Farmer's lung is the most common type of hypersensitivity pneumonitis. Farmer's Lung is a disease caused by breathing in spores from mouldy hay. It affects people who work with hay a lot. Farmer's lung is an hypersensitivity pneumonitis induced by the inhalation of biologic dusts coming from mouldy hay or other agricultural products. The immune response is most often initiated by exposure to thermophilic actinomycetes, which generates IgG-type antibodies that circulate in the bloodstream. The symptoms are serious, and once the allergic reaction begins, the person will always have the potential for symptoms with hay mould exposure.

Farmer's lung presents itself with a typical two-step allergic reaction. The first exposure occurs when spores are inhaled and the body responds by sending antibodies to the lungs where the spores have deposited. When a second exposure occurs, antigens react with the antibodies, causing white blood cells to gather in the lungs to attack the antigens. The white blood cells release chemicals, causing the blood flow to increase and lung tissue near the antigens are destroyed. This type of hypersensitive reaction causes swelling, inflammation, tissue damage, and difficulty breathing. There are three stages of Farmer's lung: Acute, Sub-Acute, and Chronic.

The signs and symptoms of Farmer's lung will vary depending on the stage of the disease. The signs and symptoms of Farmer's lung are the most severe within a 12 to 48 hour time period after exposure to the moldy crop. In some cases, symptoms may last for two weeks, or in cases where repeated exposure to moldy crops has occurred, farmers may develop chronic farmer's lung. Farmer's lung can be controlled. In milder cases that are detected early, avoiding contact with the molds will prevent further lung damage. Systemic corticosteroid administration and avoidance measures constitute the primary treatment for farmer's lung.

The only way to prevent this condition is to avoid contact with dust from moldy plant material. Avoid dusty work in confined areas. The only proven treatment for chronic farmer's lung victims is the avoidance of contact with moldy crop materials. When constructing new farm buildings or modifying older structures, keep facilities as open as possible. If necessary, a farmer's lung victim should quit farming, rather than becoming permanently disabled.

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