In Japan, farmer's Lung is managed by occupational medicines. Farmer's lung is a hypersensitivity pneumonitis caused by inhalation of thermophilic actinomycete and fungal antigens released from moldy hay, grain, silage, or compost. The dominant antigen is Saccharopolyspora rectivirgula, which thrives in damp organic material heated to 40-60 °C.
Farmer's lung (ICD-10: J67.0) is the prototypical agricultural form of hypersensitivity pneumonitis (HP), an immune-mediated interstitial lung disease caused by repeated inhalation of organic dust containing thermophilic actinomycetes (chiefly Saccharopolyspora rectivirgula, formerly Micropolyspora faeni and Thermoactinomyces vulgaris) and fungi (Aspergillus, Penicillium, Wallemia sebi). Inhaled antigens trigger a combined IgG-mediated immune complex deposition and T-cell-mediated granulomatous response in the small airways and alveoli. The ATS/JRS/ALAT 2020 guideline redefined HP into two clinical phenotypes — non-fibrotic (inflammatory) and fibrotic — replacing the older acute/subacute/chronic classification. Non-fibrotic disease is reversible with antigen avoidance and corticosteroids; fibrotic HP behaves like a slowly progressive fibrosing interstitial lung disease and carries substantial mortality.
The key symptoms of Farmer's Lung are: Fever, chills, myalgia, and malaise developing 4-8 hours after exposure to moldy hay, grain, silage, or compost — the classic acute farmer's lung episode., Dry cough that becomes productive of scanty sputum, often described as paroxysmal and worse during or shortly after farming activities., Progressive exertional breathlessness, initially with strenuous work and ultimately at rest in advanced fibrotic disease., Tight chest and inspiratory crackles at the lung bases on auscultation (positive in 60-80% of chronic cases), sometimes with mid-inspiratory squeaks suggesting bronchiolitis., Weight loss of 5-15 kg over months in chronic disease, often initially attributed to working hard and eating less., Unintended episodes resembling 'recurrent pneumonia' that respond partially to antibiotics — a major diagnostic clue when occurring in the same farmer multiple times., Clubbing of the fingers and toes in 20-50% of long-standing fibrotic farmer's lung patients, similar in appearance to that seen in idiopathic pulmonary fibrosis..
Diagnosis of farmer's lung follows the ATS/JRS/ALAT 2020 algorithm combining exposure history, clinical features, imaging, bronchoalveolar lavage, and serology. A compatible occupational history is essential — quantifying years of farming, types of fodder, storage conditions, ventilation, and timing of symptoms relative to specific tasks. The Lacasse 2003 clinical prediction criteria support diagnosis when 6 of the following are met: known antigen exposure, positive precipitating IgG antibodies, recurrent episodes of symptoms, inspiratory crackles, symptoms 4-8 hours after exposure, and weight loss. Pulmonary function tests typically show restrictive ventilatory defect with reduced total lung capacity, reduced diffusing capacity for carbon monoxide (DLCO), and resting or exertional hypoxia. High-resolution CT (HRCT) is the imaging cornerstone. Non-fibrotic disease shows ground-glass opacities, ill-defined centrilobular nodules, and mosaic attenuation with expiratory air-trapping (the 'three-density sign'). Fibrotic disease adds reticulation, traction bronchiectasis, and sometimes honeycombing, typically in mid- and upper-lung zones — differentiating it from the basal predominance of IPF. Bronchoalveolar lavage with cell count and differential is recommended in most cases: a CD8-predominant lymphocytosis above 30% (often above 50%) is characteristic. Specific IgG antibody panels against Saccharopolyspora rectivirgula, Thermoactinomyces vulgaris, Aspergillus, and Penicillium support exposure but are not specific for disease. Lung biopsy (transbronchial cryobiopsy or surgical) is reserved for ambiguous cases and shows poorly formed non-necrotizing granulomas, lymphocytic bronchiolitis, and giant cells. Multidisciplinary discussion with radiology and pathology improves diagnostic confidence.
Outcome depends primarily on disease phenotype, exposure status, and adherence to antigen avoidance. Non-fibrotic farmer's lung managed with complete antigen avoidance plus corticosteroids has excellent prognosis: 70-85% achieve full lung function recovery and 5-year mortality is comparable to age-matched controls. Fibrotic farmer's lung carries a 5-year mortality of 27-40%, similar to or slightly better than idiopathic pulmonary fibrosis when antigen avoidance is achieved. Continued exposure roughly doubles 5-year mortality versus successful avoidance. Predictors of worse outcome include older age, lower baseline FVC and DLCO, honeycombing on HRCT, low BAL lymphocyte percentage, ongoing antigen exposure, and progression in spite of treatment. The INBUILD trial showed antifibrotic therapy slows lung function decline in progressive fibrotic HP; transplant offers a second window of treatment for end-stage disease with 5-year post-transplant survival 50-55%.
Pulmonology, occupational medicine, and an interstitial lung disease (ILD) multidisciplinary clinic should jointly manage farmer's lung. Pulmonology directs investigation and pharmacotherapy; occupational medicine quantifies exposure, supports antigen avoidance, and certifies fitness-for-work and compensation claims. Radiology and pathology contribute to multidisciplinary discussions that establish diagnosis and progression patterns. Lung transplant programs assess severe fibrotic disease.
Find specialists →Acute episodes resolve within 24-72 hours of removal from exposure and symptom-directed steroids. HRCT ground-glass opacities clear over 4-12 weeks. Lung function recovery follows over 3-6 months. Fibrotic disease does not regress; antifibrotic therapy aims to slow further decline, with measurable benefit by 6-12 months in FVC trajectory. Pulmonary rehabilitation produces walking-distance gains within 8-12 weeks.
Structured pulmonary rehabilitation followed by maintenance exercise is recommended for all patients with breathlessness or exercise limitation. Aim for 150 minutes per week of moderate aerobic activity (walking, cycling, swimming) with resistance training twice weekly. Patients with exertional desaturation should use prescribed oxygen during exercise. Avoid working in high-antigen-density environments even when feeling well.
Choose a pulmonologist or ILD specialist at a center with multidisciplinary ILD review, HRCT expertise, and access to bronchoalveolar lavage, antifibrotic prescribing, and pulmonary rehabilitation. For occupational compensation, an occupational physician familiar with national agricultural disease schemes is essential. Where lung transplantation is contemplated, refer early (FVC under 80% with progression) to a high-volume transplant center.
Medically reviewed by AIHealz Medical Editorial Board · May 13, 2026
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