Farmer’s Lung

What is farmer’s lung?

Farmer’s lung is an inflammatory disorder of the lung caused by an immune reaction. It is not an infection but more of an allergic reaction. Farmer’s lung is one type of hypersensitivity pneumonitis associated with an occupational hazard. In farmer’s lung, the reaction arises with inhalation of hay, straw and compost that are tainted with molds (fungi). Although the condition is mainly seen in farm workers, especially with prolonged exposure to contaminated material, it can be occur in a person who is suddenly exposed to large amounts of tainted matter. Chronic cases may lead to permanent lung damage.

How common is farmer’s lung?

Farmer’s lung is only one type of hypersensitivity pneumonitis. Overall, hypersensitivity pneumonitis affects up to 7% of the farming population in the US. Farmer’s lung accounts for 11% of reported cases of hypersensitivity pneumonitis in the United States. The incidence has been declining due to a greater awareness about the condition. However, in some years over 500 cases per 100,000 persons have been reported among the farming community.

Lung Disease in Farmer’s

Molds are a broad term to refer to various types of fungi. As with most fungi, it tends to thrive on organic material that is dead. In farmer’s lung, the molds are inhaled into the lungs and the immune system reacts to these foreign proteins (antigens). However, the fungi do not cause an infection of the lungs. Instead their presence serves as a trigger for increased immune activity. Although any person may be exposed to these antigens in the course of their life, the development of hypersensitivity pneumonitis depends on the duration and intensity of exposure. Farm workers due to the nature of their occupation are exposed to a larger quantity of these antigens, more frequently and over a longer period of time.

The immune system forms antibodies against these antigens in a bid to coordinate a defensive immune response in the event of an infection. Binding of the antibodies to these antigens and the formation of the antigen-antibody complex activates the complement system of immunity. It attracts immune cells to the area to fight off the foreign agent. This is known as a type III hypersensitivity reaction and inflammation sets in almost immediately. After a period of time, a certain type of immune cell known as the T-cell, then specifically targets these antigens and elicits inflammation. This is known as type IV hypersensitivity reaction. It persists for prolonged periods and is responsible for permanent lung damage due to extensive scarring (pulmonary fibrosis) associated with chronic inflammation.

Types of Farmer’s Lung

Farmer’s lung can be classified by three stages – acute, subacute or chronic.

  • Acute farmer’s lung arises with short term exposure to a large amount of  hay or compost contaminated with the causative molds. Any person, even non farm workers may develop this type of farmer’s lung. It starts up as soon as 12 hours after exposure and resolves once exposure is discontinued.
  • Subacute farmer’s lung develops gradually often with little or no symptoms until weeks or months. It is more likely to arise in farm worker’s but can also affect non farm worker’s who have repeated contact with contaminated material like hay or compost on a regular basis.
  • Chronic farmer’s lung develops with continuous exposure to the antigens over a prolonged period of time. This is almost entirely seen in farm workers.  It is often marked by recurrent episodes of acute attacks and ultimately leads to permanent lung damage.

Signs and Symptoms

In pneumonitis, the lung tissue (air sacs) and the most terminal bronchioles are inflamed and with time irreversibly damaged. Symptoms of respiratory inflammation and damage are therefore present, along with the effects of the decreased lung function and generalized symptoms.

Acute Farmer’s Lung

The most intense symptoms are seen with acute farmer’s lung and it presents in a similar manner to respiratory tract infection. These symptoms include :

  • Fever
  • Chills
  • Tight chest
  • Difficulty breathing
  • Cough (non-productive)
  • Wheezing (sometimes)
  • Headache
  • Malaise

Symptoms resolve spontaneously provided that exposure to the antigen is eliminated. If not, acute farmer’s lung can progress to subacute or even chronic cases. In rare instances, and with exposure to a very large amount of the antigen, acute respiratory failure can arise.

Subacute Farmer’s Lung

  • Chronic cough
  • Dyspnea
  • Loss of appetite
  • Weight loss
  • Fatigue

Chronic Farmer’s Lung

  • Persistent cough
  • Severe breathing difficulties :
    – Shortness of breath with activity (exertional dyspnea) initially.
    – Dyspnes even at rest in later stages.
  • Clubbing
  • Generalized fatigue

Causes of Farmer’s Lungs

Farmer’s lung is not an infectious condition. Pneumonitis refers to any type of lung inflammation and includes pneumonia, a term usually reserved for a lung infection. However, pneumonia is often the preferred term these days for lung inflammation and is also used in instances of non-infectious lung inflammation as is seen with aspiration.

In farmer’s lung, it is the presence of molds that is inhaled which is the cause of the condition. It is mainly the Actinomyces and Aspergillus species of fungi that is responsible for farmer’s lung. The thermophilic actinomycetes, apart from the Aspergillus species, which are the main causative factors includes Saccharopolyspora rectivirgula, Thermoactinomyces vulgaris, Thermoactinomyces viridis and Thermoactinomyces sacchari.

Thermophilic fungi tend to thrive in hot and humid environments. However, the condition often tends to occur in the colder periods like winter when stored hay is used. The fungi by this season had sufficient time to taint large amounts of the stored hay. Although it is also seen in compost and sugar cane, hay is by far the most common medium. Grain may also be tainted with molds that can lead to farmer’s lung. Livestock farmers are a greater risk and farmer’s lung is more often seen among dairy farmers and farm workers who directly handle the hay.

Tests and Diagnosis

The symptoms of farmer’s lung may be indicative of various other more common respiratory conditions. Occupational asthma, pneumonia (bacterial, fungal or viral) and sarcoidosis need to be ruled out as possible causes due to the similarity in clinical presentation. Blood tests, sputum cultures and imaging studies such as a chest x-ray and CT scan are used to exclude other diseases and note lung changes like pulmonary fibrosis that may seen in chronic cases. Farmer’s lung is diagnosed once other respiratory conditions are excluded, and certain major and minor criteria are met based on the findings of a clinic examination and diagnostic investigations.

Farmer’s Lung Treatment

There is no specific treatment for farmer’s lung and the cornerstone of managing the condition is avoidance of the antigens. Corticosteroids are the only medication indicated for the treatment and management of farmer’s lung. Oxygen therapy may be necessary for patients with respiratory failure and severe impairment of lung function. In the acute stages treatment may not be necessary as the condition resolves spontaneously with removal of the antigens. However, in severe acute cases, corticosteroids may help until the condition resolves.

Long term use of corticosteroids is needed in chronic stages of farmer’s lung. Patients with chronic farmer’s lung need to be aware that the condition is irreversible since there is scarring of the lung tissue, which is permanent. Nevertheless they should be encouraged to avoid the antigen as far as possible. Patients with extensive lung damage may have a poor response to corticosteroids and there is a death rate of about 90% in 5 years in these patients. Farmers and farm workers who smoke often have a worse prognosis than non-smokers.


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