Farmer's Lung (Agricultural Hypersensitivity Pneumonitis)

Farmer's lung (also called farmer's hypersensitivity pneumonitis, hay sickness, harvest lung) is the prototypical and historically most studied form of hypersensitivity pneumonitis (HP), occurring in agricultural workers exposed to organic dust from moldy plant materials. First described in detail by Campbell in 1932 in Scottish farmers handling moldy hay, farmer's lung remains a significant occupational lung disease worldwide, although incidence has declined with modernization of farming practices (drier hay storage, mechanization, ventilation, mold-resistant crops). Etiology and pathogenesis: hypersensitivity pneumonitis is an immune-mediated lung disease (combined Type III and Type IV immune reactions) caused by repeated inhalation of organic antigens, with farmer's lung specifically caused by thermophilic actinomycetes (heat-loving bacteria) and fungi that grow in moisture-rich vegetable matter undergoing self-heating fermentation. The principal causative antigens are: (1) Saccharopolyspora rectivirgula (formerly Micropolyspora faeni) — the most common and historically classic etiology, identified by Pepys and Jenkins in the 1960s. (2) Thermoactinomyces vulgaris and T. sacchari. (3) Streptomyces species. (4) Aspergillus fumigatus and other Aspergillus species. (5) Penicillium species. (6) Wallemia sebi (mold). Less common antigens include endotoxins from gram-negative bacteria, mycotoxins, and residual chemical residues from agricultural sprays. The thermophiles grow at temperatures of 40-60°C in moist conditions (moldy hay, grain stored damp, silage, manure, compost piles), producing spores 1-3 micrometers in diameter that are easily aerosolized when the material is moved or processed (turning hay, mucking out animal pens, opening grain bins). The pathogenesis involves: (1) Antigen sensitization with development of specific IgG (precipitins) and cellular immunity. (2) On re-exposure, immune complex formation in alveoli (Type III hypersensitivity) and T-cell mediated response (Type IV hypersensitivity, predominantly CD8+ cytotoxic T cells in HP). (3) Granulomatous inflammation with poorly formed non-caseating granulomas, lymphocytic alveolitis, and bronchiolitis obliterans-like pattern. (4) In chronic disease, ongoing inflammation leads to fibrosis with usual interstitial pneumonia (UIP) or non-specific interstitial pneumonia (NSIP) pattern.

Three clinical patterns: (1) Acute farmer's lung: develops 4-8 hours after intense exposure to highly contaminated material (e.g., turning moldy hay, opening contaminated grain bin). Symptoms include fever (often high, 38-40°C), chills, malaise, profound fatigue, myalgia, headache, dry cough, dyspnea, sometimes nausea and vomiting. Patient often appears unwell and may be misdiagnosed with viral or bacterial pneumonia (Monday morning syndrome — symptoms develop after weekend exposure to farm work, may be mistaken for influenza). Crackles on chest exam. Tachypnea and hypoxemia. Symptoms resolve within hours to days after exposure cessation, but recur predictably with re-exposure. Repeated acute episodes can lead to chronic disease. (2) Subacute farmer's lung: develops gradually over weeks to months from continuous low-level antigen exposure. Symptoms: progressive dyspnea on exertion, productive or dry cough, fatigue, weight loss, low-grade fever, anorexia. Less dramatic than acute episodes but more progressive. May go unrecognized if exposure history not obtained. (3) Chronic farmer's lung: months to years of continuous low-dose antigen exposure (long-term farmers, often 20+ years of exposure). Symptoms: insidious progressive dyspnea, dry cough, weight loss, fatigue, finger clubbing in advanced disease. Crackles on examination. Restrictive lung physiology with decreased DLCO. End-stage disease with extensive fibrosis, pulmonary hypertension, cor pulmonale, respiratory failure. May progress despite antigen avoidance, sometimes mimicking idiopathic pulmonary fibrosis. Workup: detailed occupational and exposure history is critical (current and prior exposure to moldy hay, grain, silage, livestock, type of farm work, ventilation, presence of similar symptoms in coworkers, temporal relationship between exposure and symptoms). Chest imaging: chest radiograph in acute disease shows fine reticular and nodular pattern in mid-lower zones (often subtle), in chronic disease shows reticular pattern with upper-lobe predominant fibrosis. High-resolution CT (HRCT) is critical: acute/subacute HP shows centrilobular ground-glass nodules (3-5 mm), ground-glass opacities, and air trapping (mosaic attenuation on expiratory imaging) — predominantly upper and mid lung zones with sparing of bases (distinct from IPF which involves bases). Chronic HP shows reticular abnormalities, traction bronchiectasis, honeycombing, with upper and mid zone predominance (key feature distinguishing from IPF), and persistent areas of ground-glass and air trapping. Pulmonary function tests: restrictive pattern (decreased TLC, FVC), decreased DLCO, with decreased oxygen saturation on exertion in chronic disease.

