Farmer's lung is considered a type of hypersensitivity pneumonitis that was initially identified in the 1700s by Italian researcher Bernardino Ramazzini. He examined exposure risks across multiple professions and found that the disease process was most abundantly found in breeders and farmers who were exposed to organic dust. Interest and knowledge of the disease were rekindled in 1932 when a county tuberculosis officer identified 5 cases of acute respiratory failure seemingly caused by moldy hay during the springtime. Hypersensitivity pneumonitis, as we understand it today, encompasses a wide breadth of exposures that may illicit a delayed allergic response, specifically in the smaller airways. Farmers, in particular, are exposed to various agents in the fields of their workplace. These include organic agents, inorganic agents, gasses, methane, pesticides, fertilizers, and more. The syndrome has great variability in symptom severity and presentation due in large part to the variable exposure durations and innumerable offending agents. Spread between people is nonexistent, however, common exposure history can lead to similar disease states.
Farmer's lung or hypersensitivity pneumonitis is the immunologic reaction that occurs in response to an inhaled antigen. To understand the most common exposures that would contribute to hypersensitivity pneumonitis, and by extension farmer's lung, it is important first to understand that chronic exposure to anything that the host body may find offensive and subsequently trigger an immune response can be a possible source. There are numerous varieties of hypersensitivity pneumonitis, often named after the profession or exposure for which they are associated. Examples include woodworker's lung, sauna taker's disease, and gerbil-keeper's lung, to name a few. Farmer's lung, a subgroup of hypersensitivity pneumonitis, has a variety of pathogens. The most common organism leading to farmer's lung is thermophilic actinomycetes like Micropolyspora faeni, Thermoactinomyces vulgaris, and fungi like Aspergillus. However, etiology varies greatly from region to region. The degree of regional variation of the dominant organism causing farmer's lung across the world necessitates extensive epidemiological evaluation and corresponding preventive measures.
Tracking epidemiological data for hypersensitivity pneumonitis and, by extension, farmer's lung remains a technical challenge as the condition is simultaneously underdiagnosed and influenced by a myriad of factors such as geographical conditions, season, local customs, and proximity to industrial exposure. In general, there is a higher prevalence of respiratory symptoms in the agricultural population during non-dry climates in an indoor setting. Region-specific data varies, and the prevalence of farmer's lung was found to be higher in Asian countries as compared to European countries.
Inhaled antigens may elicit inflammation in the small airways and interstitial spaces, leading to fever, myalgias, and diffusion limitation leading to hypoxia. This inflammation is mediated by two processes, type III and type IV hypersensitivity reactions. The inhaled antigen initially causes an immune complex-mediated (type III) hypersensitivity reaction, which is responsible for granuloma formation. During ongoing acute exposure, IgG antibodies specific to the inhaled antigen can be detected in the serum in relatively high titers. As exposure to the offending antigen continues and becomes chronic, a T-cell mediated (type IV) hypersensitivity reaction is elicited, which is responsible for fibrosis.
The clinical features of hypersensitivity pneumonitis range from an acute febrile reaction to an insidious onset. The clinical picture varies according to the degree of exposure and chronicity of the disease. During an acute episode, symptoms such as dry cough, shortness of breath, fevers, myalgias, and joint pains can be observed. These symptoms can be mistaken for a viral illness. Though there is not a globally recognized established diagnostic criteria, these findings are included in various diagnostic criteria that speak to the prevalence of these symptoms. A detailed history that includes geographical locations, identifies exposure to risk in a systematic fashion, and chronicity of symptoms is essential in making a diagnosis.
A high index of suspicion, along with exposure history, high-resolution computerized tomography (HRCT) scan, and marked lymphocytosis are the main factors leading to the diagnosis of farmer's lung. HRCT is a useful investigation in evaluating acute and chronic cases. Acute features are diffuse ground-glass opacities, centrilobular nodules, mosaic pattern, and air trapping. Chronic features include septal thickening, ground-glass opacities, fibrosis, bronchiectasis, usually with upper lobe predominance, and signs of pulmonary hypertension. Some patients may present with emphysema, and this may be a confounding feature in smokers.
