Etiology and properties of pathogens
The causative agents of chromoblastomycosis belong to the group of dematiaceous fungi. These are dimorphic microorganisms, among which several main species are distinguished: Fonsecaea (compacta, pedrosoi), Phialophora verrucosa, Cladophialophora carrionii, Exophiala jeanselmei, Rhinocladiella aquaspersa, and Rhinosporidium seeberi.
The main feature of these fungi is the presence of melanin in the cell wall. It is melanin that gives the fungal elements and their colonies a characteristic brown-black hue.
The morphology of the pathogen depends on environmental conditions:
- In body tissues (in vivo): fungi exist as clusters of round, dividing cells approximately 10 µm in diameter.
- In culture (in vitro): when inoculated on Sabouraud agar, they form fluffy, dark brown colonies.
Epidemiology and pathogenesis
The natural reservoir of infection includes soil, plants, and rotting wood. Infection occurs via contact: the pathogen penetrates tissues through skin microtrauma. The most vulnerable areas (portals of entry) are the feet and lower legs.
The infection is predominantly prevalent in regions with tropical and subtropical climates. It is important to note that an infected person poses no danger to others, as the disease is not contagious.
Pathogenesis is characterized by a prolonged course (months or years). Specific changes develop in the affected area:
- Wart-like nodules form.
- Abscesses develop, followed by scarring.
- Satellite lesions form around the primary focus, giving the affected area a "cauliflower-like" appearance.
The host immune response is accompanied by the appearance of specific antibodies and the development of delayed-type hypersensitivity (DTH).
Microbiological diagnostics
Laboratory diagnosis of chromoblastomycosis is based on detecting the pathogen and isolating it in pure culture.
- Direct microscopy (native preparation)
The test material is pre-treated with a 10% KOH solution. The key diagnostic sign is the detection of so-called "sclerotic cells" (Medlar bodies). These are brown, round cells with septa. Differential diagnosis must account for the following exceptions:
- Exophiala jeanselmei forms septate hyphae.
- Rhinosporidium seeberi (the causative agent of rhinosporidiosis) forms sporangia and sporangiospores.
- Culture examination
Inoculation of pathological material is performed on Sabouraud agar. Incubation is carried out at 20–25 °C. Pathogens grow slowly—colony formation takes from 5 to 30 days. Morphologically, growth is represented by black septate mycelium with various types of conidia.
Treatment Management
Treatment of chromoblastomycosis requires a comprehensive approach.
- Medical treatment: systemic antifungal drugs are used (itraconazole, 5-flucytosine).
- Surgical treatment: removal of affected tissues is indicated (excisions of nodules and abscesses).
- Prevention: specific measures (vaccines) have not been developed to date.
Briefly: answers to topical questions
Which diseases require differential diagnosis with chromoblastomycosis?
Differential diagnosis of chromoblastomycosis during direct microscopy must be performed with the following diseases and their causative agents:
- Infection caused by Exophiala jeanselmei — this pathogen forms septate hyphae, which distinguishes it from the sclerotic cells characteristic of chromoblastomycosis.
- Rhinosporidiosis (Rhinosporidium seeberi) — the causative agent of this disease forms sporangia and sporangiospores.
The key sign of chromoblastomycosis itself is the detection of brown, round cells with septa (Medlar bodies).
Which systemic antifungal drugs are used to treat chromoblastomycosis?
The following antifungal drugs are used for the medical treatment of chromoblastomycosis:
- Itraconazole — a synthetic triazole derivative classified as a systemic antifungal agent.
- 5-Flucytosine (flucytosine) — a synthetic antifungal agent belonging to the antimetabolite group.
These medications are used as part of a comprehensive management strategy, which may also include surgical removal of affected tissues.
Chromoblastomycosis can be visualized as a "cauliflower" growing on a leg from rotting wood — this helps remember the appearance of lesions, localization, and the natural reservoir of infection.
Class and exam questions
Why do colonies of chromoblastomycosis agents appear black or brown?
The specific color of the colonies and tissue forms of the fungus is due to the presence of melanin pigment in their cell wall.
What structures should be looked for during direct microscopy?
The main diagnostic marker is Medlar bodies (sclerotic cells). These are large, round, brown cells with septa.
Does a patient with chromoblastomycosis need to be isolated?
No isolation is required. The pathogen is transmitted only via contact from the environment (soil, wood); an infected person is not contagious.
What else to study on this topic
- Morphological features of dimorphic fungi in vivo and in vitro
- Mechanism of Medlar bodies formation in human tissues
- Differential diagnosis of chromoblastomycosis and rhinosporidiosis
- The significance of delayed-type hypersensitivity (DTH) in fungal infections
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