Pulmonary Aspergillosis: Pathologic and Pathogenetic Features Richard S. Fraser Aspergillosis is a disease of worldwide distribution caused by species of the dimorphic fungus Aspergillus. Although it can occur in virtually any organ, the lungs are by far the most commonly affected. Within them, the tissue reaction to the presence of fungus and the associated clinical and radiological manifesta­ tions are more varied than those associated with any other infectious agent (Table 1), with the possible exception of Mycobacterium tuberculosis. The pur­ pose of this review is to document and illustrate these varied pathologic manifes­ tations of pleuropulmonary aspergillosis and to briefly discuss their patho­ genesis and the mechanisms of clinical and radiographic disease. Other features of the various disease states and of the fungus itself are discussed in detail in several other sources.1-7 THE ORGANISM Although approximately 300 species of Aspergillus have been described,5 only a few cause human disease. By far the most common is A fumigatus; A niger, A flavus, and A glaucis are responsible infrequently and other species rarely. In tissue, the organism typically grows as regular septate hyphae, 2 to 5 p.m in diameter, which often appear to extend from a common point in a roughly parallel orientation, resulting in a fan-like appearance (Fig. 1A). Charac­ teristically, the hyphae branch dichotomouslv at an angle of 45° (Fig. IB). They are usually easily seen in H&E preparations, although small, fragmented forms may be difficult or impossible to identify; periodic acid Schiff (PAS) and silver 231 2 3 2 R.S. FRASER TABLE 1. CLINICOPATHOLOGIC FORMS OF PLEUROPULMONARY ASPERGILLOSIS General Form Specific Disease Saprophytic aspergillosis Airway colonization Aspergilloma (Fungus ball) Invasion of necrotic tissue Allergic aspergillosis Hypersensitivity pneumonitis Loeffler’s syndrome (eosinophilic pneumonia) Allergic bronchopulmonary aspergillosis Invasive aspergillosis Acute bronchopneumonia Angioinvasive aspergillosis Acute tracheobronchitis Miliary aspergillosis Aspergillus pleuritis and empyema Chronic necrotizing aspergillosis stains demonstrate them particularly well. Although these histological features are very suggestive of Aspergillus species, they cannot by themselves be consid­ ered diagnostic. Other fungi, such as Pseudoallescheria boydii, resem ble Aspergillus very closely in tissue sections. In addition, degenerated hvphae of Aspergillus may swell and become quite irregular in shape, resembling fungi of the order Mucorales (Fig. 2A). Thin hvphae, especially if disrupted, can be mistaken for the pseudohyphae of Candida species (Fig. 2B). In the absence of the results of culture or immunologic investigations,8 9 definitive identification or exclusion of Aspergillus as the cause of disease thus must be made with care. However, two specific features seen on routine light microscopy can help make a confident diagnosis. Certain Aspergillus species, particularly A niger, are associated with the production of calcium oxalate, manifested morphologically as irregularly shaped, angulated, strongly refractile crystals (Fig. 3).1011 These are believed to result from the combination of oxalic acid produced by the organism and calcium derived from the host and have been seen in both invasive and saprophytic forms of disease. Their identifica­ tion in tissue, pleural fluid, 12 or bronchial secretions13 is good evidence that a fungal infection is truly caused by Aspergillus. The documentation of oxalic acid in fluid obtained by bronchoalveolar lavage also may aid in specific diagnosis.14 A second histological finding confirming the presence of Aspergillus is the conidiophore. This is the asexual reproductive organ of the fungus and consists of an elongated hvpha that terminates in a swollen vesicle of variable diameter from which emanate numerous spore-producing tubes called phialides or sterigmata (Fig. 4). A variable number of more or less round conidia (spores), 1 to 3 /am in diameter, are usually present adjacent to the sterigmata. Conidiophores are seen uncommonly, usually when the site of infection is exposed to air; thus, they are present most often in aspergillomas and in invasive tracheobronchitis. Aspergillus organisms are ubiquitous, being found in soil, water, and decay­ ing organic matter throughout the world. As indicated above, asexual reproduc- Figure 1. Aspergillus hyphae: lypical morpnoiogy. A. lypical radiating tan ap­ pearance of aspergillus hyphae in tissue. B. Magnified view showing uniform hyphae with parallel walls, septa, and dichotomous branching at an angle of 45°. (Grocott silver methenamine: A. x45, B. x650) 233 Figure 2. Aspergillus hyphae: Atypical morphology. A. Degenerated hyphae show­ ing irregular size and shape and tocal marked swelling, an appearance suggestive of Mucorales. B. Very thin, somewhat beaded forms that might be mistaken for the pseudohyphae of Candida sp. (normal appearing hyphae are present on the bot­ tom). (H&E x400) 2 3 4 PULMONARY ASPERGILLOSIS 2 3 5 * , • «• » *;<* " • ‘ , ■' * ' * . " X * • . 