Aspergillus niger
Summary
Aspergillus niger is a filamentous fungus and is ubiquitous in the environment, in both the soil and as airborne spores. It is an economically important pathogen of crops and causes allergic and infectious diseases in humans. A niger allergens are important in occupational allergic disease and the pathophysiology of respiratory allergic disease, where there is a significant health burden. Exposure to the allergen is by the inhalational route. Although several A. niger specific allergenic proteins have been proposed, they are presently poorly understood. Cross-reactivity with other species of the Aspergillus genus is thought to be commonplace.
Epidemiology
Worldwide distribution
The incidence of Aspergillus sensitization in patients with allergic respiratory disease has been reported between 15.3%– 38.0 % and a risk factor for a more severe clinical disease course. It is estimated that up to 70% of patients with severe asthma are sensitized to fungi, compared with 10% of patients with milder disease and 5% of the general population. For patients with cystic fibrosis (CF), a meta-analysis reported a pooled prevalence of sensitization to Aspergillus spp of 39.1%. It should be noted that observed prevalence was higher with skin prick test than specific IgE assay. ABPA occurs in approximately 7-9% of CF patients and 1-2% asthmatic patients.
A study from India found a high proportion of patients with asthma were sensitized to A. niger, with 68.7% having A. niger specific circulating IgE and 14.7% returning positive skin prick test. Another study from the UK reported A. niger had a prevalence of 8% of the total positive fungal culture from patients with non-cystic bronchiectasis.
With its widespread use in industry, A. niger allergens are also a well-described pathogenic cause of occupational asthma (OA) and hypersensitivity pneumonitis, with baker’s and animal feed producers most at affected. One study showed that 87.5% (n=7/8) bakers had IgE raised against an A. niger allergen, xylosidase.
Route Of Exposure
Main
The primary route of exposure to A. niger is allergen inhalation. Aspergillus spp. have small conidiospores (2–5 µm), which means they can reach the terminal airways. However, their presence in the upper respiratory tract can also provoke a hypersensitivity reaction.
Clinical Relevance
The spectrum of allergic diseases caused by Aspergillosis species includes asthma (with or without rhinitis), allergic rhinosinusitis and hypersensitivity pneumonitis. The significance of hypersensitivity to Aspergillus spp. in the pathogenesis of asthma is well described. Patients with asthma sensitized to Aspergillus spp. are more likely to have more severe clinical signs. Although type III and IV hypersensitivity is implicated in mold allergy, the main burden of the disease is the consequence of type I hypersensitivity.
Severe disease is associated with increased morbidity and early mortality. The eosinophilic endotype associated with Aspergillus hypersensitivity is primarily seen in adults as late-onset asthma but can also be secondary to existing airway disease, such as cystic fibrosis.
Allergic bronchopulmonary aspergillosis (ABPA) is diagnosed when a specific set of clinical parameters are fulfilled, fixed airways obstruction, bronchiectasis, and lung fibrosis. ABPA can cause irreversible lung damage, and so there is an imperative to obtain a diagnosis as soon as practicable. Patients with ABPA are commonly sensitized to the Aspergillus fumigatus allergens, Asp f 1 and 2, which are helpful for diagnosis.
Cross-Reactivity
There is evidence of significant cross-reactivity between allergens of different Aspergillus spp. and it has been proposed that for many patients with ABPA who are positive for A fumigatus specific IgE, their clinical signs may be caused by another member of the Aspergillus genus Although co-sensitization to both Aspergillus species may have occurred simultaneously.
A study has shown that all patients positive to IgEs specific to A. niger, A. flavus or A. terreus also had A. fumigatus specific IgE. However, it is thought that A. niger is less cross-antigenic with A. fumigatus than A. flavus. It has also been shown that A. niger has no cross-reactivity with Alternaria alternata, a clinically significant cause of fungal hypersensitivity.
References
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