Sputum cultures, antigen detection, and serum antibodies evaluation by ELISA can also be used. Serological studies are usually not useful markers, especially in chronic cases, as they may indicate mere exposure. One study shows that antibodies can persist even after quitting farming for ten years. Some patients may have antibodies without clinical features. This indicates a lack of consistent value of antibodies in diagnosis and followup.
Bronchoscopic evaluation with bronchoalveolar lavage (BAL) can be useful in some cases and may suggest a gross lymphocytosis and CD4/CD8 ratio reversal. Transbronchial biopsy (TBLB) may not be useful in all cases since it may miss the disease process. In nearly half of the cases, findings may be nonspecific. Transbronchial cryobiopsy (TBLC) is utilized for the evaluation of fibrotic diffuse interstitial lung diseases. Compared to TBLB, TBLC had a higher diagnostic yield for hypersensitivity pneumonitis.
Surgical lung biopsy will be the final diagnostic choice but may not be possible in a chronic patient who has fibrotic lung and hypoxia.
This highlights the usage of history, clinical examination, imaging, and bronchoscopic evaluation, indicating a lack of definite, standard criteria, and the necessity of an integrated approach.
In acute stages, removal of the patient from the surroundings is the most important step. Identifying other insults like pesticide poisoning is essential since treatment may differ. Steroids for the short term might give symptomatic improvement, but no functional improvement is seen in the longterm. Recent trials of usage of mycophenolate in hypersensitivity pneumonitis have resulted in improvement in DLCO, but further studies are needed.
Chronic cases may need supportive therapy like vaccinations, oxygen supplementation, diuretics, and noninvasive ventilation. The final treatment option would be lung transplantation, which may prolong life expectancy. Preventive aspects are important and grossly underutilized. The powered dust respirator helmet has proved to be useful not only for symptoms but also for functional improvement. Avoidance of work is the best measure but may not be possible in all due to financial issues. Persistence of symptoms in 2/3 of patients even after five years after acute episode indicates the need for strict primary preventive measures. Leaving the farm offers the best possible solution. Patients with five or more symptom recurrences usually have progressive and significant lung damage.
Differential diagnosis of farmer's lung is broad and challenging.
The prognosis of farmer's lung depends on the stage of identification of the disease as avoidance of further antigen exposure may result in the prevention of progression of the disease. Exacerbations have been shown to occur in 8% of cases after acute episodes. A mortality rate of 1% has been reported. The average lifespan was eight years after diagnosis.
The most common clinical features of hypersensitivity pneumonitis are fever, cough, and dyspnea. However, it can result in a myriad of complications. As a worst-case scenario, the patient can present with respiratory failure and shock and require ventilatory support and vasopressors.
The prevalence of farmer's lung can be reduced by the regular application of preventive measures, along with financial incentives. Farmworkers should be educated about processes such as efficient drying of cereals and hay before storage, using silage extensively, improving the ventilation of farm buildings, and introducing mechanical feeding systems. Individual farmworkers should also be taught how to spot the early symptoms of farmer's lung. All farmworkers should be encouraged to wear respirators and masks when handling moldy fodder.
Farmer's lung disease is an occupational illness of dairy and cattlemen in many countries. The diagnosis of the disease is usually overlooked because of the lack of patient or clinician awareness. It often poses a diagnostic dilemma as the patients may exhibit non-specific signs and symptoms such as dry cough, shortness of breath, fevers, myalgias, and joint pains during an acute episode. These symptoms can be mistaken for a viral illness. While history and physical exam may point towards interstitial lung disease, the underlying cause is difficult to know without proper imaging studies. The pulmonologist is usually the focal person involved in the treatment of patients with farmer's lung. It is, however, essential to involve an interprofessional team, including a radiologist, laboratory technicians, and nurses. The radiologist plays a vital role in determining the diagnosis. The interprofessional team should work together and coordinate with each other to optimize patient education and treatment. The outcomes of farmer's lung depend on the stage of identification of the disease, and avoidance of further antigen exposure may result in the prevention of progression of the disease. However, to improve outcomes of farmer's lung, consultation with an interprofessional group of specialists should be considered.
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