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Epidemiology and Etiologies
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Hypersensitivity pneumonitis (HP), or extrinsic allergic alveolitis, is a spectrum of interstitial, alveolar, and bronchiolar lung diseases resulting from immunologically induced inflammation in response to inhalation of a wide variety of different materials that are usually organic or low–molecular-weight chemical antigens (or haptens), which may lead to irreversible lung damage. Despite the terms hypersensitivity and allergic, HP is not an atopic disease and is not associated with increased IgE or eosinophils. The prevalence of HP is quite variable in different populations, presumably because of differing intensity, frequency, and duration of inhalation exposure, and also probably due to host factors that have yet to be identified. Once thought to be a relatively rare disease, it is becoming more frequently recognized as awareness is of the limitations of classical diagnostic criteria has increased. Among pigeon breeders, 8% to 30% of members of pigeon-breeding clubs who participated in surveys exhibited evidence of HP, so-called pigeon breeder’s disease (Fig. 58-1). Among farmers, 0.5% to 5% have symptoms compatible with HP, so-called farmer’s lung disease. The prevalence of symptoms is lower in farms that use hay-drying methods that decrease exposure to the responsible antigens and increased after a wet summer season.
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The population at risk and the season of exposure vary with the type of HP. For example, most cases of farmer’s lung disease occur in cold, damp climates in late winter and early spring, when farmers (usually male) use stored hay to feed their livestock. Pigeon breeder’s disease occurs chiefly in men in Europe and the United States but predominantly in women in Mexico, owing to differing patterns of exposure, but without a seasonal preference in either population. Bird fancier’s disease in Europe and the United States occurs in subjects who keep domestic birds and does not exhibit a predilection to either sex. Japanese summer-type HP occurs mostly in women without an occupation outside the home in June to September in warm, moist parts of the country. The disease has been reported in children as well, though rarely.
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In contrast to other pulmonary diseases, there is a curious predominance (80%–95%) of nonsmokers in all examples of HP, which is substantially higher than the proportion of nonsmokers in similarly exposed individuals without HP.1 The mechanisms of this phenomenon are unknown, but could include ...