heals without leaving a mark, ecthyma does leave scars. Because the infection damages the dermis, the body repairs the area with scar tissue rather than regenerating normal skin. The scars are usually small and flat but permanent, and they may appear lighter or darker than the surrounding skin.
Systemic symptoms like fever are rare with ecthyma, but complications can develop if the infection spreads. The bacteria can move into surrounding tissue, causing cellulitis (a spreading skin infection) or lymphangitis (infection of the lymph vessels, visible as red streaks traveling from the wound). In uncommon cases, the bacteria can enter the bloodstream.
One complication worth knowing about is post-streptococcal glomerulonephritis, a kidney problem that can develop after a streptococcal skin infection. It causes blood in the urine, swelling, and elevated blood pressure, typically appearing one to three weeks after the skin infection. Notably, treating ecthyma promptly with antibiotics does not appear to reduce the risk of this kidney complication. It’s driven by the immune system’s response to the bacteria rather than by the infection itself, so it can occur even after the skin has fully healed.