Aspergillus is readily isolated from samples of soil, decaying vegetation, water, and air worldwide. Aspergillus fumigatus, followed by Aspergillus flavus, Aspergillus niger, and Aspergillus terreus, are the most common species that cause human disease. Aspergillosis follows exposure of a susceptible host to the ubiquitous conidia (spores). Germinating conidia form hyphae, the invasive form of the fungus. Aspergillus hyphae average 2 to 4 μ in diameter and are septate, with dichotomous (Y-shaped) branching (Figure 171.1). The spectrum of diseases caused by the aspergilli is wide and profoundly influenced by the underlying immune status of the host.

CLINICAL MANIFESTATIONS AND DIAGNOSIS OF INVASIVE ASPERGILLOSIS

Although inhalation of conidia is common, invasive disease is relatively rare. The vast majority of affected patients are severely immunosuppressed. Major risk factors include profound, prolonged neutropenia due to cytotoxic chemotherapy and macrophage dysfunction due to high doses of corticosteroids. In patients who have undergone hematopoietic stem cell transplantation, additional risk factors are graft-versus-host disease and cytomegalovirus infection. Invasive aspergillosis also is particularly common in individuals with chronic granulomatous disease (CGD), a rare genetic disorder characterized by a defective phagocyte respiratory burst. Finally, recent studies have suggested that critically ill patients are at risk for invasive aspergillosis, even without aforementioned risk factors.