Version May 2024
INTRODUCTION — Hypersensitivity pneumonitis (HP), also called extrinsic allergic alveolitis, is a syndrome characterized by diffuse inflammation of lung parenchyma and airways in response to the inhalation of antigens to which the patient has been previously sensitized. Numerous inciting agents have been described including, but not limited to, agricultural dusts, bioaerosols, and certain reactive chemical species. (See "Hypersensitivity pneumonitis (extrinsic allergic alveolitis): Epidemiology, causes, and pathogenesis".)
Both environmental and host factors are involved in the production of the HP syndromes, so management theoretically can involve modification of the environment or of the host immune response [1]. As the pathogenesis of HP is incompletely understood, emphasis on environmental control remains the cornerstone of therapy [2]. The treatment, prognosis, and prevention of HP will be reviewed here. The epidemiology, causes, clinical manifestations, and diagnosis of HP are reviewed separately. (See "Hypersensitivity pneumonitis (extrinsic allergic alveolitis): Epidemiology, causes, and pathogenesis" and "Hypersensitivity pneumonitis (extrinsic allergic alveolitis): Clinical manifestations and diagnosis".)
TREATMENT — Antigen avoidance is the cornerstone of treatment for symptomatic hypersensitivity pneumonitis and usually results in regression of disease [1,3-5]. Additional treatment may be required in more severe or progressive disease. The best studied forms of HP are farmer's lung and bird fancier’s lung; treatment of other types of HP largely is extrapolated from the experiences in these populations.
Antigen avoidance — Patients with HP should be encouraged to completely avoid (preferred) ongoing exposure to the provocative antigen(s) [1,3-6]. Patients with progression of disease in the context of ongoing exposure should be advised in the strongest terms to avoid the antigen completely and by whatever means necessary [5,7-9].
As an exception, some patients, such as those with farmers’ lung who have prompt resolution of initial symptoms, may do well despite continued low level exposure, making it difficult to determine the degree to which antigen avoidance is required in such patients. The risk of recurrence is unclear in this setting, and drastic measures to avoid inciting antigens may not always be indicated [10-13].
Antigen avoidance depends on identification of the antigen(s) responsible for HP in the individual patient (table 1) (see "Hypersensitivity pneumonitis (extrinsic allergic alveolitis): Epidemiology, causes, and pathogenesis"). A standardized questionnaire which elicits exposure characteristics is helpful in increasing diagnostic confidence of chronic HP [14]. Sometimes, antigen avoidance is straightforward, such as removing birds or feather bedding from the household, avoiding hot tubs and water flume slides, or sterilizing humidifiers and vaporizers [1,3,15]. For other patients, complete avoidance may require relocation to a new job or home, although such drastic steps may not guarantee improvement. Additional measures to minimize ongoing antigenic exposure are described below. (See 'Mitigation and prevention' below and "Hypersensitivity pneumonitis (extrinsic allergic alveolitis): Epidemiology, causes, and pathogenesis".)
Farmer's lung poses particular problems because of the emotional and financial ramifications of leaving the farm environment. While a change of employment and residence may be necessary in persons with severe disease, several studies show that most farmers with HP do not progress clinically even if they do not modify their employment [10,11,16].
Persons with bird fancier's lung are frequently reluctant to give up their birds. Even after removal of the birds and environmental cleanup, recovery is not assured because high levels of bird antigens can be detected in the home environment for prolonged periods of time [17].
In addition to treatment of the index patient, antigen identification and avoidance are important in preventing sensitization of co-workers and family members.
Acute HP with minimal or transient symptoms — For most patients with acute HP and mild or intermittent respiratory symptoms, we advise complete antigen avoidance (see 'Antigen avoidance' above), rather than oral glucocorticoids. Antigen avoidance, such as removal of feather bedding, cleaning a contaminated humidifier, or workplace mitigation, is usually sufficient treatment. We re-evaluate patients after 6 to 12 weeks to ensure improvement in symptoms and lung function, or sooner if symptoms are not improving.
