Farmer's lung, one of the most common forms of hypersensitivity pneumonitis, develops due to inhalation of thermophilic microorganisms (present in hay or grain dust) that produce a proinflammatory reaction. Depending on the severity of exposure, patients can suffer from acute, subacute, or chronic respiratory (dyspnea, cough) and constitutional symptoms, as well as fever. Recognition rests on identifying the patient's profession and associated risk factors, whereas imaging and immunology studies are necessary to confirm the diagnosis.
Farmer's lung, as a separate clinical entity, is one of the subtypes of hypersensitivity pneumonitis (HP), an autoimmune-mediated disorder of the lung parenchyma  . The pathogenesis starts with inhalation of dust from hay or grain containing thermophilic microorganisms (temperatures between 40-60 degrees celsius are optimal for their growth), most commonly occurring under two circumstances - in late winter months when farmers handle very large quantities of hay for feeding purposes; and in very damp and humid conditions (from heavy rain), which are favorable for thermophilic microbial growth  . Thermoactinomyces species (T. sacchari, T. vulgaris, T. viridis) and Saccharopolyspora rectivirgula are notable bacterial pathogens, whereas several fungal species (Aspergillus and Penicillium) have been linked to farmer's lung as well  . When these microorganisms are inhaled, a hypersensitivity reaction leads to the formation of antigen-antibody immune complexes and the appearance of symptoms . On the basis of several factors (severity of exposure, duration, etc.), three distinct clinical courses have been described   :
The diagnosis of farmer's lung can be made only if adequate clinical suspicion is raised, which should be based on information obtained from a detailed patient history and thorough physical examination. During history taking, the physician must gather data about the patient's occupancy, in order to assess potential exposure to thermophilic pathogens . The physical examination may further aid in making a presumptive diagnosis by detecting dry crackles when auscultating the lungs . After a careful assessment of signs and symptoms, imaging studies should be ordered. Plain radiography is usually the first radiograph employed, showing diffuse infiltrates in a ground glass pattern and patchy opacities, but high-resolution computed tomography (HRCT) is the gold standard for evaluation of the lung parenchyma in patients suffering from hypersensitivity pneumonitis  . In addition to imaging studies, laboratory testing for precipitating immunoglobulin (Ig) G antibodies, skin prick testing, bronchial challenge testing using appropriate antigens, and spirometry are constitutive features of the workup   . Some authors advocate that a transbronchial biopsy (TBB) with subsequent histopathological examination should be obtained in order to solidify the diagnosis .