Microsporia

Pathogenesis and pattern of hair involvement

Fungi of the genus Microsporum exhibit a marked tropism for keratin-containing tissues, colonizing smooth skin and the scalp. A crucial pathogenetic feature of microsporum is the specific type of hair shaft involvement, which in mycology is termed "ecto-endothrix".

This term means that fungal elements actively develop both inside the hair structure and on its external surface. Microscopic examination of the affected hair reveals that fungal spores arrange themselves in a characteristic mosaic pattern. Around the hair shaft, they form dense clusters that visually resemble specific sheaths or cuffs. It is this extensive destruction of the hair structure that leads to its breakage, which gave the disease its common name — ringworm.

Epidemiological classification of pathogens

Depending on the main ecological reservoir and source of infection, all pathogens of microsporum are divided into three large groups: anthroponotic, zoonotic (zooanthroponotic), and geophilic.

Anthroponotic microsporum is transmitted exclusively from human to human. This group includes M. audouinii and M. ferrugineum.

Zoonotic microsporum is one of the most frequent forms. It is caused by the species M. canis. The main reservoir of infection is domestic animals, predominantly cats and dogs. It is important to note that the infection in animals can be subclinical: the fungus is often present in their fur as asymptomatic carriage, posing a threat to people interacting with them.

Morphological features of cultures

For accurate laboratory identification of microsporum pathogens, the morphology of grown cultures is assessed:

• M. audouinii forms a broad septate mycelium with a diameter of 4 to 5 µm. The culture contains arthrospores and large chlamydospores (up to 30 µm in diameter), while macroconidia and microconidia are very rare.
• The culture of M. ferrugineum is distinguished by the formation of a branched septate mycelium, which also forms arthrospores and chlamydospores.
• M. canis is characterized by a septate mycelium and rounded chlamydospores. The main 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 gardeners' microsporum

Geophilic fungi are a special group of pathogens (M. cookei, M. fulvum, M. nanum, M. gypseum) whose natural habitat is soil. Human infection occurs via contact, typically when handling soil with bare hands without protective gloves.

The most striking clinical example of infection from this group is "gardeners' microsporum", caused by M. gypseum. The disease is quite severe: a pronounced purulent-inflammatory process develops on the patient's scalp, forming a so-called kerion. The course of this form of microsporum takes an average of about 8 weeks and, unlike many other superficial mycoses, concludes with moderate scarring of the affected tissue.

Class and exam questions

What else to study on this topic

• Morphological differences of macro- and microconidia in various species of the genus Microsporum
• Features of mycelium and chlamydospore structure in M. audouinii and M. ferrugineum
• Mechanisms of asymptomatic M. canis carriage formation in animals
• Pathogenesis of kerion formation upon infection with geophilic fungi
• Complete list of geophilic pathogens and their ecological niches

Other topics in the section "Epidermophytoses (microsporia, trichophytosis, favus, rubroophytosis, etc.)"

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