Acute or subacute HP with persistent symptoms — For patients with acute or subacute HP and recurrent or persistent symptoms and lung function impairment, avoidance of causative antigens remains the most important intervention (see 'Antigen avoidance' above). Addition of oral glucocorticoids can accelerate initial recovery.
Glucocorticoids — Based upon experience from observational studies of farmer's lung and bird fancier's lung [4,18-20], we suggest a short course of oral glucocorticoids in patients with subacute HP and moderate respiratory impairment to accelerate improvement. However, the long-term outcome appears unchanged by glucocorticoid treatment.
●Patient selection – Glucocorticoids are usually prescribed for patients with persistent symptoms (eg, dyspnea, cough, fatigue, weight loss), abnormal lung function tests (eg, FVC or DLCO <80 percent predicted), hypoxemia, or radiographic evidence of extensive lung involvement [21,22].
●Dosing and duration – Therapy is usually initiated with prednisone, 0.5 mg/kg per day (up to 30 mg per day), given as a single dose each morning [1,4], although dose and duration have not been formally studied. This dose is maintained for one to two weeks and then tapered over the next two to four weeks.
Maintenance doses are not usually required, particularly if the patient has been removed from exposure. Inhaled glucocorticoids (eg, nebulized budesonide) may be effective in treating or preventing recurrence, but this approach has not been well studied [23].
●Efficacy – Evidence in favor of glucocorticoid therapy comes from studies such as the following:
A trial of 36 persons with acute farmer's lung randomly assigned patients to eight weeks of treatment with either prednisolone or placebo [19]. After one month, there was a significant improvement in diffusing capacity for carbon monoxide (DLCO) in the prednisolone-treated group, but no statistically significant differences in forced vital capacity (FVC), forced expiratory volume in one second (FEV1), or DLCO were found after that time until follow-up was terminated at five years.
One study of 93 patients with farmer's lung compared patients treated with glucocorticoids for 4 or 12 weeks with a group of control patients treated only with antigen avoidance [18]. Symptom resolution was more rapid among the glucocorticoid-treated groups, but no differences were noted in the course of disease or lung function among the three groups after six months. The 12 week glucocorticoid regimen offered no advantage over the four week regimen. Limitations of this report are that the nonglucocorticoid-treated group had less severe disease and the study was not blinded.
●Adverse effects – Systemic glucocorticoids cause a variety of side effects, and may not be tolerated by some patients. (See "Major adverse effects of systemic glucocorticoids".)
An additional concern is that glucocorticoid treatment may lead to an improved sense of well-being, causing the patient to be less stringent about measures to limit future antigen exposure [24].
Chronic fibrotic HP — The optimal management of chronic fibrotic HP is not well-established because of a paucity of clinical trial data. In the absence of such data, the following is a practical approach based on clinical experience and expert opinion [3,4].
Do not neglect antigen avoidance — For patients with chronic or fibrotic HP, identification and avoidance of culprit antigens are associated with improved survival in several observational studies [1,3,7,25-27]. For example, in one cohort of 377 patients with fibrotic HP, combined suspected antigen identification and avoidance was associated with greatly improved long-term transplant-free survival (approximately 20 versus 40 percent at five years, adjusted hazard ratio [HR] 0.47, 95% CI 0.31-0.71) [27]. Conversely, patients with suspected antigens without avoidance or with unidentifiable antigens demonstrated a similar increased risk of mortality (adjusted HR 2.22 and 2.09, respectively) compared with the antigen-avoidant group. In a separate study, patients with chronic bird-related HP were more likely to deteriorate over time when exposed to higher amounts of avian antigen, while those exposed to lower amounts were more likely to have stable disease [7]. (See 'Antigen avoidance' above.)
Supportive care — Supportive care should be provided given the chronicity of disease and includes smoking cessation, seasonal influenza and pneumococcal vaccinations, pulmonary rehabilitation, and supplemental oxygen as needed. (See "Overview of smoking cessation management in adults" and "Seasonal influenza vaccination in adults" and "Pneumococcal vaccination in adults" and "Pulmonary rehabilitation" and "Long-term supplemental oxygen therapy".)
