17th January 2024

Background

Epidemiology

Allergic bronchopulmonary aspergillosis (ABPA) is a type of hypersensitivity immune response respiratory disorder specifically associated with the Aspergillus fungus. The main species responsible is usually Aspergillus fumigatus, but other species, such as Aspergillus flavus, have also been identified as causative agents of ABPA. Other associated hypersensitivity respiratory disorders include HP and IgE mediated asthma. ABPA normally only occurs in those with cystic fibrosis (CF) or asthma– in those with asthma, prevalence ranges from 1-5%, and in those with CF, prevalence is approximately 1-9%. The main affected age group tends to be teenagers (if they have CF) or young adults (with asthma). It can also affect children and older patients, but occurrence is less common within these age groups.

ABPA is an emerging condition in India, and there is high prevalence in the Southwest areas of America also. Under recognition of signs and symptoms of the disease means rates may be much higher worldwide than is currently known.

Pathophysiology

ABPA is an IgE and IgG immune-mediated hypersensitivity response to Aspergillus fumigatus fungus. It is not a systemic nor invasive fungal infection, and the fungi can be found in multiple environmental sources, including soil, grass and sewage. The spores are airborne and must be inhaled.

Exposure in individuals without atopy does not usually lead to antibody formation. However, in those with atopy, such as asthmatics, antibody formation after exposure to the fungal allergen is common. Conditions like asthma and CF tend to cause impaired mucus secretion and clearance, increasing the likelihood that spores will become trapped in these individuals’ airways and allow colonisation.

Once in the airway, the presence of fungal spores triggers an intense inflammatory response on top of existing inflammation caused by underlying asthma or CF. Th2 cells are recruited to release interleukin (IL)-5, which stimulates eosinophil and B cell recruitment. The presence of fungal enzymes causes IL-8 release, which stimulates neutrophil recruitment. These interleukins also stimulate synthesis of IgG and IgE antibodies, as do B cells. The inflammatory products released by the eosinophils and neutrophils recruited cause the bronchi and bronchioles to fill with eosinophilic plugs and hyphae, which leads to bronchiectasis and obstruction, and the signs and symptoms associated with the disease.