Micheli (1729) identified the genus Aspergillus, naming it after the perforated globe used to shower holy water during Catholic religious’ rites (aspergus te). Aspergillosis is an opportunistic invasive mycosis caused by hyaline molds of the genus Aspergillus (A.fumigatus, A.flavus, A.terreus, A.niger) and, less commonly, other Aspergillus species. Aspergillosis has a wide spectrum of clinical manifestations, including:
- Invasive aspergillosis:
- The most common clinical manifestation of pulmonary aspergillosis is tracheobronchitis.
- Extrapulmonary dissemination includes bones and joints, cardiac, cerebral abscess, cutaneous, endocarditis, esophageal and gastrointestinal, ocular, and renal.
- Colonizing aspergilloma
- Allergic bronchopulmonary aspergillosis
- Other clinical manifestations include endocarditis after prosthetic heart valve implantation, mycotoxin poisoning, otomycosis, and sinusitis, including allergic and invasive forms.
The fungus Aspergillus is one of the most ubiquitous in the human environment. Virchow described the first examples of Aspergillus sp. infection in humans in 1856, with precise microscopic illustrations of the organism in vivo. Virchow noticed that the organism he had seen in human illness (most likely A. fumigatus) was closely linked to the ‘parasitic’ vegetative structures that grew in live animals.
Invasive aspergillosis is more common in immunocompromised people, such as those who have had a hematologic malignancy, stem cell transplants, or lung transplants. With the introduction of extended spectrum triazole treatment (e.g., voriconazole), better diagnostic imaging, and an antigenemia test, the steepest surge occurred in the early 1990s, followed by a steady reduction.
Habitat and Transmission
The intake of airborne conidia causes infection. Conidia alighting on damaged skin, penetrating injuries, and fomites such as contaminated or nonsterile tape used to fix bandages or catheters are also less frequent ways for infection to develop.
The most prevalent pathogen, Aspergillus fumigatus, has historically been the most common cause of invasive aspergillosis, accounting for up to 90% of isolates. The organism can be found in a variety of places in nature, including soil, rotting plants, the air, and drinking water.
Aspergillus flavus is a fungal fungus that may be found in soil and rotting plants. Aspergillus terreus is found in tropical and subtropical environments and is becoming more well recognized as a cause of invasive infection in immunocompromised individuals. Aspergillus niger is found in soil and on plants, as well as in foods like pepper.
Pathogenesis and Host Defenses
Infection with Aspergillus niger is usually contracted by inhaling conidia into the lungs, although alternative routes of infection, such as oral or aerosol contact to contaminated water, can also occur. Cutaneous infections can occur as a result of cutaneous exposure from surgical wounds, infected intravenous catheters, or arm boards.
Toxins produced by Aspergillus species, such as aflatoxins, ochratoxin A, fumagillin, and gliotoxin, can cause clinical symptoms after exposure and may contribute to virulence in certain situations. Gliotoxin, for example, has a major influence on macrophage and neutrophil activity, however in experimental models, gliotoxin synthesis was disrupted and neutropenic animals survived. Gliotoxin gene deletion resulted in longer life in nonneutropenic animals, suggesting that it may be more relevant in invasive infection in nonneutropenic patients. Other pathogenic determinants include pathogenic strains’ production of a wide range of proteases and phospholipases.
Table below summarizes the innate and adaptive immune responses in the lung to inhaled Aspergillus conidia. In the immunocompromised host, the majority of these functions are downregulated or missing.