Glucocorticoids — Based on expert opinion and observational data from studies of farmer's lung and bird fancier's lung, we suggest a trial of oral glucocorticoids for patients with chronic HP [3,18-20]. The usual initial dose is the equivalent of prednisone 0.5 mg/kg per day (up to 30 mg per day) for four to eight weeks, followed by tapering to 10 to 20 mg per day by three months [3,28]. We use the following features to help decision-making, although this approach is based on clinical experience rather than trial data [3].
●Patients with inflammatory features – Patients with inflammatory features, such as ground-glass opacities on high-resolution computed tomography (HRCT), bronchoalveolar lavage (BAL) lymphocytosis (eg, >20 percent), and/or histopathologic cellular interstitial pneumonia or granulomas, are more likely to experience improvement with glucocorticoid therapy [3]. If symptoms, pulmonary function, and HRCT show improvement over the course of approximately three months, prednisone is tapered gradually to 15 mg/day, as tolerated.
If symptoms do not improve substantially, a second immunosuppressive agent can be added. Similarly, if symptoms and/or lung function recur during or after tapering prednisone, the dose can be returned to the level previously associated with disease control. Addition of a second immunosuppressive agent may enable tolerance of a lower dose of prednisone. (See 'Additional immunosuppressive agents' below.)
●Patients without inflammatory features – Patients without inflammatory features (eg, no ground-glass opacities on HRCT, absence of BAL lymphocytosis, absence of histopathologic cellular interstitial pneumonia or granulomatous inflammation) are less likely to improve with glucocorticoids, so a reasonable alternative is not to give glucocorticoids, particularly if symptoms and lung function impairment are mild. For those who pursue a trial of glucocorticoids, we use a similar initial dose, but rapidly taper and discontinue after a three month trial in the absence of clear benefit [3]. Cytotoxic agents are unlikely to be of benefit in patients who do not respond to glucocorticoids. Other options include clinical trial participation and lung transplantation. (See 'Lung transplantation' below and 'Antifibrotic agents' below.)
Long-term glucocorticoid treatment is associated with a variety of adverse effects (eg, hyperglycemia, adrenal suppression, opportunistic infection, weight gain, cataracts, osteoporosis), so careful and ongoing assessment of risks and benefits is essential. (See "Major adverse effects of systemic glucocorticoids".)
Additional immunosuppressive agents — Azathioprine (AZA) and mycophenolate mofetil (MMF) have been used in patients with chronic HP who have not responded to antigen removal and systemic glucocorticoid use, but these agents have not been examined in clinical trials of HP [3,4,29-32].
Observational data suggest that these agents lead to improvements in lung function (DLCO) and appear to have fewer adverse events compared with glucocorticoid therapy alone [29,30]. In a retrospective study, 51 patients with chronic HP were treated with MMF and 19 with AZA; after 11 months, no improvement was noted with either agent in FEV1, but DLCO increased by 4 percent in both groups [29]. Six months after initiation of AZA or MMF, prednisone decreased by almost 4 mg/day from a prior mean dose of 12 mg/day. In another retrospective study, 30 patients with chronic HP treated with MMF or AZA therapy showed improvement in DLCO and reduction in glucocorticoid dose [33]. Retrospective analysis of 62 patients on AZA treatment for up to two years shows an improvement in lung function [34].
In a separate observational study that included 282 patients with fibrotic HP stratified by leukocyte telomere length, the use of AZA or MMF was not linked to improved survival in patients with normal telomere length (≥10th percentile for age). Specifically, the two-year transplant-free survival was 86 percent in those treated with AZA or MMF compared with 88 percent in untreated patients (adjusted HR 0.8, 95% CI 0.3-2.0) [35]. Conversely, in patients with short telomeres (<10th percentile for age) treatment with AZA or MMF was strongly associated with worse transplant-free survival (48 versus 83 percent in untreated patients, adjusted HR 0.15 95% CI 0.06-0.42). The significant interaction between telomere length and poor outcome with immunosuppressants suggests that measuring telomere length in patients with chronic HP may help identify a population who might benefit more from alternative management strategies.
