The first describer of farmer's lung was the 17th century Italian researcher Bernadino Ramazzini. Around 1932, interest around the disease reignited when an official noticed 5 cases of acute respiratory failure (Zergham 2022).
According to Sennekamp (2019), farmer's lung can even be traced back to the 1500s.
Farmer's lung (FLD) is an immunological reaction to inhaled antigen (Zergham 2022) from hay or straw dust (Sennekamp 2006).
Farmer's lung is one of the recognized occupational lung diseases (Wiesmüller 2015).
Human-to-human transmission has not been observed to date (Zergham 2022).
FLD can occur acutely or subacutely as so-called ASF = acute or subacute farmer's lung and chronically as so-called CF = chronic farmer's lung (Cano- Jimenez 2016).
FLD is a rare form of lung disease (Kreuter 2016). Only about 8% of exposed farmers are affected (Saloga 2006).
More non-smokers than smokers are affected by the disease (Wiesmüller 2015).
The (acute / subacute) FLD occurs seasonally, predominantly in the months of March - April (Cano- Jimenez 2017). In farmers, the prevalence is estimated at 0.2 - 1.5% (Soumagne 2017). However, the disease is often underdiagnosed (Cano- Jimenez 2016).
Exogenous allergic alveolitis (EAA) is second most commonly caused by microorganisms found in moldy grain (Kasper 2015), hay, or silage that can lead to farmer's lung disease (FLD) (Herold 2022). Little is known about the epidemiology (Cano- Jimenez 2016).
FLD also represents the most common form of hypersensitivity pneumonitis (HP) (Bellanger 2010).
Farmer's lung is caused by contact with the following antigens from moldy grain, hay, or silage (Kasper 2015):
- Absidia corymbifera
- Eurotium amstelodami
- Saccharopolyspora rectivirgula (Herold 2022)
Saccharopolyspora rectivirgula represents the best known and most common pathogen of exogenous allergic alveolitis such as farmer's lung (Zahradnik 2018).
- Thermoactinomyces vulgaris (Herold 2022).
Thermophilic actinomycetes and Aspergillus sp. are found as bioaerosol (Wiesmüller 2015).
The pathobiology has not yet been fully elucidated. Individual and genetic factors certainly also play a role (Schweisfurth 2019).
Inhaled antigens cause inflammation in the airways and interstitial spaces. As a result, fever and myalgias occur. Hypoxia occurs due to restriction of diffusion (Zergham 2022).
Inflammation is initially triggered by immune complex-mediated hypersensitivity(type III reaction). This results in the appearance of granulomas. If acute exposure persists, specific IGG- antibodies to the inhaled antigen are detectable in serum in sometimes high titers (Zergham 2022).
If the exposure continues, a chronic course in the form of a T-cell mediated type IV reaction occurs. This leads to fibrosis (Zergham 2022).
In Asian countries, the prevalence is significantly higher than in European countries. In addition to geographical conditions, season, local customs, and proximity to exposure play a role (Zergham 2022).
Climatic conditions in northern Spain also favor the development of the disease (Cano- Jimenez 2016).
Clinical symptoms vary, depending on the length and severity of contact (Cano- Jimenez 2016).
- Acute exposure to high antigen concentrations (Cano- Jimenez 2016):
- Fever
- Dyspnea (Duprez 2020)
- Myalgias
- hypoxia
- dry cough (Zergham 2022)
Acute symptoms typically do not appear until 4 - 8 hours after exposure (Kasper 2015).
- Chronic exposure over a prolonged period:
- progressive dyspnea
- cough
- fatigue
- weight loss (Kasper 2015)
In this case, signs of pulmonary emphysema are also found (Kreuter 2016).
To date, there is no globally recognized established diagnostic criterion of FLD. A detailed history together with exposure history, high-resolution computed tomography (HRCT) and marked lymphocytosis are the main criteria.
In addition, should be performed:
- enzyme-linked immunosorbent assay (ELISA) for serum antibody evaluation (antibodies are detectable for up to 10 years after cessation of exposure).
- antigen detection
- sputum cultures
- transbronchial biopsy (TBLB) is not always useful because half of the findings are non-specific
- transbronchial cryobiopsy (TBLC) is used to assess fibrotic changes (Zergham 2022)
- auscultation
Sometimes inspiratory crackles (Soumagne 2017).
- HRCT
With this examination it is possible to differentiate between acute and chronic courses (Zergham 2022):
- Acute course
- diffuse milk glass opacities
- centrilobular nodules
- air inclusions
- mosaic pattern
- Chronic course
- Milky glass opacities
- Evidence of pulmonary hypertension
- Dominance of the upper lobe
- Thickening of the septum
- sometimes emphysematous changes
- fibrosis
- Antigen tests
These show sensitivity to specific antigens of exogenous allergic alveolitis (Cano- Jimenez 2016). IgG antibodies to Aspergillus fumigatus are found most frequently, at 63%, followed by 57% to S. rectivirgula (Zahradnik 2018).
- Bronchoalveolar lavage
This typically shows lymphocytic alveolitis (Cano- Jimenez 2016).
In the chronic, persistent course, lymphocytic alveolitis usually disappears (Soumagne 2017).
- Lung function
In lung function, there is
- an impairment of TLCO (transfer factor for carbon monoxide)
- Airway obstruction (Soumagne 2017).
- FEF (forced mid-expiratory flow) decreased at 25-75% (Zergham 2022).
In the chronic course of FLD, pulmonary fibrosis is often found (Soumagne 2017).
The differential diagnoses are numerous in FLD:
- organic dust toxic syndrome
- acute viral diseases
- COPD in active smokers
- combined pulmonary fibrosis and emphysema in smokers (Zergham 2022)
- Pulmonary emphysema
This was detectable in 48.5% of patients with active FLD in the SOPHIA study. It is localized predominantly centrilobularly and in the upper lobes. Pulmonary hyperinflation was not found in these patients. Risk factors in active Farmer's lung are likely a longer duration of exposure at a higher level (Soumagne 2017).
- Fibrotic changes
These are numerous in the literature in the chronic form of progression, but the risk for developing emphysema is significantly higher according to epidemiological studies (Soumagne 2015).
Treatment consists primarily of permanent avoidance of the antigens (Cano- Jimenez 2016).
- 1. acute course:
The administration of corticosteroids can only lead to a more rapid cure of the acute form. An influence on the progression of the disease or a reduction in lethality cannot be achieved with this (Cano- Jimenez 2016).
To date, there are no evaluations of therapy with immunosuppressants (Cano- Jimenez 2016).
Zergham (2022) describes an improvement in TLCO with treatment with mycophenolate. However, further studies are needed here.
- 2. chronic course:
Chronic progression may require:
- Oxygen supplementation
- vaccinations
- non-invasive ventilation
- Lung transplantation as the last treatment option (Zergham 2022).
The prognosis depends primarily on the stage of the disease. Therefore, early diagnosis is of crucial importance.
Avoidance of antigen exposure can prevent progression of the disease.
In up to 2/3 of patients, symptoms persist 5 years after the acute episode. Affected individuals with 5 or more symptom recurrences usually have progressive and significant lung tissue damage (Zergham 2022).
The lethality rate is 1%, and the average life expectancy after diagnosis is 8 years (Zergham 2022).
Up to now, preventive measures have generally been neglected. These include, for example:
- motorized dust helmet
- Avoidance of exposure (Zergham 2022)
Smoking has been shown to have a protective effect on farmer's lung (Bellanger 2020).