hypersensitivity pneumonitis / bird fancier's lung / farmer's lung

Introduction

hypersensitivity pneumonitis (HP, formerly called EAA extrinsic allergic alveolitis) is a form of interstitial lung disease with T cell lymphocytic infiltration and granulomatous inflammation in the bronchioles and alveoli
hypersensitivity pneumonitis is caused by inhalation of an allergen and the acute form may present 4-6hrs after heavy exposure to the provoking antigen with cough, SOB, fever, chills, rash, headache which may last days after cessation of exposure
a subacute form is more gradual in onset, less severe and takes longer to resolve
a chronic form may be more insidious with cough and progressive SOB, desaturation with exercise, LOW and 50% develop clubbing and develop additional findings of chronic interstitial inflammation and alveolar destruction (honeycombing) associated with dense pulmonary fibrosis and may progress to hypoxia at rest.

bird fancier's lung
- especially pigeons, budgerigars, parrots, poultry
- the main agent that causes the allergy is the bloom on the feathers (the dust that comes off which keeps the feathers sliding nicely over each other). It comes from the preen gland (also known as oil gland or uropygial gland) found under the base of the birds' tail and is spread over the feathers with the beak
- the droppings can also be a problem if they dry and are inhaled
- certain small mammals kept as pets have the same or similar proteins in their fur and faeces and may contribute to the hypersensitivity
- affects up to 20% of bird fanciers
farmer's lung
- mouldy hay - Aspergillus species, thermophilic actinomycetes, Saccharopolyspora rectivirgula, Micropolyspora faeni
- affects 0.4–7% of the farming population
many other forms
- see wikipedia

CXR is usually normal but may show fibrosis in chronic cases
HRCT is the usual radiologic modality of choice
- typically shows ground-glass opacities with areas of decreased attenuation on expiratory images because of air trapping and bronchiolitis
- see https://radiopaedia.org/articles/bird-fancier-lung-1
a blood test is available to check for antibodies to avian proteins which is suggestive of disease and the IgG levels can indicate the degree of ongoing exposure
lung function tests

remove allergens - avoiding repeated exposure is important for recovery and avoidance of chronic irreversible impairment
inhaled steroids may be useful
in patients with severe disease, prednisolone may be given for several weeks before gradually reduced according to response