In a multicenter, retrospective, noninterventional study involving 344 patients with chronic HP, the effects AZA or MMF treatment were evaluated across various types of antigen exposures: avian (27 percent), mold (26 percent), other (4 percent), and unknown (43 percent). The study found no significant differences in patient characteristics, lung function, or disease progression among the different antigen groups. However, a notable finding was the observed worsening survival in patients with unknown antigen exposure who were treated with immunosuppression (HR 2.7; 95% CI, 1.01-6.9). This suggests a variable response to immunosuppressive therapy in chronic HP based on the type of antigen involved [36].
Prospective clinical trials are needed to clarify the role of AZA and MMF in the long-term management of chronic HP.
Information about the administration of AZA and MMF and potential adverse effects are discussed separately. (See "Pharmacology and side effects of azathioprine when used in rheumatic diseases" and "Mycophenolate: Overview of use and adverse effects in the treatment of rheumatic diseases".)
Antifibrotic agents — For patients with chronic hypersensitivity pneumonitis and progressive fibrosis, treatment with the antifibrotic nintedanib may improve disease progression. The alternative antifibrotic agent pirfenidone has been less well-studied in this setting but is sometimes offered off-label if nintedanib cannot be used. When initiated, antifibrotic therapy is often added to immunosuppressive therapies, if tolerated. Dosing, administration, and monitoring of antifibrotic therapies are covered elsewhere. (See "Treatment of idiopathic pulmonary fibrosis", section on 'Medical therapies'.)
Antifibrotic therapies, such as pirfenidone and nintedanib, have been found to slow disease progression in idiopathic pulmonary fibrosis (see "Treatment of idiopathic pulmonary fibrosis", section on 'Nintedanib' and "Treatment of idiopathic pulmonary fibrosis", section on 'Pirfenidone'). This finding has led to considerable interest in the role for these therapies in other fibrosing interstitial lung diseases, including chronic fibrotic hypersensitivity pneumonitis. Although nintedanib shows a potential benefit in terms of lung function decline, whether antifibrotic therapies improve patient-important outcomes in those with chronic fibrotic hypersensitivity pneumonitis is less certain.
One trial compared nintedanib (150 mg twice daily) with placebo in 663 patients with fibrotic interstitial lung diseases (other than IPF) that affected at least 10 percent of the lung on high-resolution computed tomography (HRCT); 26 percent of participants had chronic hypersensitivity pneumonitis [37]. Requirements for trial entry included a relative decline in the FVC of at least 10 percent of the predicted value; a relative decline in the FVC of 5 to <10 percent of predicted and worsening of respiratory symptoms or an increased extent of fibrosis on HRCT; or worsening of respiratory symptoms and an increased extent of fibrosis. After 52 weeks, the adjusted rate of decline in forced vital capacity was -80.8 mL/year in the nintedanib group and -187.8 mL/year in the placebo group with a between-group difference of 107.0 mL/year (95% CI 65.4 to 148.5). Between group differences in other end-points such as composite measures of activity and symptoms, and also frequency of acute exacerbations or death did not meet significance. The most common adverse event was diarrhea, reported in 67 and 24 percent of patients treated with nintedanib and placebo, respectively.
Pirfenidone has been assessed in two placebo-controlled trials that were terminated early due to slow recruitment. In the first trial, 127 patients with a variety of fibrotic interstitial lung diseases (45 percent of whom had chronic hypersensitivity pneumonitis) received pirfenidone or placebo for 48 weeks [38]. There was no clinically meaningful reduction in the rate of FVC decline or in progression-free survival. The other trial, which assessed pirfenidone (versus placebo) in 40 patients with fibrotic hypersensitivity pneumonitis, was underpowered to detect improvement or delayed decline in lung function [39].
Lung transplantation — For the patient who develops advanced lung disease due to HP, lung transplantation may be an option [3,40]. The indications for lung transplantation in HP are similar to those for other interstitial lung diseases and are discussed separately. (See "Treatment of idiopathic pulmonary fibrosis", section on 'Indications and choice of procedure' and "Lung transplantation: General guidelines for recipient selection".)
