What is Extrinsic allergic alveolitis (EAA) or hypersensitivity pneumonitis (HP)

Hypersensitivity pneumonitis or extrinsic allergic alveolitis is a widespread disease of the lungs, involving inflammation in both the small airways of the lung and the alveoli (air sacs). It is not dissimilar to asthma, but the latter involves the bronchi and bronchioles. It is also known as extrinsic allergic alveolitis. Breaking up these terms, extrinsic means the disease is caused by an external agent not from the body with allergic component to it; alveolitis means inflammation to the alveoli (air sacs) in the lungs.

The lung is composed of large airways (beginning with the trachea and then two main bronchi), which branch out to form smaller airways (segmental bornchi and bronchioles) which finally lead to the alveoli – the small air sacs where gas exchange takes place.

Statistics on Extrinsic allergic alveolitis (EAA) or hypersensitivity pneumonitis (HP)

This depends on exposure to the variety of predisposing factors identified for hypersensitivity pneumonitis. The disease is most common in poor, wet farming areas of the world, sometimes affecting 1 in 10 of the farmers. In the West, the cases appear to be declining as more mechanised farming procedures are introduced.

The cases of hypersensitivity pneumonitis vary widely in different countries, although it is thought to be low.

One of the commonest form of hypersensitivity pneumonitis is farmer’s lung. The cases of farmer’s lung in different countries is detailed below:


  • US: 420-3000 per 100,000 population
  • UK: 12-2300 per 100,000 population
  • France: 4370 per 100,000 population
  • Finland: 1400-1700 per 100,000 population


Risk Factors for Extrinsic allergic alveolitis (EAA) or hypersensitivity pneumonitis (HP)

A number of causes have been identified, involving exposure to substances that lead to allergic response in some people. The following occupations place the workers at risk of developing hypersensitivity pneumonitis.

  • Forking mouldy hay – farmers
  • Handling pigeons, cleaning lofts or budgie cages – bird fanciers
  • Turning germinating barley – malt-workers
  • Contaminated humidifying systems in air conditioners or humidifiers in factories – some factory workers
  • Turning mushroom compost – mushroom workers
  • Grain and flour processing and loading workers
  • Lumber milling, construction, wood stripping, paper and wallboard manufacturing
  • Plastic manufacture, painting, electronics industry, other chemicals
  • Textile workers

Interestingly, cigarette smoking is associated with a decreased risk of hypersensitivity pneumonitis. Although smoking seemed to be protective, once the disease is established, smoking does not reduce the disease severity.

Progression of Extrinsic allergic alveolitis (EAA) or hypersensitivity pneumonitis (HP)

This disease is similar to asthma in that it occurs due to a hypersensitivity response to an inhaled substance. However, the difference is that in this case, the allergic response occurs mainly in the alveoli and small vessels supplying them.

Lung symptoms of hypersensitivity pneumonitis can occur in 3 different stages:


  • acute: symptoms occur 4-6 hours after substance exposure
  • subacute: characterised by gradual cough development and other symptoms over 10-14 days
  • chronic: insidious onset where the patient may not complain of any symptoms until the disease reaches advanced stage.


If substance exposure continues on a long term basis, fibrosis begins to replace the inflammation, resulting in progressive, irreversible, fibrosis of the lungs. Fibrosis means that fibrous tissue (scar) replaces the inflammed tissue in the lungs.

How is Extrinsic allergic alveolitis (EAA) or hypersensitivity pneumonitis (HP) Diagnosed?

On admission, usually general routine blood tests will be done although they are not very useful for diagnosis of hypersensitivity pneumonitis. However these blood tests can determine the severity of disease when there are any abnormalities.

Prognosis of Extrinsic allergic alveolitis (EAA) or hypersensitivity pneumonitis (HP)

Prognosis is good if disease, and exposure to causative agent, is identified early. Hence further exposure can be avoided. Established lung scarring, however, will not resolve, and these patients may progress inexorably to lung failure.

Overall outcome is excellent, where most patients recover completely after ceasing exposure to the offending agent. However, bird fancier’s lung appears to have a worse prognosis than farmer’s lung.

How is Extrinsic allergic alveolitis (EAA) or hypersensitivity pneumonitis (HP) Treated?

It is important to diagnose the disease early, as it may be effectively cured at this stage.

The mainstay of treatment for hypersensitivity alveolitis is avoiding exposure to the causative substance. For example, the use of silage for animal fodder, drier storage of hay and grain may prevent or reverse early farmer’s lung. Individuals may be forced to change jobs or to give up bird rearing as a hobby. Usually patients are very reluctant to do this.

Medication such as a course of steroid (usually prednisolone) is often necessary to reverse early stages of disease. This medication may have significant side effects if taken long term, hence needing specialist supervision. In the chronic form of disease, steroid therapy is unable to reverse fibrosis once it has been established.

Extrinsic allergic alveolitis (EAA) or hypersensitivity pneumonitis (HP) References

  1. Cotran RS, Kumar V, Collins T. Robbins Pathological Basis of Disease. 6th ed. New York: WB Saunders Company. 1999.
  2. eMedicine: hypersensitivity pneumonitis [online]. 2005. [Cited 2005 October 14th]. Available from: URL: http://www.emedicine.com/ped/topic2577.htm
  3. McSharry, C, Banham, SW, Boyd, G. Effect of cigarette smoking on the antibody response to inhaled antigens and the prevalence of extrinsic allergic alveolitis among pigeon breeders. Clin Allergy 1985; 15:487.
  4. Up to Date: Epidemiology and causes of hypersensitivity pneumonitis (extrinsic allergic alveolitis) [online]. 2005. [Cited 2005 October 14th]. Available from URL: http://www.utdol.com/application/topic.asp?file=int_lung/12651&type=A&selectedTitle=3~24

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