Hypersensitivity Pneumonitis Profile

Hypersensitivity pneumonitis (HP) is an interstitial lung disease that is characterized by a complex immunological reaction of the lung parenchyma in response to repetitive inhalation and subsequent sensitization to a wide variety of inhaled organic dusts.^1-7 HP is associated with progressive pulmonary disability, irreversible lung damage, and mortality in some cases. HP can be classified as Acute/Inflammatory (symptoms less than six months) and Chronic/Fibrotic (symptoms more than six months) based on clinical, radiologic and pathologic characteristics.⁸ The name previously used for this condition, extrinsic allergic alveolitis, has been largely abandoned because inflammation involves more than just the alveoli and can extend to the bronchioles as well. The severity of the disease and clinical presentation varies depending on the quantity and type of inhaled antigen causing the condition. Numerous antigens have been found to cause HP.

Diagnosis of HP can be challenging and requires a combination of detailed history, radiologic evaluation, pathological examination and laboratory testing. Acute exposures to inciting antigens typically cause abrupt onset of nonproductive cough, dyspnea, and chills with arthralgias or malaise within a few hours of heavy exposure to a specific antigen.^1-4,8 Symptoms usually resolve within a few days of avoiding exposure. Coughing is a predominant symptom due to airway-centered nature of inflammation.Patients also report shortness of breath, malaise, weight loss. No single laboratory testis diagnostic for hypersensitivity pneumonitis.^1,6-8 Double diffusion (Ouchterlony) assays are used to determine antigen-specific IgG antibodies. The appearance of precipitin arcs confirms the presence of precipitating antibodies to specific antigens.

A number of antigens have been found to cause HP but only a small proportion of the people who are exposed to these antigens develop HP.^1-6,8,9 Exposures to the causative antigens can be associated with specific occupations or hobbies but can also occur in the home and general environment.

Bird or Pigeon Fancier’s Lung: Globally, this is the most commonly reported form of HP and is caused by exposure to organic antigens in bird (particularly pigeon) excreta.^1,10,11 Indirect exposure from feather bedding or down comforters have also been reported to cause disease. Avian antigen can exist in the indoor environment regardless of antigen avoidance.¹² The presence of avian antigen in the indoor environment can be attributed to wild birds found outdoors.¹²

Farmer’s Lung: Caused by exposure to moldy hay, compost or grain stored in conditions of high humidity in the agricultural workplace.^2,13 IgG precipitins commonly associated with Farmer’s Lung including Aspergillus fumigatus, Thermoactinomyces sacchari, Thermoactinomyces vulgaris and Saccharopolyspora rectivirgula (formerly called Micropolyspora faeni).

Humidifier/Sauna Taker’s Lung: HP secondary to occupational exposure to moldy water from heating/ventilation/air-conditioning systems has been described in adults.^14-16 Non-occupational exposure to molds including Aspergillus fumigatus and aureobasidium pullulans via home saunas or water damage has also been shown to cause HP.^17-20

Early diagnosis of HP is critical to avoid the development of extensive pulmonary fibrosis or restrictive lung disease has occurred.¹ Identification of the offending agentis critical in diagnosing HP and implementing preventive measures.^5,7 If diagnosed early enough, complete avoidance of inciting antigen results in total recovery of lung function in the majority of patients. If not promptly diagnosed and treated, HP can progress to pulmonary fibrosis and progressive respiratory failure. Presence of fibrosis and honeycombing have been associated with higher mortality. Primary prevention should aim to reduce exposure to known organic antigens.