In general, patients with HP have excellent medium-term survival following lung transplantation. In a case series, 31 patients with HP underwent lung transplantation and were compared with 91 patients with idiopathic pulmonary fibrosis (IPF) transplanted at the same center [40]. The inciting allergen was identified in 12 patients; eight were exposed to birds, three to mold, and one to wood bark. Among patients with HP, survival at one, three, and five years post transplantation was 96, 89, and 89 percent, respectively, which compared with 86, 67, and 49 percent for patients with IPF. Two patients experienced recurrence of HP in the allograft: one had heavy occupational exposure to trees, possibly sequoias, and lived in a wooded area; the other had heavy exposure to birds and molds with positive serology to avian and fungal precipitins, and post-transplant HP followed a repeat exposure to pigeons.
Future directions
Rituximab — Rituximab, a B cell depleting monoclonal antibody, has been used in individual cases of refractory hypersensitivity pneumonitis [41,42]. Given that chronic progressive HP has a mixed immunopathogenesis that involves T cells and fibrosis in addition to antibody-mediated disease, B cell-depleting therapy may not be broadly beneficial. Among six patients with HP and severe respiratory impairment that was refractory to oral glucocorticoid and at least one other immunosuppressive agent, treatment with rituximab was associated with stabilization or improvement in three and progression in three [42]. A retrospective study in 20 patients with chronic HP showed that rituximab treatment of six months' duration may lead to stabilization or improvement of lung function in some patients [43].
Leflunomide — A retrospective study of leflunomide treatment in 40 patients with chronic HP showed improvement in pulmonary function (FVC) at 12 months [32]. Non-fibrotic chronic HP patients had the largest gain in pulmonary function; patients with fibrotic chronic HP did not improve. Leflunomide treatment was associated with significant adverse effects leading to discontinuation of therapy in 40 percent of patients [32].
MONITORING — A specific monitoring schedule has not been established for HP [3]. For patients with an acute presentation of HP that resolves promptly, a follow-up visit is desirable to determine the success of antigen mitigation and resolution of disease, especially if antigen avoidance has not been successful. Continued low levels of exposure may lead to subclinical progression of lung injury.
For patients with subacute or chronic HP treated with glucocorticoids or additional immunosuppressive agents, follow-up visits to assess symptoms, potential adverse effects, and lung function (spirometry, pulse oximetry, and diffusing capacity for carbon monoxide [DLCO]) are reasonable at one to two month intervals [3]. Once prednisone is tapered to <20 mg daily, the interval can often be lengthened to three to six months, depending on the severity and trajectory of symptoms and potential for medication adverse effects. Repeat high-resolution computed tomography (HRCT) is based on symptoms and lung function but is often obtained three to six months after initiation of treatment.
PROGNOSIS
Predictive features — The long-term outcome of HP varies and depends on factors such as the specific causal antigen, duration of antigen exposure, and host response. Patients with acute HP who have complete avoidance of the causal antigen tend to experience near total recovery of lung function, although full recovery may take several years after the inciting exposure ceases [4,44]. Bird fancier's HP appears to have a worse prognosis than farmer's lung. The prognoses of other varieties of HP are less well described. In general, patients with evidence of pulmonary fibrosis on surgical lung biopsy have a poorer prognosis than those without such changes [45-48]. In fact, in one large cohort of patients with fibrotic lung disease, approximately 60 percent of both those with chronic fibrotic HP and those with idiopathic pulmonary fibrosis demonstrated disease progression (defined by forced vital capacity [FVC] decline of more than 10 percent, a 5 to 10 percent FVC decline with worsening respiratory symptoms or an increase in radiographic fibrosis, both worsening symptoms and radiographic fibrosis, lung transplant, or death) over two years [49].
