This patient had a long history of raising birds.

Hypersensitivity pneumonitis is an interstitial lung disease caused by an immune response to chronic inhalation of a variety of antigens, such as bacterial, fungal, animal proteins or chemicals. In chronic cases, progressive fibrosis develops and is often characterized by honeycombing which could mimic UIP or NSIP.

Chronic hypersensitivity pneumonitis is a legacy term not recommended in the most recent guidelines (c. 2020)¹. In the recent guidelines, hypersensitivity pneumonitis is subtyped into non-fibrotic hypersensitivity pneumonitis and fibrotic hypersensitivity pneumonitis

In the chronic/fibrotic HP, there is radiological evidence of fibrosis and represents the end-stage of repeated or persistent pneumonitis

Typical radiological features for fibrotic/chronic hypersensitivity pneumonitis include:

• ground-glass opacities
• mosaic attenuation with areas of air-trapping, secondary to bronchiolar obstruction
• areas of pulmonary fibrosis with a reticular pattern, honeycombing and traction bronchiectasis

There is often a middle or upper zone predominance of CT findings with sparing of the lung bases, unlike non-specific interstitial pneumonia (NSIP) or usual interstitial pneumonia (UIP), which show a lower zone predominance.