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Epidemiology and causes of hypersensitivity pneumonitis (extrinsic allergic alveolitis)

Talmadge E King, Jr, MD
Section Editor
Kevin R Flaherty, MD, MS
Deputy Editor
Helen Hollingsworth, MD


Hypersensitivity pneumonitis (HP), also called extrinsic allergic alveolitis, is a complex syndrome of varying intensity, clinical presentation, and natural history rather than a single, uniform disease [1-3]. It represents an immunologic reaction to an inhaled agent, particularly an organic antigen, occurring within the pulmonary parenchyma. Numerous inciting agents have been described, including, but not limited to, agricultural dusts, bioaerosols, and certain reactive chemical species.

The epidemiology and etiologic agents of HP will be reviewed here. The major classes are discussed in the text, while the individual causes are listed in the tables. The clinical features, diagnosis, and management of HP are discussed separately. (See "Classification and clinical manifestations of hypersensitivity pneumonitis (extrinsic allergic alveolitis)" and "Treatment, prevention, and prognosis of hypersensitivity pneumonitis (extrinsic allergic alveolitis)".)


The epidemiology of HP remains largely unknown. The prevalence and incidence are thought to be low, but few population-based studies have been conducted to address this question. Much of the epidemiologic information regarding HP has been derived from studies of farmers and bird fanciers.

Based upon these studies, the prevalence and incidence of HP appear to vary considerably depending upon case definitions, intensity of exposure to inciting antigens, season, geographical conditions, local practices and customs, proximity to certain industries, and host risk factors [2,4]. Many persons with mild or subclinical HP escape detection or are misdiagnosed as suffering from viral illnesses or asthma, either of which may have nonspecific clinical findings which mimic HP.

Only a small proportion of exposed individuals develop clinically significant HP, and genetic factors have been postulated to play a major role in determining an individual's risk of disease. As an example, one study of 44 patients with HP related to pigeon exposure, 50 exposed but asymptomatic individuals, and 99 healthy, unexposed controls found significant associations between HP and the presence of a number of specific MHC class II alleles [5]. It is likely that the immunologic abnormalities which underlie HP reflect the interplay of multiple genes involved in the immune response.

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Literature review current through: Nov 2017. | This topic last updated: Sep 09, 2016.
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