Hypersensitivity Pneumonitis

How hypersensitivity pneumonitis develops

When some people repeatedly breathe in fine particles of certain organic material, their immune system becomes sensitized. Further exposure triggers inflammation in the bronchioles and alveoli, the tiny air sacs where oxygen enters the blood. Unlike asthma, which mainly affects the airways, HP affects the lung tissue itself and is a type of interstitial lung disease.

Only a minority of exposed people develop HP, suggesting genetic and other factors matter. It differs from asthma, sarcoidosis and mineral-dust diseases such as pneumoconiosis, although they can look similar. Smoking, oddly, is associated with a lower risk, but it is still harmful.

Common causes

In up to half of cases, no specific trigger can be identified despite careful searching.

Types and symptoms

Non-fibrotic (inflammatory) HP

Often follows intermittent, heavy exposure. Symptoms begin 4 to 8 hours after exposure:

• cough and breathlessness
• chest tightness
• fever, chills, muscle aches and tiredness

It is often mistaken for flu or pneumonia, and settles within days if exposure stops, then recurs with re-exposure.

Fibrotic HP

Follows long-term, low-level exposure. Symptoms develop gradually over months to years:

• increasing breathlessness on exertion
• dry cough
• fatigue and weight loss
• crackles heard in the lungs
• clubbing of the fingers in some people

Scarring (pulmonary fibrosis) may be permanent and can progress even after exposure stops.

Diagnosis

• Detailed exposure history: at home, work and hobbies, including birds, feather bedding, humidifiers, hot tubs, water damage and occupation.
• High-resolution CT scan: shows patterns typical of HP, such as ground-glass areas, small nodules and air trapping.
• Lung function tests: usually show reduced lung volumes and reduced gas transfer.
• Blood tests: for specific antibodies (precipitins) to suspected antigens; a positive test shows exposure, not necessarily disease.
• Bronchoscopy with lavage: a high proportion of lymphocytes supports HP.
• Lung biopsy: in uncertain cases.

Diagnosis is often made by a multidisciplinary team of lung specialists, radiologists and pathologists.

Treatment

• Avoid the exposure: the cornerstone of treatment. Remove birds and feather bedding, fix water damage, clean or remove humidifiers and hot tubs, and use protective equipment or change jobs if needed. Professional remediation may be required.
• Corticosteroids: for significant or worsening symptoms.
• Steroid-sparing immunosuppressants: such as mycophenolate or azathioprine for longer-term control.
• Antifibrotic medicine: nintedanib for progressive fibrotic lung disease.
• Supportive care: oxygen if levels are low, pulmonary rehabilitation, vaccinations, and stopping smoking.
• Lung transplant: for advanced fibrosis in selected people.

Outlook

Non-fibrotic HP usually improves substantially, often completely, once the trigger is removed. Fibrotic HP has a more variable outlook; some people stabilize, while others progress. Early diagnosis and complete avoidance of the trigger give the best chance of preserving lung function.

Frequently asked questions

What is hypersensitivity pneumonitis?

Hypersensitivity pneumonitis is inflammation of the lungs caused by an immune reaction to something repeatedly inhaled, such as bird proteins, mold or bacteria in hay or hot tubs. It can cause flu-like attacks or gradual breathlessness and scarring.

What are the symptoms of hypersensitivity pneumonitis?

Acute symptoms include cough, breathlessness, chest tightness, fever, chills and aches 4 to 8 hours after exposure. Chronic HP causes gradually worsening breathlessness, dry cough, fatigue and weight loss.

Is hypersensitivity pneumonitis reversible?

Often, when caught early. Inflammatory HP usually improves after the trigger is removed. Once scarring (fibrosis) has developed, damage is usually permanent, though treatment can slow progression.

What is farmer's lung?

Farmer's lung is hypersensitivity pneumonitis caused by inhaling dust from moldy hay, grain or straw containing heat-loving bacteria and molds. It causes flu-like symptoms and breathlessness after exposure, and can lead to lung scarring with repeated exposure.