Extrinsic Allergic Alveolitis

Synonym: Hypersensitivity Pneumonitis. It includes farmer's lung and bird fancier's lung, including pigeon fancier's and budgerigar fancier's lung.

See related article Occupational Asthma

Inflammation of the small airways or alveoli is caused by an immunological reaction to an inhaled bioaerosol (a mist or dust of biological particles), or some organic chemicals. The clinical outcome is highly variable in terms of severity and natural history. It was initially thought to be an immune complex disease but now cell mediated immunity is thought to be more important.^1,2

Farmer's lung is the classical form of this disorder but it is associated with many occupations and hobbies. Other examples include metalworking-fluid lung, cheese washer's lung, mushroom worker's lung, doghouse disease, wood pulp worker's lung, rodent handler's lung and woodworker's lung.³ More recently characterised conditions include hot tub lung (due to presence of Mycobacterium avium in poorly-maintained hot tubs), humidifier lung, compost lung and peat-moss worker's lung.

The provoking antigen can be difficult to identify with certainty and the presence of antibodies does not confirm disease.

• This condition accounts for 2% of occupational lung disease in the UK.
• 50% involve farm work and 15% workers in material, metal or electrical processing.
• Bird fancier's lung may be the commonest form in the whole population due to popularity of keeping budgerigars and a mild form may be present in 0.5-7.5% of budgerigar fanciers.⁴
• The sex incidence is roughly equal or a slight male excess.⁵
• Bird fancier's lung can occasionally present in children.⁶

Pre-existing lung disease, occupations, bird keeping and other hobbies, regular use of hot tubs.

Sensitisation occurs after a period of exposure to the antigen varying from weeks to years:

Acute form

• In the acute form there are recurrent, paroxysmal episodes of flu-like illness with fever, chest tightness, dry cough and dyspnoea.
• It starts usually 4-8 hours after exposure to the sensitizing antigen.
• Associated symptoms include malaise, chills, headache, generalised aches and pains, anorexia and tiredness.
• Symptoms are directly related to level of exposure and low-level acute exposure may produce mild symptoms that last for just a few hours.
• In very severe cases, patients may develop life threatening respiratory failure with cyanosis, respiratory distress at rest and high fever.

Chest examination is often unremarkable apart from scattered inspiratory crackles, particularly at the lung bases. Wheeze is rare.

Subacute or intermittent form

The clinical presentation is similar to the acute form except that symptoms are less severe but last longer. The subacute, or intermittent, form produces more well-formed noncaseating granulomas, bronchiolitis with or without organizing pneumonia, and interstitial fibrosis.

Chronic form

• In the chronic form there is often no systemic symptom except weight loss and gradual diminution of exercise tolerance due to dyspnoea.
• Eventually there may be chronic hypoxaemia and pulmonary hypertension with right heart failure.

This form is typical of keepers of a single budgerigar, particularly if the person is confined to the home.

Cyanosis and clubbing may develop.

The following criteria are good diagnostic predictors:

• Exposure to a known offending antigen
• Positive precipitating antibodies to the offending antigen
• Recurrent episodes of symptoms
• Inspiratory crackles on physical examination
• Symptoms occurring 4-8 hours after exposure
• Weight loss

Making the diagnosis depends upon having suspicion and this requires a good occupational history and possibly enquiry about hobbies and pets.²

• Infection per se or it may co-exist
• Connective tissue disorders causing interstitial lung disease
• Other causes of pulmonary fibrosis including pneumoconiosis and asbestosis
• Sarcoidosis
• Drug-induced interstitial lung disease
• Q fever
• In bird keepers:
  • Viral infection
  • Mycoplasma infection
  • Chlamydial infection

• Spirometry shows restrictive defect with reduced gas transfer.
• Oxygen saturation may be reduced.
• Chest X-ray usually shows no abnormality until symptoms are moderately severe in the acute form.
  • If abnormalities are present, they are widespread, with a ground-glass appearance or alveolar filling pattern especially in the lower and mid-zones.
  • In the sub-acute form small reticular opacities follow same distribution.
  • High resolution CT also shows increased ground-glass density or reticular/nodular infiltration.
  • Hilar lymphadenopathy is rare.
• In the chronic form, both plain X-ray and CT show irreversible fibrotic process mainly affecting the upper zones.
• Antibody tests (ELISA) may demonstrate the presence of precipitating serum IgG antibodies to the antigen but this indicates exposure but is not diagnostic.
• Inhalation challenge test where temperature, circulating neutrophil and lymphocytes, FVC and exercise tests are performed after inhaling the suspected allergen can aid diagnosis.⁷
• Analysis of bronchoalveolar lavage fluid and transbronchial or open lung biopsy may show characteristic cytological or histopathological features. Recent research has shown that diagnosis can be made using clinical prediction rules, without the need for these invasive tests.⁸

• In acute severe cases with significant respiratory distress and/or the presence of cyanosis, immediate referral to hospital for further assessment.
• Supplemental oxygen should be given to treat hypoxaemia.
• In less severe acute cases, or in the chronic form, avoidance of exposure to the allergen, along with investigations to confirm or refute the diagnosis, usually in conjunction with a respiratory specialist.
• Once the diagnosis is made it is important to avoid allergen exposure. This may require a change of job and before such a drastic step, consultation with a respiratory consultant or specialist in occupational medicine may be advised.
• In the acute form, simply avoiding further exposure will result in recovery without medication.
• Corticosteroid therapy may aid recovery in severe disease but the long-term prognosis is not affected. Prospective studies have shown improvement at 1 or 2 months, but not on longer-term follow-up.
• Steroids are standard treatment for severely ill patients with all varieties of the disease. The optimum dose and duration is uncertain. Treatment starts with prednisolone 40-60mg daily for 2-4 weeks and then tapered over a 4-8 weeks period while evaluating clinical response, pulmonary function, and radiological improvement. Maintenance doses are not always required, particularly if the patient is removed from exposure.

With continued exposure, a significant minority (25-40%) will develop progressive pulmonary fibrosis. Rarely pneumothorax may occur in the fibrotic stage.

• Most patients recover lung function completely, but this may take several years.
• Most patients with farmer's lung recover with only minor functional abnormalities and very few progress to disability. A significant number of farmers develop mild chronic lung impairment, which is predominantly obstructive airflow disease associated with mild emphysematous changes on high-resolution CT scan images.
• Bird fancier's lung is less well studied but it appears to have a much worse prognosis than farmer's lung. The poorer outcome may be due to higher antigenic exposure and persisting avian antigens in the home environment, even after birds are removed. These factors may account for the substantial 5-year mortality rate of 30%.
• Pigeon fanciers tend to have a chronic and fairly benign course despite the fact that many choose to continue to keep pigeons.⁹

Health and Safety measures at work, avoiding pastimes which provoke the illness, adequate maintenance of hot tubs/indoor swimming pools and humidifiers.