Pathogenesis and Pattern of Hair Involvement
Fungi of the genus Microsporum exhibit a marked tropism for keratin-containing tissues, colonizing glabrous skin and the scalp. A critical pathogenetic feature of microsporia is the specific type of hair shaft invasion, known in mycology as "ecto-endothrix".
This term indicates that fungal elements actively proliferate both inside the hair structure and on its outer surface. Microscopic examination of an affected hair reveals that fungal spores arrange themselves in a characteristic mosaic pattern. Around the hair shaft, they form dense clusters that visually resemble sheaths or sleeves. This extensive destruction of the hair structure leads to hair breakage, which gave the disease its common name—ringworm.
Epidemiological Classification of Pathogens
Depending on the primary ecological reservoir and source of infection, all microsporia pathogens are divided into three major groups: anthroponotic, zooanthroponotic, and geophilic.
| Microsporia Group | Source of Infection | Main Pathogens |
|---|---|---|
| Anthroponotic | Humans only | M. audouinii, M. ferrugineum |
| Zooanthroponotic | Animals and humans | M. canis |
| Geophilic | Soil | M. gypseum, M. cookei, M. fulvum, M. nanum |
Anthroponotic microsporia is transmitted exclusively from human to human. This group includes M. audouinii and M. ferrugineum.
Zooanthroponotic microsporia is one of the most frequent forms. It is caused by M. canis. The primary reservoir of infection consists of domestic animals, predominantly cats and dogs. Notably, the infection can be subclinical in animals: the fungus is frequently present in their fur as asymptomatic carriage, posing a threat to humans in contact with them.
Morphological Features of Cultures
For precise laboratory identification of microsporia pathogens, the morphology of cultured isolates is evaluated:
- M. audouinii forms a broad, septate mycelium with a diameter of 4 to 5 µm. The culture contains arthrospores and large chlamydoconidia (up to 30 µm in diameter), whereas macroconidia and microconidia are very rare.
- The M. ferrugineum culture is characterized by the formation of a branched septate mycelium, which also gives rise to arthrospores and chlamydoconidia.
- M. canis features a septate mycelium and rounded chlamydoconidia. The primary diagnostic marker of this species is the presence of specific macroconidia: they are spindle-shaped, thick-walled, multicellular, and densely covered with spines.
Geophilic Fungi and "Gardener's Microsporia"
Geophiles are a specific group of pathogens (M. cookei, M. fulvum, M. nanum, M. gypseum) whose natural habitat is the soil. Human infection occurs via contact, typically when handling soil with bare hands without protective gloves.
A prominent clinical example of this group is "gardener's microsporia", caused by M. gypseum. The disease runs a rather severe course: a pronounced suppurative inflammatory process develops on the patient's scalp, forming a so-called kerion. The course of this form of microsporia lasts on average about 8 weeks and, unlike many other superficial mycoses, concludes with moderate scarring of the affected tissue.