Morphology and cultural properties of the pathogen
The causative agents of aspergillosis are septate mold fungi. They have a branching mycelium and reproduce asexually by forming spores — conidia.
- Microscopic structure: The spore-bearing hypha ends with a characteristic swelling (conidiophore), on which sterigmata (phialides, metulae) are arranged in one or two rows. Chains of conidia extend from them. Under a microscope, this complex structure strongly resembles a watering can.
- Staining: The color of the colonies directly depends on the color of the formed conidia. They can be black, green, white, or yellow.
- Cultivation: Aspergillus species are obligate aerobes. They grow excellently on standard nutrient media (wort agar, Czapek-Dox agar, Sabouraud agar) at an optimum temperature of 24–37 °C. Initially, fluffy white colonies grow on the medium, which acquire a species-specific color within 2–4 days.
Epidemiology and pathogenicity factors
Fungi of the genus Aspergillus are ubiquitous: their conidia are constantly present in soil, water, atmospheric air, and on decaying plant remains. Out of approximately 200 known species, about 20 are considered pathogenic to humans, among which the leading roles are played by A. fumigatus, A. niger, A. flavus, A. terreus, and A. nidulans.
The main route of infection is aerogenous (inhalation of conidia). Contact transmission is less common. Risk groups include individuals of certain professions (working with dust, moldy paper — the so-called "garbage collector's disease" or "ragpicker's disease"). Of immense importance is the iatrogenic factor — invasive medical interventions such as bronchoscopy, punctures, or vascular catheterization.
Key pathogenicity factors of Aspergillus:
- Aggressive enzymes: the pathogen synthesizes protease, elastase, collagenase, and acid phosphatase, which destroy host tissues.
- Toxins: some species (especially A. flavus and A. parasiticus) produce potent aflatoxins. When accumulated in food products, they cause alimentary aflatoxicoses, which have pronounced carcinogenic effects and can lead to severe liver cirrhosis.
Pathogenesis and clinical forms
The development of severe forms of aspergillosis is closely associated with immunodeficiency. Normally, cellular defense factors (macrophages and granulocytes simply digest them) successfully cope with fungal conidia, and delayed-type hypersensitivity (DTH) is also formed in the patient. However, when immune system failures occur, the pathogen is capable of generalization with involvement of the central nervous system, endocardium, and paranasal sinuses.
Main clinical variants:
- Invasive pulmonary aspergillosis. The most severe form, most commonly caused by A. fumigatus. It is characterized by rapid invasive growth of the fungus, invasion into blood vessels, and their thrombosis.
- Allergic bronchopulmonary aspergillosis. The disease proceeds in the manner of bronchial asthma. It is characterized by the development of allergic alveolitis and persistent blood eosinophilia.
- Aspergilloma (aspergillary mycetoma). This is a specific form of granuloma, most often localized in the lung tissue. It represents a dense tangle of mycelium surrounded by a thick fibrous wall.
- Cutaneous aspergillosis. It develops either from direct contamination of an open wound with spores or via hematogenous dissemination. Hyperemic plaques appear on the skin and subsequently transform into ulcers.
Diagnostics, treatment, and prevention
The basic and primary method of laboratory diagnostics is microscopy. Smears of pus, sputum, or affected tissues (using Gram staining) are examined for septate mycelium and characteristic chains of conidia. Native microscopy of a crushed drop in 10% KOH is widely used. Additionally, culture inoculation for isolating a pure culture, serological tests (ELISA, precipitation reaction), and skin-allergy tests are employed.
- Treatment: Based on the use of antimycotics (amphotericin B, itraconazole, caspofungin). It is important to remember: the species A. terreus has natural resistance to amphotericin B! In the treatment of localized forms (e.g., aspergilloma), surgical excision of affected tissue areas is often necessary.
- Prevention: Based on strict sanitary and hygienic measures. To prevent nosocomial (iatrogenic) infection, absolute control over the sterility of medical equipment and maximum indoor air purity are required.
Briefly: answers to topical questions
What specific Aspergillus antigens are determined in blood and bronchoalveolar lavage fluid using ELISA?
Galactomannan, a specific Aspergillus antigen, is determined in blood and bronchoalveolar lavage (BAL) fluid. Its detection is officially included in the diagnostic criteria for invasive aspergillosis. Galactomannan testing in BAL has high sensitivity and specificity (over 80%) and is considered more effective than blood serum analysis. To confirm the diagnosis, the optical density index is used: a positive result in blood is ≥ 0.5, and in BAL fluid — ≥ 1.0.
What classes of immunoglobulins are determined in the serological diagnosis of aspergillosis?
Immunoglobulins of classes E (IgE) and G (IgG) are determined in the serological diagnosis of aspergillosis.
As part of laboratory tests, the following are evaluated:
- Total IgE — used for screening and monitoring of allergic bronchopulmonary aspergillosis (ABPA); its elevation serves as an early sign of exacerbation.
- Specific IgE (Aspergillus fumigatus) — detected at the onset or exacerbation of ABPA.
- Specific IgG (Aspergillus fumigatus) — determined when ABPA and chronic pulmonary aspergillosis (CPA) are suspected.
An isolated elevation of specific IgG without clinical and radiological criteria does not confirm infection.
What specific conditions and diseases are the main risk factors for the development of invasive aspergillosis?
Risk factors for the development of invasive aspergillosis are conditions associated with immunodeficiency.
Confirmed source factors include:
- neutropenia lasting 10 days or more;
- severe neutropenia with a granulocyte count < 0.5 × 10⁹/L;
- glucocorticoid intake;
- therapy with fludarabine or alemtuzumab;
- status post lung transplantation against the background of immunosuppressive therapy.
Invasive aspergillosis occurs almost exclusively in immunocompromised individuals, especially patients with severe neutropenia.
It is very easy to memorize the characteristic appearance of the Aspergillus conidiophore under a microscope through association with an ordinary watering can: it is a swollen spore-bearing hypha with rows of sterigmata, from which chains of conidia extend like streams of water.
Class and exam questions
What is "ragpicker's disease" or "scavenger's disease"?
This is occupational aspergillosis, which occurs in people with regular contact and inhalation of dust from old moldy paper, rags, or organic debris.
Which specific species causes severe invasive lung lesions?
The most dangerous forms of invasive pulmonary aspergillosis, accompanied by vascular thrombosis, are caused by the species Aspergillus fumigatus.
What is the danger of aflatoxins?
Species A. flavus and A. parasiticus secrete potent toxins that accumulate in food products. Their consumption leads to alimentary poisonings capable of provoking liver cirrhosis and exerting a carcinogenic effect.
What else to study on this topic
- Mechanisms of cellular immunity and DTH in fungal infections
- Structure of the conidiophore (phialides and metulae)
- Iatrogenic routes of Aspergillus infection in a hospital setting
- Application of ELISA and precipitation reaction in mycology
- Mechanism of development of allergic bronchopulmonary aspergillosis
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