Makindo Medical Notes

Also called Hypersensitivity Alveolitis or Hypersensitivity Pneumonitis. The spores that cause Farmer's Lung are not infectious; instead, they trigger an allergic reaction. Bird fancier's lung generally has a worse prognosis than Farmer's lung.

About

• Farmer's lung is an allergic reaction rather than an infection.
• It is one of the most common occupational forms of Extrinsic Allergic Alveolitis (EAA).
• The condition arises from exposure to a group of microbes that form mould on stored vegetable matter.

Aetiology

• Exposure to organochlorines and carbamate pesticides may be risk factors for developing Farmer's lung.
• Primarily caused by exposure to mouldy hay, with Saccharopolyspora rectivirgula being the major antigen.

Background

• During the handling of mouldy straw, hay, or grain, especially in poorly ventilated spaces, inhalation of spores is likely.
• The water content of crops, heating through mould production, and microbial growth are critical factors that influence spore production and EAA development.

Clinical

• Acute Illness: Occurs 4 to 8 hours after inhaling a large amount of dust from mouldy crops. Symptoms include sudden onset of shortness of breath, dry cough, malaise, fever, chills, and tachycardia.
• Sub-Acute Farmer's Lung: Develops with continuous low-level exposure to mouldy dust. Symptoms include cough, breathlessness, mild fever, chills, arthralgia, myalgia, malaise, and loss of appetite.
• Chronic Farmer's Lung: Develops over years with progressive exposure. Symptoms include increasing dyspnoea, weight loss, cyanosis, and clubbing, with permanent lung damage.

Investigations

• FBC: Usually normal; CRP and ESR may be elevated.
• CXR: Diffuse micronodular interstitial shadowing. Subacute forms show micronodular or reticular opacities in mid/upper lung fields, while chronic forms show fibrosis and loss of lung volume.
• HRCT: Provides detailed imaging, showing typical changes and the extent of fibrosis.
• Antigen-specific IgG Antibodies: Supportive evidence for diagnosis.
• Pulmonary Function Testing: May show a restrictive defect in acute/subacute forms and a mixed restrictive/obstructive pattern in chronic cases.
• Bronchoalveolar Lavage: Shows lymphocytosis with a reduced CD4/CD8 ratio (less than 1.0).
• Biopsy: Transbronchial or open lung biopsy may show characteristic histopathological features.

Management

• Acute Management: Administer oxygen for hypoxemia; corticosteroids may be needed for severe cases.
• Long-term Management: For chronic or resistant cases, steroid-sparing agents like azathioprine and mycophenolate mofetil can be used. Systemic corticosteroids are the primary treatment but do not alter long-term outcomes.
• Prevention: Dry crops adequately before storage, ensure good ventilation, and wear respiratory protection when handling mouldy crops.
• Early diagnosis and removal from exposure can prevent progression and improve outcomes.

References

• Predictors of survival in coexistent hypersensitivity pneumonitis with autoimmune features