4* ' V . **' 4 , hT » • * « *» ** * ■» * I % • * * * v i * % ?v. * *<* " - ■ ‘ «•*' * *• • * * , / - «r» * - ^ -/■• ; v -. ** * « - , " .# * #, "** Jf** ** # r •» / * - /8 *• * * * -V ' f l 4 ^ ' • v ’v*; * ■ ; .* # * p *#‘-t * ^ »*> •- f * '« * * * £ ■ •* . i -4 •_• - / n *# « #»* «,*• <# * * «» %» ** M* * ^ ow, * * •a'* « T ■ * * • * * 3# **■■ * ^ *« * «f «* ^ ' * * f*"» I! I. * * , , y *' > * * / " * + ( ’ . f V * / . * ■ -* 1 , - „■. , . ■»...*«» % «,... - * * I* ** * # # * *. ' •■ -» ' j . v » * j.* it- .• " 'A * >v » *2 » . ^ v ‘'** %. • % * * %•>* * w J* - * v , * * *»• * ♦**• * * *'*• V* i .'**,*■“ v , ••. * * **>. ■ t»r*" V . " * ?* • .* i. M*.* ** * .♦• * * 4 * V 1 « m « Figure 3. Aspergillus abscess with calcium oxalate. High power view of a pulmo­ nary abscess showing several irregularly shaped clumps of what appear to be crystalline foreign material (arrows) admixed with neutrophils and necrotic debris. Although hyphae are not visible in the photomicrograph, they were easily demon­ strated by silver stain throughout the abscess. (H&E x375) tion is by means of conidiophores, and it is by inhalation of conidia that the vast majority of infections occur. Conidia are present virtually everywhere in the atmosphere (including hospital wards),15 although the number varies somewhat with geographic location and season. Because of this, many infections probably are derived from general atmospheric “contamination.” Others, however, un­ doubtedly are related to the presence of organisms in specific sites, including contaminated marijuana cigarettes,16 and such hospital sources as pigeon ex­ creta located near a ventilation system17 and dust created during renovation.18 Some instances of invasive pulmonary aspergillosis also originate in an endoge­ nous focus of colonization or saprophytic growth. This is well documented in occasional cases of aspergilloma (see below) and also may occur after coloniza­ tion of the nose or upper airways.19 GENERAL PATHOGENETIC FEATURES As with other pulmonary infections, the risk of developing disease after contact with Aspergillus depends on the interplay between virulence20 and inoculating 2 3 6 R.S. FRASER Figure 4. Aspergillus conidiophores. Typical appearance of conidiophores as seen in tissue sections. V, vesicle; long arrow, sterigmata; short arrows, conidia. (Grocott silver methenamine x640) dose21 of the organism, and the ability of the host to resist infection,22 The last feature is influenced most importantly by the competence of local and systemic host defense. Aspergillus species produce a variety of toxic substances that undoubtedly are of importance in the pathogenesis of disease. Both A fumigatus and A fla vu s have been shown to produce an endotoxin that is lethal after injection in several animals.23 A variety of proteases, including elastase, also are produced; the presence of the latter enzyme has been shown to be related to the development of invasive disease in mice.24 The ability of different species and strains to produce these toxic substances may at least partially explain their variable pathogenicity.24 The conidial diameter also may be a factor influencing the risk of disease. For example, conidia of A fumigatus measure 2 to 3 yum in diameter, a size ideal for inhalation and deposition in the distal respiratory tract. By contrast, conidia of the relatively less common pathogens A niger and A flavus are 4 to 5 /xm in diameter, a size more likely to be associated with proximal airway deposition and, possibly, more rapid mucociliary clearance. This, however, cannot be the sole explanation for variation in species pathogenicity, as some species that are rare causes of human disease have a conidial diameter similar to that of A fum igatus. PULMONARY ASPERGILLOSIS 2 3 7 The precise mechanisms of host defense against Aspergillus species are not well understood. It is likely that clearance of inhaled conidia by alveolar macrophages and the tracheobronchial mucociliary escalator is important as a local defense. In situations in which such clearance appears to be deranged, as in bronchiectasis or the cavities of chronic tuberculosis, the likelihood of fungal colonization and growth is greatly increased. Although efficient pulmonary clearance also may be important in preventing invasive disease, a variety of observations suggest that adequate inflammatory and immunologic reactions are of even greater significance. The most convincing of