Features of pathogenesis and localization
The group of superficial mycoses, also known as keratomycoses, is characterized by an extremely low degree of contagiousness. This means that the risk of transmitting the infection from person to person is minimal.
The pathological process in these diseases is strictly limited. Keratomycetes develop only in dead, keratinized tissues. Their main zones of habitation and reproduction are the superficial layers of the stratum corneum of the epidermis (smooth skin), as well as the cuticle (surface) of the hair. At the same time, the pathogens are unable to penetrate the deep layers of the epidermis, dermis, hair follicle, or hair cortex, which largely determines the mild clinical presentation and specific treatment.
Main causative agents of keratomycoses
Depending on the localization of the lesion, keratomycetes can be divided into two main groups: those affecting smooth skin and those affecting hair. In the affected tissues, they form specific morphological structures.
Fungi affecting the epidermis of smooth skin:
- Malassezia furfur. The causative agent of pityriasis versicolor. Microscopic examination of the stratum corneum reveals short, slightly curved hyphae combined with yeast-like cells.
- Phaeoanellomyces werneckii. Causes a disease known as tinea nigra. In the tissues of the stratum corneum, this pathogen is represented by dark septate hyphae and budding cells.
Fungi affecting hair (causative agents of piedra):
- Piedraia hortae. The causative agent of black piedra. Clinically manifested by the formation of dense black nodules on the hair surface. Microscopically, asci are identified inside these nodules.
- Trichosporon beigelii. Causes white piedra. Characteristic yellowish nodules form around the hair shaft. They consist of mycelium fragments and arthroconidia.
Principles of therapy for superficial mycoses
Given the superficial localization of the pathogen, the treatment of keratomycoses is based on a combination of two approaches: mechanical removal of the affected tissues and local pharmacotherapy.
The main strategy consists of active exfoliation of infected scales using keratolytic agents. The following drug groups are used in treatment regimens:
- Keratolytics and antiseptics. Preparations containing selenium disulfide, sodium thiosulfate, or salicylic acid. They promote the loosening and removal of the stratum corneum along with fungal elements.
- Antifungal antibiotics. A representative of the group is amphotericin B.
- Synthetic antifungals. Miconazole nitrate is the most actively used.
Mechanism of action of synthetic antifungals (using miconazole nitrate as an example) is the direct inhibition of ergosterol synthesis. Ergosterol is a vital structural component of the fungal cell membrane; without it, the membrane loses its integrity, leading to the death of the pathogen.
In specific hair infections caused by Piedraia hortae or Trichosporon beigelii, pharmacotherapy may be insufficiently effective due to the formation of dense nodules. In such cases, the most effective measure is mechanical shaving of the affected hair combined with careful adherence to personal hygiene rules.
Briefly: answers to topical questions
What laboratory diagnostic methods are used to confirm the diagnosis of pityriasis versicolor?
To confirm the diagnosis of pityriasis (versicolor) versicolor, the microscopic method of examining pathological material is used. This approach allows visualizing the pathogen directly in the lesion area.
- Fungal elements (Malassezia furfur) — microscopy of scales from the stratum corneum of the epidermis reveals yeast-like cells and pseudomycelium, which looks like short, slightly curved filaments or hyphae.
What are the main clinical manifestations (symptoms) of pityriasis versicolor on smooth skin?
The main clinical manifestations of pityriasis versicolor are skin changes on the trunk, neck, arms, as well as on the chest and in the armpits.
- Spots — pinkish-yellow non-inflammatory elements; they can be either hyperpigmented or hypopigmented. On tanned skin, they may appear as depigmented spots 0.5–1 cm in size.
- Desquamation — scraping the lesions separates bran-like scales.
To remember the causative agents of piedra: Piedraia hortae sounds harsh and starts with "P" — this is black piedra (black nodules with asci). Trichosporon beigelii — white piedra (yellow nodules with arthroconidia).
Class and exam questions
Do the causative agents of superficial mycoses penetrate into the hair follicle?
No, keratatomycetes localize exclusively on the hair surface (cuticle) and do not penetrate into the hair follicle or the cortex.
What is the mechanism of action of miconazole in the treatment of keratomycoses?
Miconazole nitrate inhibits the synthesis of ergosterol, which is a critically important component of the fungal cell membrane, leading to the death of the microorganism.
How to most effectively treat piedra (hair infection)?
The most effective measure for infections caused by Piedraia hortae and Trichosporon beigelii is shaving the affected hair and maintaining personal hygiene.
What else to study on this topic
- Morphological differences between Malassezia furfur and Phaeoanellomyces werneckii hyphae under a microscope
- Life cycle of keratomycetes in the stratum corneum of the epidermis
- Properties and mechanism of action of salicylic acid as a keratolytic in mycoses
- Structure of asci in Piedraia hortae and arthroconidia in Trichosporon beigelii
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