|Mucociliary elevator||Clears Aspergillus conidia from the airways.|
|Alveolar macrophages||Conidia that have made their way into the alveoli are phagocytosed. (Before T lymphocytes boost them, macrophages are not fully activated.) TNF- is produced by macrophages, and it enhances vascular permeability and endothelial adhesiveness for leukocytes and platelets, enabling fluid, cells, and proteins to enter the body and help in host defense. PAMPs on Aspergillus niger bind to TLR4 and Dectin-1 on the macrophage surface. This stimulates transcription factors, which cause inflammatory cytokines to be released and T lymphocytes to become activated.|
|Opsonization||Conidia are opsonized by complement activation and IgG before and after germination, assisting in their phagocytosis.|
|PMNs||PMNs, active killers of Aspergillus that use reactive oxygen species (ROS) and nonoxidative microbicidal peptides, attack conidia that escape macrophage phagocytosis or germinate and burst through macrophages.|
|Plasminogen||Local blood clots keep the infection contained, but they must be dissolved to avoid infarctions. Activation of plasminogen is used to attain it.|
|Adaptive immunity, CD4+ T-lymphocytes||IFN-γ and the CD40 ligand, which sensitizes macrophages via interaction with CD40 on the macrophage, are two signals that T cells deliver to activate macrophages. T cells generate interleukin-18 (IL-18), interleukin-12 (IFN-γ), and interleukin-12 (IL-12). The Th1 response is characterized by the presence of IL-18, which promotes the production of IFN-γ.|
|Activated macrophages||They efficiently fuse their lysosomes to phagosomes, allowing lysosomal hydrolases to digest ingested Aspergillus conidia and hyphal components. Activated macrophages produce oxygen radicals and nitric oxide (NO), both of which have antibacterial properties.|
The clinical manifestations of Aspergillus species illnesses vary, but they all reflect the host’s underlying immunological state and reaction to the fungus. Asymptomatic colonization, superficial or saprophytic infection, allergic reactions to the organism, and acute or subacute invasive illness are all examples of aspergillosis syndromes.
Aspergillus fungal balls can colonize preexisting lung cavities in chronic pulmonary diseases including healed TB, sarcoidosis, and emphysema. An Aspergillus species causes a fungus ball or mycetoma in the lung, which is characterized as a mass of fungal hyphae embedded in a matrix of cell debris and fibrin, enclosed in a preexisting pulmonary cavity. The hollow wall may thicken with time, allowing the fungus ball to move freely within the cavity. Cough and hemoptysis are two clinical findings. The fungal colonization is tolerated by the majority of patients, and the yearly chest film shows minimal change.
Following a penetrating injury or ocular surgery, Aspergillus species are an uncommon causative cause of this locally invasive corneal infection. Agricultural workers have a higher risk of developing Aspergillus keratitis. Ocular discomfort and the risk of vision loss are common symptoms of this illness. In contact lens wearers who do not follow adequate antiseptic cleaning methods, Aspergillus species are an uncommon cause of fungal keratitis.
Otomycosis is a harmless infection caused by Aspergillus, which grows on debris in the external auditory canal (especially A. niger).
Allergic Fungal Sinusitis: Allergic sinusitis is more common in atopic people. Allergic rhinitis, nasal polyps, recurrent sinusitis, and asthma are some of the symptoms. Although Aspergillus species have been linked to allergic sinusitis, melanized molds are thought to be more common.
Invasive Sinusitis: Invasive sinusitis is a unique illness that necessitates extensive diagnostic work-up and antifungal medication when the host is immunocompromised by Stem cell transplantation and neutropenia. Hyphae can infiltrate mucosa and bone, causing bleeding, infarctions, and extend into the orbit of the eye and the brain. Because some Aspergillus species involved in invasive sinusitis may have decreased sensitivity to AmB so biopsy, culture, and antifungal susceptibility tests are critical.
Aspergillosis causes mucoid casts containing Aspergillus hyphae to form in the trachea and bronchi, causing airway obstruction and presenting clinically as bilateral lower lobe infiltrates. AIDS patients and lung transplant recipients are both susceptible to obstructing bronchial aspergillosis. Bronchoscopy is required for diagnosis, and it may also be therapeutic if mucus plugs can be removed.
Invasive Pulmonary Aspergillosis
Fever, pleuritic chest discomfort, and severe pneumonia are common symptoms of invasive pulmonary aspergillosis, although the clinical presentation varies. Invasive fungal sinusitis can lead to pneumonia. Because of Aspergillus‘ angioinvasive tendency, hemoptysis can accompany pneumonia due to pulmonary infarction.
Cerebral Invasive Aspergillosis
Rather than developing as a direct extension of sinonasal illness, cerebral invasive aspergillosis is most commonly the consequence of hematogenous dissemination from a pulmonary site.
Osteomyelitis and Septic Arthritis
After lung, sinus, and brain infections, osteomyelitis is the fourth most frequent site of infection for aspergillosis. Although Aspergillus osteomyelitis can be classified as vertebral, skull, rib, or long bone infections, the treatment guidelines have remained consistent. Due to hematogenous dispersion, Aspergillus osteomyelitis typically affects immunocompromised individuals, although it can also affect immunocompetent people as a result of surgery, IV medication usage, trauma, and prior aspergillosis.
