Those who speak of a fungal infection of the skin usually refer to athlete’s foot. But there are many other areas of the skin on the body where the microbes settle. In bad cases, patients infected with dermatophytes must take special medications for months to heal the inflamed areas.
What are dermatophytes?
Dermatophytes are filamentous fungi (hyphomycetes). The tiny microbes cause skin infections (dermatophytoses). There, they nest in the upper or even lower layers of the skin and feed on the keratins of the dead skin scales. The tiny pathogens have enzymes such as keratinase to break down the particles. Currently, 38 species of dermatophytes are known in humans and animals. They are classified into three genera: Trichophyton, Microsporum and Epidermophyton floccosum. The vast majority of skin fungal diseases are caused by filamentous fungi. Most of them affect only the epidermis. However, some of them are specialized on nutrients located in dermis and subcutis. As a rule, they infest parts of the body covered with horny cells. Other dermatophytes prefer scalp and nails. Epidermophytes are rarely causative agents of fungal infections of the skin.
Occurrence, distribution, and characteristics
Filamentous fungi occur, outside of humans, in soil and on the skin of animals. These are mostly domestic animals (dogs, cats, rodents). Because humans have particularly intense contact with them, animal-to-human transmission is relatively common. Skin fungi can also be transmitted by rats and mice. Soil-to-human infections are rather rare and usually only affect occupational groups that have frequent contact with soil. However, wet areas in public swimming pools and saunas can also be infectious. If visitors then do not wear bathing slippers, infections often occur. Human-to-human infection, on the other hand, is more common. It occurs through close physical contact and shared surfaces (pool floor) and objects (combs, brushes). Skin inflammation caused by dermatophytes can take different forms depending on the type and size of the affected skin area. In case of heavy infestation, even large parts of the acid mantle of the skin can be destroyed. Some hyphae attach themselves to the skin cells with the aid of special adhesive devices, while other types of fungus prefer to colonize hard-to-reach areas of the body (interdigital spaces). Depending on the preferred site of colonization, they have enzymes such as elastases (break down the elastin layer of the skin), keratinases (specialize in horny scales) and collagenases (feed on collagen). Their metabolic products then cause the fungal infection (tinea). Filamentous fungi not only infect the skin, but also hair and nails. Trichophytes also occur in pets and can infect nails, skin and hair. Microsporum species also readily colonize animals that live in close contact with humans. They prefer hair and skin. Epidermophyton floccosum, if it appears at all, specializes in nails and skin.
Diseases and symptoms
Prerequisites for infection with filamentous fungi are poor skin barrier condition, high degree of moisture, prolonged contact with the pathogen, and the strength of the fungi’s infectious potential. In infections caused by filamentous fungi, there are three types (skin, nail and hair mycoses) depending on the area of spread. Skin mycoses (tinea corporis) appear in the form of reddened, scaly and sharply demarcated skin blossoms (ring lichens) with dark red scaly edges, spreading from the trunk. Therefore, they also occur on the arms and in the groin and anal regions. Causative agents of tinea corporis are trichophytes and Microsporum (from dogs). The efflorescences itch and spread from the inside to the outside, and the metabolic products secreted by them cause an annular wall that is highly infectious. The area within heals more rapidly, becoming progressively lighter in color. Larger areas affected by the skin lichen also grow into each other. If there is hair on the infected areas, it usually falls out quickly. In patients with a weakened immune system, tinea corporis usually spreads over the entire body. An infectious special form is the tinea rubrum syndrome: Trichophyton rubrum, which normally only causes athlete’s foot, spreads over the entire body.The syndrome runs in families. Even the nails can be infected. Hand fungus infections (tinea manuum) are usually restricted to members of certain occupational groups, such as agricultural workers and gardeners. They are caused by the soil-dwelling Microsporum gypseum. Inflammation of this type is usually confined to a single palm. It is scaly, cracked and thickened by horny layers. Nail mycoses (tinea unguium) cause brownish brittle nails. Epidermophyton floccosum or Trichophyton species are to blame. Nail fungus infestation often occurs together with athlete’s foot. The infection of the interdigital space of the toes spreads to the toenails. Nail mycoses occur frequently in patients with diabetes and circulatory disorders of the legs. According to estimates, athlete’s foot (tinea pedis) now affects one in five German citizens. All three genera of filamentous fungi contribute to its development, but especially Trichophyton rubrum, which is widespread in Europe. Hair mycoses occur either on the head as Tinea capitis or in the beard (Tinea barbae). The causative agents are trichophytes and microspores, which preferentially live on the skin of dogs. The infected areas on the scalp are circular, covered with scales and have the typical “mown meadow”: the hairs there are all broken off at the same height. Tinea barbae leaves bald patches that are sometimes crusted and covered with foci of pus.
