What is Extrinsic Allergic Alveolitis (Farmer's Lung)?

Extrinsic allergic alveolitis (EEA) is a group of diseases that involve a hypersensitivity reaction in the lungs to irritants in the air. The most common type is known as Farmer’s lung, which involves exposure to and inhalation of species in mouldy hay, but there are several other types:

• Farmer's lung: exposure to mouldy hay.
• Bird fancier's lung: exposure to avian proteins in birds.
• Cheese worker's lung: exposure to cheese mould.
• Malt worker's lung: exposure to mouldy malt.
• Hot tub lung: exposure to bacteria in hot tubs.
• Chemical worker's lung: exposure to chemical antigens
• Mushroom worker's lung: exposure to mushroom compost.

Terminology

The term “extrinsic allergic alveolitis” describes the cause and nature of the disease, as follows:

• Extrinsic refers to an external cause
• Allergic refers to the hypersensitivity reaction that is characteristic of the disease
• Alveolitis refers to inflammation of the alveoli in the lungs

Epidemiology

Farmer’s lung is one of the most common types of hypersensitivity pneumonitis, but it only accounts for less that 2% of all cases of interstitial lung disease.

The incidence depends greatly on the population sample, as it primarily affects individuals that are exposed to the causative species, usually farmers that handle hay. It may affect up to 7% of all farmers, ranging from 2.5 to 540 cases per 100,000 farmers per year.

Pathophysiology

Extrinsic allergic alveolitis is caused by inhalation of antigens that cause a hypersensitivity reaction in the respiratory system.

Exposure to the thermophilic Actinomyces speciesis most commonly responsible for causing the condition in Farmers. This may include exposure to Saccharopolyspora rectivirgula, Thermoactinomyces vulgaris, Thermoactinomyces viridis, or Thermoactinomyces sacchari. These species are usually found in decaying organic matter, such as compost, hay or sugar cane.

With chronic exposure to the causative antigens, an inflammation and immune response contributes to changes in the lung tissue and the eventual presentation of symptoms.

Presentation

The symptoms of EEA may include chest tightness, cough, breathlessness and unexplained weight loss. In more severe cases, flu-like symptoms and fever may be evident.

Diagnosis can be made based upon consultation with the patient, with particular attention to potential irritants and risk factors the individual may have been exposed to. For example, the occupation is an important factor as some jobs are at an increased risk for the condition.

The diagnosis can then be confirmed with lung function tests, blood examinations and diagnostic imaging techniques, such as X-ray and computed tomography (CT) scans. This is important to differentiate the condition from other conditions of the respiratory tract, such as infections, interstitial lung disease, asthma, sarcoidosis and histoplasmosis.

Management

The management of EEA depends on the severity of symptoms and the individual circumstances of the case. Acute cases with respiratory distress may require emergency medical attention, with the administration of oxygen to treat hypoxia, if present.

Chronic cases can often be managed by avoidance of suspected allergen, although this can be difficult when it is a present in the occupational environment of the individual.

Some individuals benefit from treatment with corticosteroid medication for acute forms of the disease, although this is less effective for chronic EEA.

Prevention of the condition by avoiding or reducing exposure to the allergen is the best course of action. Simple techniques such as allowing hay in a farming environment to dry adequately before storage, preventing the growth of allergens, can help to reduce symptoms.

References

• http://emedicine.medscape.com/article/298811-overview#showall
• http://patient.info/doctor/Extrinsic-Allergic-Alveolitis.htm
• http://www.hse.gov.uk/lung-disease/extrinsic-allergic-alveolitis.htm
• https://www.nlm.nih.gov/medlineplus/ency/article/000109.htm