Diagnosis: 2020 ATS/JRS/ALAT guidelines for HP diagnosis based on combination of clinical features, exposure history, imaging, BAL, and pathology. Confidence levels: definite, high confidence, low confidence, indeterminate. Key components: (1) Exposure history: detailed occupational/environmental history identifying causative antigen, often supplemented by inspection of suspected exposure source. (2) Serum precipitating antibodies (precipitins): IgG against suspected antigens (S. rectivirgula, T. vulgaris, A. fumigatus, etc.) detected by immunodiffusion or ImmunoCAP. Present in 50-90% of patients, but not pathognomonic — also positive in 10-50% of exposed asymptomatic individuals. Useful when positive but absence does not exclude diagnosis. (3) Bronchoalveolar lavage (BAL): lymphocytic alveolitis (over 30% lymphocytes, often 40-70%) with CD4/CD8 ratio less than 1.0 (often less than 0.5) is highly suggestive of HP. (4) Lung biopsy: surgical lung biopsy if non-invasive workup inconclusive — shows triad of (a) bronchiolocentric cellular infiltrate (lymphocytes, plasma cells), (b) poorly formed non-necrotizing granulomas, and (c) chronic bronchiolitis. Chronic disease shows fibrosis with UIP-like or NSIP-like pattern, often distinguishable from IPF by airway-centered fibrosis and bridging fibrosis. Treatment: (1) ANTIGEN AVOIDANCE is the cornerstone of treatment for all stages and the most important intervention. Patients should be advised to avoid further exposure: change jobs (often required), use of personal protective equipment (powered air-purifying respirators with HEPA filtration during exposure), modify farm work (drier hay, mechanization to reduce dust generation, ventilated indoor spaces, replace animal feed sources, fix water leaks contributing to mold growth). Cessation of exposure may be sufficient for acute and subacute disease. (2) Corticosteroids: prednisone 0.5-1 mg/kg/day (typically 40-60 mg/day) for 1-2 weeks then taper over 4-12 weeks for acute severe disease and may speed recovery, but does not appear to alter long-term outcome. Pulse intravenous methylprednisolone (1000 mg/day for 3 days) for severe acute disease. Long-term low-dose corticosteroids (5-10 mg/day) for chronic fibrotic disease that progresses despite antigen avoidance. (3) Immunosuppressants: mycophenolate mofetil, azathioprine considered for chronic fibrotic disease as steroid-sparing agents (limited evidence). (4) Antifibrotic therapy: pirfenidone and nintedanib (originally developed for idiopathic pulmonary fibrosis) approved for chronic fibrotic HP based on INBUILD trial (nintedanib showed slower FVC decline vs placebo). (5) Lung transplantation for end-stage disease in eligible patients. Pulmonary rehabilitation, oxygen therapy as needed, vaccinations (influenza, pneumococcal, COVID-19). Prognosis: acute and subacute farmer's lung with antigen avoidance has good prognosis with full recovery. Chronic farmer's lung has variable prognosis depending on extent of fibrosis at diagnosis, with overall median survival 5-10 years from diagnosis if fibrotic; better prognosis if non-fibrotic. Continued exposure is associated with worse outcomes.