Sputum, bronchoalveolar lavage fluid, fine needle aspirates, thorascopic biopsy, and other biopsy materials are examples of direct examination specimens. Surgical blades, blunt platinum spatulas, or calcium alginate swabs soaked in trypticase soy broth are used to collect specimens in the event of fungal keratitis. On microscopic inspection of KOH preps of fresh clinical material, the morphology of hyphal components may be observed. Hyaline, tightly septate, and dichotomously branched at an acute angle, Aspergillus hyphae in sputum or nasal scrapings.
Mycotic thrombi, which may be seen in tissue, are caused by Aspergillus species invading blood arteries. The fungus proliferates in tissue in parallel or radial arrays when there is little host inflammatory response, such as in immunocompromised hosts with invasive aspergillosis. The hyphae may be twisted and pushed into strange “varicose” forms, similar to those of the Mucorales, in reaction to the host’s inflammatory response.
In most clinical microbiology laboratories, the original SDA is the media of choice for mold isolation. When the specimen comes from a nonsterile location, antibiotics such as chloramphenicol and gentamicin are added to the medium. Aspergilli do not develop in cycloheximide-containing media, such as “Mycosel” and “Mycobiotic” medium. At 30–37°C, sporulation occurs in 48–72 hours on SDA-Emmons agar. Tease mounts are preferred, although slide cultures are also useful for examining asexual reproductive features, such as sporing heads, if they are not revealing.
Depending on the species, Aspergillus has a wide range of colony shape. Most species start off as white colonies that quickly become green, yellow, orange, black, or brown. Colonies develop in 3–5 days and are fluffy to velvety. Some create concentric color circles, while others are colorless.
Aspergillus fumigatus is thermotolerant, growing and sporulating at temperatures as high as 48°C and as low as 20°C. On SDA-Emmons agar, typical isolates grow quickly and are blue-green to gray-turquoise in hue, with the reverse being uncolored to cream. Conidia pigmentation is responsible for the hue. The texture of the colony is velvety and woolly. The region of vigorous mycelial development on the colony’s outer border is white. Conidial heads are uniseriate (no metulae), with phialides concentrated on the top surface of the vesicle (in contrast to A. flavus). Conidia are widely distributed, although they can form lengthy chains on occasion. Phialoconidia are 2.4–3.0 µm long, spherical to broadly ellipsoidal, and finely roughened to rough on the surface.
A. flavus, the second most commonly seen species in clinical practice, is a major plant disease. A. flavus is morphologically similar to a number of other species. Because of the colored conidia, the colony color changes from olive green to yellow (parrot) green on SDA-Emmons agar at 25°C. Phialoconidia are globose to subglobose, 3.8–5 µm in diameter, and have a finely rough to rough surface texture at high power.
Cinnamon-brown Aspergillus terreus colonies on SDA-Emmons are uncommonly vivid orange-brown. The back of the colony has a light golden color. Conidial heads are long, fan-shaped, and compact, with a diameter of 30 to 50 μm and a length of 150 to 500 μm. Smooth-walled conidia are brown, spherical to broadly ellipsoidal, and 1.5–2.5 μm in diameter.
Conidia turn the white mycelium of Aspergillus niger black. Brown conidia fill the whole vesicle. Conidia are subspherical, 3.5–5.0 µm in diameter, and have warts and ridges.
Antibody testing are helpful in the diagnosis of allergic bronchopulmonary illness and aspergilloma. Galactomannan (GM) detection in serum and, more recently, other bodily fluids, particularly BAL, has been critical in the serological diagnosis of invasive aspergillosis.
The detection of serum (1–>3) beta-D-glucan using an amebocyte limulus lysate test is another serological-based technique that has been authorized for clinical usage Because this test detects cell wall beta-glucans, it is not specific for Aspergillus and can be positive in infections caused by other fungus, such as Candida and molds other than Aspergillus.
Invasive aspergillosis has also been studied using the use of PCR to create a molecular foundation for diagnosis. Although circulating PCR product levels in serum or blood are minimal, significant results have been found when used in conjunction with other diagnostic techniques.
For most patients with invasive aspergillosis, voriconazole, a powerful, broad-spectrum triazole antifungal, has become the recommended first-line treatment.
- Essentials of Clinical Mycology (Mary et al.,)
- Fundamentals of Medical Mycology (Frrol et al.,)
- Atlas of clinically important fungi