●ILD-GAP model – Decline in lung function over time is a poor prognostic sign, particularly for patients with chronic HP [50]. Similar to the GAP model used for predicting mortality in patients with IPF [51], the interstitial lung disease (ILD)-GAP model, which combines age, gender, and measures of pulmonary function (FVC and diffusing capacity for carbon monoxide [DLCO]) with a variable to account for the ILD subtype, can help to predict mortality in chronic HP [52]. (See "Prognosis and monitoring of idiopathic pulmonary fibrosis", section on 'Predictors of mortality'.)
●Imaging – While chest radiographs at the time of diagnosis do not correlate with outcome, high-resolution computed tomography (HRCT) patterns may be useful in predicting prognosis in chronic HP [47,53-59]. In particular, a greater severity of traction bronchiectasis and increased extent of honeycombing are associated with increasing mortality [54,60]. Similarly, among 117 patients with biopsy-proven HP, HRCT-based characterization correlated with nonfibrotic HP (no traction bronchiectasis, or honeycombing), had a longer event-free survival (14.73 years) than nonhoneycomb fibrosis (7.95 years) or honeycomb fibrosis (2.76 years) [59]. By comparison, event-free survival was 2.81 years among 161 patients with idiopathic pulmonary fibrosis (IPF).
A model derived from fully automated extraction of HRCT features performs comparably to models based upon patient demographics and pulmonary function testing results [61].
●Histopathology – Certain histopathologic patterns appear predictive of mortality and shorter transplant-free survival [45-48]. In a longitudinal cohort of 119 patients with chronic HP, surgical lung biopsies were reviewed prospectively [48]. Histopathologic patterns of fibrotic nonspecific interstitial pneumonitis (NSIP), bronchiolocentric fibrosis, and usual interstitial pneumonitis (UIP) were associated with a poor prognosis compared with the patterns of cellular NSIP and peribronchiolar inflammation with poorly-formed granulomas (PI-PFG). The presence of fibroblast foci or dense collagen fibrosis were independent predictors of death or lung transplant.
●Pulmonary hypertension – Among patients with chronic HP, development of symptomatic pulmonary hypertension correlates with the severity of interstitial lung disease and portends a worse survival [62,63]. In most patients, pulmonary hypertension is attributable to the interstitial lung disease, but a small portion have type 2 pulmonary hypertension related to left heart dysfunction.
Farmer's lung — Many investigators have studied the clinical course of farmer's lung and have reached a similar conclusion: most farmers recover from their original illness with only minor, if any, functional abnormalities, and very few progress to an advanced, debilitated state [10,11,24,64,65]. Approximately 50 percent of farmers develop chronic lung impairment, but the alterations are usually minor [64]. Most have obstructive abnormalities, often associated with emphysematous changes on high-resolution computed tomography (HRCT) [66,67]. As an example, one 14 year study of 89 patients with farmer's lung and 84 controls matched for age, sex, and smoking habits found a higher incidence of airflow obstruction in those with farmer's lung (33 versus 17 percent) [66].
In general, PaO2 improves more rapidly than the forced vital capacity (FVC), while recovery of the diffusing capacity of carbon monoxide (DLCO) takes longest [65]. The pulmonary function of farmer's lung patients continues to improve for up to two years after the initial acute episode, and this improvement may be seen even if exposure continues.
Attempts to identify factors that influence prognosis have yielded conflicting results [16,24,68-71]. However, repeated mild attacks appear more likely to result in disability than a single, acute attack [67,72]. This may reflect better adherence to antigen avoidance measures in the latter group, as well as resolution of granulomatous inflammation before significant pulmonary fibrosis has occurred.
Bird fancier's lung — Bird fancier's lung (also called pigeon breeder's disease) has not been as well studied as farmer's lung, but its prognosis appears worse. We and others have noted that many of our chronic, progressive, and fatal cases of HP had disease secondary to bird antigen exposure [40,55,73-77]. Nevertheless, some patients do relatively well despite continued antigen exposure after an acute episode of HP [13]. In one report of 18 such patients who were followed for 10 years, only five had troublesome symptoms and four had some residual abnormalities in pulmonary function.
