Extrinsic allergic alveolitis (also known as hypersensitivity pneumonitis, farmer's lung, bird fancier's lung, and pigeon breeder's lung) is inflammation of lung parenchyma due to a hypersensitivity reaction to antigens. Pathogenesis is attributed to a combination of type III and type IV hypersensitivity reactions. Thus, a detailed exposure history for occupational or domestic exposure (ie, pets, especially birds, feather / fur exposure, water damage to living space, use of hot tubs / saunas, use of air conditioning, metalworking, hay handling, spray painting) is the first step in evaluation. Annual prevalence ranges between 1.7-2.7 per 100 000 persons, with a higher prevalence in patients above the age of 65.

Extrinsic allergic alveolitis manifests acutely with sudden-onset chest pain, cough, dyspnea, and fever following exposure to antigens. Headache, bibasilar crackles, inspiratory crackles, and malaise develop within 4-6 hours after exposure and may clear within 24 hours after removing the offending substance.

The subacute form manifests gradually and less severely and may recur infrequently. A chronic form, present in half of patients, is characterized by an insidious onset of cough, fatigue, progressive dyspnea, and weight loss. Tachypnea, respiratory distress, and clubbing portend a poor prognosis.

Longer exposure and greater intensity of exposure to bird antigens, development of digital clubbing, and older age all suggest a worsening of lung function and poorer prognosis.

The first line of therapy is avoidance of the antigen causing the exaggerated immune response. When identification of the offending antigen is made quickly and exposure has been of shorter duration, complete recovery has been reported.