Chapter 235 Zygomycosis (Mucormycosis)
Etiology
Zygomycosis refers to a group of opportunistic fungal infections caused by dimorphic fungi of the class Zygomycetes, which are primitive, fast-growing fungi that are largely saprophytic and ubiquitous. These organisms are found commonly in soil, in decaying plant and animal matter, and on moldy cheese, fruit, and bread.
This class is subdivided into 2 orders, Mucorales and Entomophthorales, each containing human pathogens. The term mucormycosis refers only to infections caused by Mucorales, which includes the genera Absidia, Apophysomyceae, Mucor, Rhizomucor, and Rhizopus and represents the more-common cause of zygomycosis in humans. Infections caused by organisms of the genera Cunninghamella, Saksenaea, and Cokeromyces are seen less often. Mucorales disease in humans is characterized by a rapidly evolving course, tissue necrosis, and blood vessel invasion in addition to subcutaneous infection. These infections are most acute and fulminant in debilitated patients. Genera of the order Entomophthorales causing infection in humans include Conidiobolus and Basidiobolus. These agents typically cause indolent sinus or subcutaneous infections in immunocompetent persons.
Epidemiology
Zygomycosis is primarily a disease of persons with underlying conditions that impair host immunity. Predisposing factors include diabetes, hematologic malignancies, persistent acidosis, corticosteroid or deferoxamine therapy, organ transplantation, prematurity, and, less commonly, AIDS. Fungi that are pathogenic in humans grow on almost any carbohydrate substrate and are able to grow at temperatures >37°C. Acidosis diminishes the phagocytic and chemotactic ability of neutrophils while increasing the availability of unbound iron. Deferoxamine-bound iron can also be used by the fungus to enhance its growth.
Pathogenesis
Macrophages and neutrophils are the main host defense against Zygomycetes and other filamentous fungi and provide almost complete immunity against Zygomycetes by phagocytosis and oxidative killing of spores, perhaps explaining the predilection for zygomycosis in patients with neutropenia or neutrophil dysfunction. The primary route of infection from Zygomycetes is inhalation of spores from the environment. In immunocompromised persons, if spores are not cleared by macrophages they germinate into hyphae, resulting in local invasion and tissue destruction. Cutaneous or percutaneous routes of infection can lead to cutaneous and subcutaneous zygomycosis. Ingestion of contaminated food or drinks has been linked to gastrointestinal disease.
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