Lichen

Etiology and key causative agents

Most causative agents of fungal skin infections belong to the phylum Ascomycota (ascomycetes). This vast group includes yeasts, molds, and dermatophytes. Classic ringworm and other true dermatomycoses are caused by dermatophytes of the genera Trichophyton, Microsporum, and Epidermophyton. Meanwhile, the sexual stages (teleomorphs) for Trichophyton and Microsporum are combined into the common genus Arthroderma.

According to the degree of aggressiveness, causative agents are divided into two categories:

• Pathogenic fungi — true causative agents capable of initiating an infectious process (including lichen) even in individuals with a fully functioning immune system.
• Opportunistic fungi — realize their potential only when the natural resistance of the host organism decreases, causing opportunistic mycoses.

Clinical classification of fungal diseases

A detailed classification of mycoses is based on the localization of the lesion, the depth of primary colonization, and the presence of toxic-allergic effects. There are 7 main groups:

1. Superficial mycoses (keratomycoses) — the fungus invades exclusively the uppermost layers of the skin and the hair cuticle.
2. Dermatomycoses (epidermophytoses) — a more severe condition in which the infection encompasses the entire epidermis, skin, and hair as a whole.
3. Subcutaneous mycoses — the invasion penetrates through the dermis into the subcutaneous tissue, fascia, and muscles.
4. Systemic (deep) mycoses — the lesion spreads to internal organs and tissues.
5. Opportunistic mycoses — develop exclusively against the background of immunodeficiency.
6. Fungal allergies — hypersensitivity reactions (dermo- and pneumoallergies).
7. Mycotoxicoses — food poisoning caused by fungal toxins (the infectious process itself is absent).

Note: Pharmacology uses a simplified scheme dividing mycoses into systemic, dermatomycoses, and candidamycoses. This triad determines the baseline choice of antimycotic therapy.

Forms of lichen by localization

In dermatology, the classification of typical dermatomycoses is built around the anatomical lesion zone:

• Smooth skin — ringworm, dermatomycosis of smooth skin (tinea corporis).
• Scalp — scalp ringworm (tinea capitis).
• Inguinal region — tinea cruris (tinea cruris).
• Feet — tinea pedis (tinea pedis).
• Nails — onychomycosis (tinea unguium).

It is extremely important to perform differential diagnosis of true dermatomycoses from infections with a similar clinical picture. For example, pityriasis versicolor is triggered not by dermatophytes, but by the opportunistic fungus Malassezia furfur (synonym — Pityrosporum ovale). Also, skin and nail lesions visually resembling lichen can be initiated by fungi of the genus Candida.

Opportunistic and HIV-associated mycoses

In immunocompromised patients, cutaneous forms can progress to deep lesions. Thus, opportunistic infections caused by the genus Candida have 5 forms of interaction with the body: from superficial candidiasis of mucous membranes and nails to chronic granulomatous, visceral (involvement of a single organ), systemic (disseminated candida sepsis) lesions, and allergy to antigens.

Against the background of severe immunodeficiencies (HIV), disseminated fungal pathologies develop, which include candidiasis, cryptococcosis, histoplasmosis, coccidioidomycosis, as well as Pneumocystis pneumonia (its causative agent Pneumocystis jirovecii is now classified as a fungus).

Fungi of no medical significance

It is worth noting that there are vast taxa whose pathogenicity is realized only against aquatic organisms and plants. These include Chytridiomycetes (Chytridiomycota) — aquatic saprophytes, and Oomycetes (Oomycetes), which were previously considered fungi and are now moved to the kingdom Chromista (related to algae). In clinical mycology, these groups are not considered as they pose no threat to humans.

Class and exam questions

What else to study on this topic

• Teleomorphic and anamorphic stages: the connection between the genera Arthroderma, Trichophyton, and Microsporum.
• Principles of pharmacological classification of mycoses and choice of therapy.
• Differential diagnosis of skin lesions caused by dermatophytes and fungi of the genus Candida.
• Clinical forms of candidiasis: from superficial manifestations to visceral involvement and sepsis.
• Pathogenesis of HIV-associated fungal infections (cryptococcosis, histoplasmosis, coccidioidomycosis).

Other topics in the "Additional Topics" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.