Wednesday 9 November 2005
Definition: Zygomycoses are angioinvasive fungal infections due to fungi belonging to the genera of Mucorales (Rhizopus sp., Mucor sp., Rhizomucor sp., Absidia sp., Apophysomyces sp., Cunninghamella sp., and Saksenaea sp.)
Zygomycosis (or mucormycosis) describes in the broadest sense any infection due to a member of the Zygomycetes.
Zygomycosis (mucormycosis, phycomycosis) is an opportunistic infection caused by "bread mold fungi," including Rhizopus, Absidia, Cunninghamella, and Mucor, which belong to the class Zygomycetes.
These fungi are widely distributed in nature and cause no harm to immunocompetent individuals, but they infect immunosuppressed patients, albeit somewhat less frequently than do Candida and Aspergillus. The major predisposing factors are neutropenia, corticosteroid use, diabetes mellitus and breakdown of the cutaneous barrier (e.g., as a result of burns, surgical wounds, trauma).
Similar to Aspergillus, zygomycetes are transmitted by airborne asexual spores. Most commonly, inhaled spores produce infection in the sinuses and the lungs, but spores can also lead to infection following percutaneous exposure or ingestion. Macrophages provide the initial defenses by phagocytosis and oxidative killing of germinating spores.
Neutrophils have a key role in killing fungi during established infection. Proteolytic and lipolytic enzymes and mycotoxins have been identified for some of the zygomycetes, but whether these contribute to disease is not yet known. The thermotolerance of the spores of some species of zygomycetes might contribute to their spread.
Zygomycetes form nonseptate, irregularly wide (6 to 50 μm) fungal hyphae with frequent right-angle branching, which are readily demonstrated in the necrotic tissues by hematoxylin and eosin or special fungal stains.
The three primary sites of invasion are the nasal sinuses, lungs, and gastrointestinal tract, depending on whether the spores (which are widespread in dust and air) are inhaled or ingested. Most commonly in diabetics, the fungus may spread from nasal sinuses to the orbit and brain, giving rise to rhinocerebral mucormycosis.
The zygomycetes cause local tissue necrosis, invade arterial walls, and penetrate the periorbital tissues and cranial vault. Meningoencephalitis follows, sometimes complicated by cerebral infarctions when fungi invade arteries and induce thrombosis.
Lung involvement with zygomycetes may be secondary to rhinocerebral disease, or it may be primary in patients with hematologic neoplasms. The lung lesions combine areas of hemorrhagic pneumonia with vascular thrombi and distal infarctions.
Zygomycetes are primitive, fast growing, terrestrial, largely saprophytic fungi with a cosmopolitan distribution.
To date, some 665 species have been described although infections in humans and animals are generally rare. Medically important orders and genera include:
1. Mucorales, causing subcutaneous zygomycoses and systemic zygomycoses (systemic mucormycoses)
2. Entomophthorales (entomophthoraceous fungi), causing subcutaneous zygomycoses (subcutaneous entomophthoromycoses)
digestive zygomycosis (gastrointestinal zygomycosis)
- intestinal zygomycosis (intestinal mucormycosis)
rhinocerebral zygomycosis (rhinocerebral mucormycosis)
pulmonary zygomycosis (pulmonary mucormycosis)
cutaneous zygomycosis (cutaneous mucormycosis)
cerebral zygomycosis (cerebral mucormycosis)
disseminated zygomycosis (disseminated mucormycosis)
Bialek R, Konrad F, Kern J, Aepinus C, Cecenas L, Gonzalez GM, Just-Nubling G, Willinger B, Presterl E, Lass-Florl C, Rickerts V. PCR based identification and discrimination of agents of mucormycosis and aspergillosis in paraffin wax embedded tissue. J Clin Pathol. 2005 Nov;58(11):1180-4. PMID: 16254108
Karanth M, Taniere P, Barraclough J, Murray JA. A rare presentation of zygomycosis (mucormycosis) and review of the literature. J Clin Pathol. 2005 Aug;58(8):879-81. PMID: 16049294