Aspergillosis is the collective term used to describe all disease entities caused by any one of ~50 pathogenic and allergenic species of Aspergillus. Only those species that grow at 37°C can cause invasive infection, although some species without this ability can cause allergic syndromes. A. fumigatus is responsible for most cases of invasive aspergillosis, almost all cases of chronic aspergillosis, and most allergic syndromes. A. flavus is more prevalent in some hospitals and causes a higher proportion of cases of sinus infections, cutaneous infections, and keratitis than A. fumigatus. A. niger can cause invasive infection but more commonly colonizes the respiratory tract and causes external otitis. A. terreus causes only invasive disease, usually with a poor prognosis. A. nidulans occasionally causes invasive infection, primarily in patients with chronic granulomatous disease.
Aspergillus has a worldwide distribution, most commonly growing in decomposing plant materials (i.e., compost) and in bedding. This hyaline (nonpigmented), septate, branching mold produces vast numbers of conidia (spores) on stalks above the surface of mycelial growth. Aspergilli are found in indoor and outdoor air, on surfaces, and in water from surface reservoirs. Daily exposures vary from a few to many millions of conidia; the latter high numbers of conidia are encountered in hay barns and other very dusty environments. The required size of the infecting inoculum is uncertain; however, only intense exposures (e.g., during construction work, handling of moldy bark or hay, or composting) are sufficient to cause disease in healthy immunocompetent individuals. Allergic syndromes may be exacerbated by continuous antigenic exposure arising from sinus or airway colonization or from nail infection. High-efficiency particulate air (HEPA) filtration is often protective against infection; thus HEPA filters should be installed and monitored for efficiency in operating rooms and in areas of the hospital that house high-risk patients.
The incubation period of invasive aspergillosis after exposure is highly variable, extending in documented cases from 2 to 90 days. Thus community acquisition of an infecting strain frequently manifests as invasive infection during hospitalization, although nosocomial acquisition is also common. Outbreaks usually are directly related to a contaminated air source in the hospital.
Global aspergillosis incidence and prevalence have been estimated (Table 241-1). However, given the inadequate diagnostic capability in almost all low- and middle-income countries, the accuracy of these estimates is uncertain. The frequency of different manifestations of aspergillosis varies considerably with geographic location; most notably, chronic granulomatous sinusitis is rare outside the Middle East and India, and fungal keratitis is particularly common in Nepal, Myanmar, Bhutan, and India (800 and 113 cases/100,000 population, respectively). The potential effects of chronic pulmonary aspergillosis after pulmonary tuberculosis have only recently been appreciated and include life-threatening hemoptysis, misdiagnosis of smear-negative tuberculosis, and general exacerbation of posttuberculosis morbidity.
TABLE 241-1Disease Frequency and Diagnostic Sensitivity for Different Manifestations of Aspergillosis