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  • • Known repeated exposure to an immunologically sensitizing organic or chemical antigen.
  • • Symptoms and signs frequently include dyspnea on exertion and inspiratory rales.
  • • Pulmonary physiology typically demonstrates a restrictive or mixed pattern with reduced diffusion capacity of the lung for carbon monoxide (DlCO).
  • • Abnormalities on chest imaging most commonly include infiltrates on radiograph and centrilobular nodules on high-resolution computed tomography (HRCT).
  • • Lung histopathology demonstrates cellular bronchiolitis and interstitial granulomas.

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Hypersensitivity pneumonitis (HP), also known as extrinsic allergic alveolitis, refers to a constellation of inflammatory lung diseases caused by repeated exposure and immunological sensitization to a variety of organic and chemical antigens. Diagnosis relies on a combination of findings including a history of repeated antigen exposure; characteristic signs and symptoms, most commonly including exertional dyspnea and inspiratory crackles on examination; pulmonary function abnormalities often including decreased diffusion capacity and restrictive changes; radiological abnormalities, classically with infiltrates on chest radiograph or centrilobular nodules on HRCT; and characteristic histological findings of cellular bronchiolitis and interstitial granulomas.

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The three general categories of antigens that cause HP are microbial agents, animal proteins, and low-molecular-weight chemicals. Microbial agents include bacteria and fungi. Multiple species of thermophilic bacteria frequently contaminate decaying vegetable matter and are causally associated with farmer’s lung, the prototypical example of HP. Thermophiles as well as other bacterial species may also contaminate ventilation systems and humidifiers and cause HP from exposure to indoor aerosols. Nontuberculous mycobacteria contaminating hot tubs and metal working fluids are also thought to cause HP. Many fungi, including Rhizopus, Penicillium, Aspergillus, and Alternaria, are causative antigens in various occupational settings such as woodworking, agriculture, and lumber milling, and as contaminants of air-handling systems or water-damaged indoor settings. Avian antigens are the most common animal proteins associated with HP. Environmental and occupational history taking should always examine the possible presence of birds in the home and participation in bird hobbies. Cases of HP have been associated with apparently trivial exposures such as feather duvets and decorations, down comforters, and dusty laundry from bird hobbyists. A few low-molecular-weight chemicals are known to cause HP, including isocyanates, the pesticide pyrethrum, Pauli’s reagent (sodium diazobenzenesulfate), and copper sulfate.

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The epidemiology of HP is problematic due to variable clinical case definitions and underrecognition of disease. Most epidemiological studies have focused on farmer’s lung; estimates of incidence and prevalence vary significantly by region. HP prevalence among dairy and cattle ranchers in Wyoming was found to be 3%, whereas prevalence rates in Wisconsin dairy farmers are as high as 12%. Prevalence of HP among bird hobbyists ranges from 0.5% to 21%. Among isocyanate workers, the prevalence is approximately 1%. Attack rates in outbreaks of HP may be quite high. HP occurred in 52% of office workers exposed to a contaminated humidification system. Attack rates of 27% and 65% in sequential HP outbreaks occurred among lifeguards exposed to microbial-contaminated aerosols at an indoor swimming pool.

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