The poorer outcome in some patients with this disorder may be due to a higher degree of exposure to HP antigens than in farmer's lung and to the persistence of avian antigens in the home environment despite attempts at decontamination [7]. Continued domestic exposure may account for the substantial mortality (29 percent at five years) seen in Mexican patients with chronic pigeon breeder's lung [75].
Several factors affect prognosis with bird fancier's lung [8,46,78-81]:
●The duration of exposure appears important. One study of nine patients found complete recovery among persons exposed to bird antigens for less than six months but documented residual abnormalities in those with longer exposures [78]. Similar findings were reported in a series of 24 patients with bird fancier's lung in which patients exposed for less than two years had a more favorable outcome than those exposed for a longer period [79].
●A wide range of histopathologic changes has been reported among patients with bird fancier's lung, and these findings may help predict the subsequent clinical course of disease [46]. In addition to hypersensitivity pneumonitis, changes associated with several forms of idiopathic interstitial pneumonia have been identified. As an example, patients with organizing pneumonia or cellular NSIP appear to have a better prognosis than those with evidence of fibrosing pneumonitis (either fibrotic NSIP or a UIP-like pattern with a distinct centrilobular fibrosis) on biopsy [46,82]. (See "Idiopathic interstitial pneumonias: Classification and pathology".)
●The presentation (acute, subacute, or chronic) may correlate with recovery or progression. In a study of 18 patients followed for a median of 11 years, an acute presentation was associated with a better prognosis [8]. This association has been noted in some [83], but not all [78] subsequent studies.
●The presence of digital clubbing predicts a worse outcome [81].
●Older patients tend to have a less complete recovery than younger patients [78].
●Greater exposure intensity results in accelerated declines in lung function [80].
MITIGATION AND PREVENTION — The incidence of HP can be reduced by diminishing exposure to provocative antigens by interventions such as minimizing contact with potential inciting agents, reducing microbial contamination of the work or home environment, and using protective equipment. For patients with HP, these interventions can help mitigate further exposure.
Reduction of antigenic burden — Alteration in the handling and storage of potential sources of microbial antigens can diminish the occurrence of HP [5,84-87]. As examples, enclosing machines and improving ventilation reduces exposure to metal working fluids [86,87]; use of antimicrobial solutions in sugar cane processing diminishes fungal growth and the development of bagassosis [88], and reducing outdoor storage of sawn wood and using a new kiln to dry wood decreases mold exposure and respiratory symptoms in sawmills [84].
Design and maintenance of facilities — Indoor microbial contamination is usually related to problems with moisture control [89]. Appropriate design of facilities may reduce stagnant water that is prone to microbial overgrowth. Thus, humidity in occupied buildings should be maintained below 60 percent, carpeting should be avoided in areas where persistent moisture is likely to be present, and water in heating, ventilating, and air conditioning systems should not be recirculated.
Preventive maintenance should be performed routinely in order to ensure that all heating, ventilation, and air conditioning equipment is properly maintained and that the indoor environment is clean. Portable humidifiers or vaporizers should be drained daily with removal of all slime and sterilized frequently with either chlorine bleach or hydrogen peroxide [89].
Water damage should be immediately repaired because microbial colonization occurs quickly and can be very difficult to eradicate. Water-damaged furnishings, drywall, and carpeting should be removed.
Protective devices — Devices to limit inhalation of inciting antigens may be useful when complete elimination of these antigens is impossible, although supportive data are limited [5,10,11,86]. As an example, the level of antigen exposure can sometimes be controlled by the appropriate use of environmental controls such as a specialized filter in the return ducts of a central air conditioning system [90].
The efficacy of various types of personal respirators in preventing antigen sensitization and disease progression has not been established. Their use is most appropriate when avoidance of the causative antigen cannot easily and rapidly be achieved [5,91,92]. Helmet-type powered air purifying respirators have been used to prevent episodic exposure in individuals with previous acute episodes of farmer's lung, but are cumbersome to wear for prolonged periods. Data on the use of dust respirators are conflicting and further research is needed to evaluate whether they are sufficiently protective [5,91-93].
SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Interstitial lung disease".)
SUMMARY AND RECOMMENDATIONS
●Patients with hypersensitivity pneumonitis (HP) should be encouraged to completely avoid (preferred) ongoing exposure to the provocative antigen(s), particularly patients with progressive disease. As an exception, some patients, such as those with farmers’ lung who have prompt resolution of initial symptoms, may do well despite continued low-level exposure. (See 'Antigen avoidance' above.)
●For patients with acute HP, mild or intermittent symptoms, and minimal to no abnormalities on pulmonary function tests (PFTs), avoidance of the culprit antigen will usually lead to resolution of symptoms and lung function abnormalities. (See 'Antigen avoidance' above.)
●For patients with symptomatic acute or subacute HP, reduced lung function, and diffuse lung disease on high-resolution computed tomography (HRCT), but no evidence of infection, in addition to antigen avoidance, we suggest a course of oral glucocorticoids, based on limited evidence that therapy will shorten the duration of symptoms (Grade 2C). (See 'Glucocorticoids' above.)
•The usual initial dose is the equivalent of prednisone 0.5 mg/kg per day (up to 30 mg/day); the initial dose is maintained for one to two weeks.
•Once the patient is symptomatically improved, prednisone is tapered over the next two to four weeks. Maintenance therapy is rarely required, usually only when a patient is unable to avoid the culprit antigen. (See 'Glucocorticoids' above.)
●Supportive care, such as smoking cessation, seasonal influenza and pneumococcal vaccinations, pulmonary rehabilitation, and supplemental oxygen) should be provided to patients with chronic HP, as needed. (See 'Supportive care' above.)
●For patients with chronic or fibrotic HP, we suggest a trial of oral glucocorticoids (Grade 2C). The usual initial dose of glucocorticoid is the equivalent of prednisone 0.5 mg/kg per day (up to 30 mg per day) for four to eight weeks followed by tapering to 10 to 20 mg per day by three months, with subsequent tapering as guided by symptoms and lung function. A reasonable alternative is not to give glucocorticoids, particularly if symptoms and lung function impairment are mild and inflammatory features are absent. For patients without inflammatory features who undergo a trial of glucocorticoid therapy, we reassess after three months and taper off of prednisone rapidly if there is no clear evidence of benefit. (See 'Glucocorticoids' above.)
●For patients with chronic fibrotic hypersensitivity pneumonitis and progressive fibrosis, we suggest treatment with the antifibrotic nintedanib (Grade 2C), which may reduce subsequent lung function decline. For patients with progressive pulmonary fibrosis who cannot use nintedanib, off-label treatment with pirfenidone is a reasonable alternative.
●Azathioprine (AZA) and mycophenolate mofetil (MMF) have been used with some success in patients with chronic HP who have not responded to antigen removal and systemic glucocorticoid therapy, but clinical trial data are lacking. (See 'Additional immunosuppressive agents' above.)
●Lung transplantation is an option for selected patients with advanced chronic HP, and has been shown to have excellent medium-term survival. Recurrent exposure may lead to HP in the allograft. (See 'Lung transplantation' above.)
●The majority of patients with acute or subacute HP experience near total recovery of lung function, which in some cases may take several years after the inciting exposure ceases. However, more fibrotic histopathologic patterns have a poorer prognosis than more inflammatory patterns. Fibroblast foci and dense collagen fibrosis may be independent predictors of mortality or need for lung transplantation. (See 'Prognosis' above.)
●Bird fancier's lung appears to have a worse prognosis than farmer's lung; this may be due to a higher concentration and persistence of avian antigens in the home environment compared with exposure to antigens that cause farmer's lung. (See 'Bird fancier's lung' above.)
●The incidence of HP can be reduced by diminishing exposure to the agricultural dusts, bioaerosols, and chemicals that are known or found to be causative. Methods to reduce exposure include reducing microbial contamination and excess humidity in the work or home environment, maintaining ventilation systems properly, and/or using protective equipment. (See 'Mitigation and prevention' above.)
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