Hypersensitivity pneumonitis is an inflammation of the lungs that occurs from exposure to certain allergens. It is a complex syndrome rather than a single disease, and symptoms can vary in appearance and intensity. Hypersensitivity pneumonitis usually affects the tiny air sacs of the lungs (alveoli).

People develop hypersensitivity pneumonitis after being sensitized during repeated exposures to an allergen. Organic dust, especially from bird feces, is the most common source of sensitization and symptoms. Other sources of this dust include animal proteins (including dander, urine and feces), molds, plants, hay and foods.

There are two forms of hypersensitivity pneumonitis:

• Acute hypersensitivity pneumonitis. Usually occurs within four to 12 hours after exposure to an allergen. Symptoms may persist for up to 18 hours or longer, but usually begin to resolve soon after exposure to the allergen ceases.
  
  
• Chronic hypersensitivity pneumonitis. A result of several bouts of the acute form of the disease, it can lead to a scarring of the lung tissue (pulmonary fibrosis) that often is not reversible.

Early diagnosis and treatment of hypersensitivity pneumonitis is essential. The prognosis is excellent if treatment begins when the disease is in its acute phase. However, once the disease becomes chronic, scarring of the lungs (fibrosis) can set in. This is often irreversible and can be debilitating for the patient.

Hypersensitivity pneumonitis is an inflammation of the lungs that usually affects the tiny air sacs (alveoli). It is most often caused by an immune system reaction to tiny organic (living) dusts or vapors in the form of bacteria, mold or fungi. Repeated exposure to the particles sensitizes the person to the allergen. In some cases, the source of these particles can also be inorganic (not derived from animals or plants).

Widespread inflammation of the primary lung tissues (parenchyma) and airways is the chief feature of hypersensitivity pneumonitis.

There are two categories of hypersensitivity pneumonitis:

• Acute hypersensitivity pneumonitis. The sensitization period to the allergen varies from months to years. Following sensitization, the symptoms usually occur within four to 12 hours after exposure to an allergen. Symptoms may persist for up to 18 hours or longer, but usually begin to resolve soon after exposure to the allergen ceases.
  
  
• Chronic hypersensitivity pneumonitis. A result of several bouts of the acute form of the disease, it can lead to a scarring of the lung tissue (pulmonary fibrosis) that often is not reversible.

Hypersensitivity pneumonitis is often found in people who keep birds as pets. People also commonly contract the condition on the job after being repeatedly exposed to certain allergens. Occupations with a high risk for the disorder include:

• Bird handlers
  
• Farmers and cattle workers
  
• Ventilation workers
  
• Veterinarians and animal handlers
  
• Grain and flour processors and loaders
  
• Lumber mill workers and paper and wallboard manufacturers
  
• Plastic manufacturers, painters and electronics industry workers
  
• Textile workers

In addition, hypersensitivity pneumonitis may be contraced through fungi and bacteria in air conditioners, heating systems, humidifiers and hot tubs.

Hypersensitivity pneumonitis is known by many different names, most of which reflect the occupation or allergen most closely related to symptoms. Examples include:

• Farmer’s lung disease. From exposure to bacteria in moldy hay.
  
  
• Pigeon breeder’s disease. From exposure to protein particles in bird droppings.
  
  
• Mushroom worker’s disease. From exposure to moldy compost.
  
  
• Cheese worker’s lung. From cheese mold.
  
  
• Malt worker’s lung. From moldy malt.
  
  
• Paprika splitter’s lung. From paprika dust.
  
  
• Chemical worker’s lung. From the manufacture of plastics, polyurethane foam and rubber.
  
  
• Mollusk shell hypersensitivity. From shell dust.
  
  
• Sauna taker’s disease. From exposure to mold in wet containers.
  
  
• Bagassosis. From exposure to moldy sugar cane.

There are several conditions triggered by inhalation of organic agents that feature symptoms similar to those of hypersensitivity pneumonitis. They include:

• Inhalation fever. Symptoms include fever, chills, headaches and myalgias (muscle pain or tenderness). However, significant pulmonary impairment is not present.
  
  
• Organic dust toxic syndrome. Results from exposure to airborne particles contaminated with toxin-producing fungi (mycotoxins). Symptoms include fever, chills and myalgias. This differs from hypersensitivity pneumonitis in that sensitization does not need to occur prior to a reaction.
  
  
• Chronic bronchitis. Often the result of chronic obstructive pulmonary disease (COPD), the most common respiratory disorder among those who work in agriculture.

Hypersensitivity pneumonitis is most often triggered by exposure to organic (living) dusts or vapors in the form of bacteria, mold or fungi. It is repeated exposure over a period of time, which can vary from months to years, that triggers the sensitization to the allergen.

Thermoactinomyces bacteria are a major source of the disease. They are found in damp hay and other vegetation, and in air-conditioning and humidifying systems. When these substances vaporize, they can circulate and trigger a form of the disease called ventilation pneumonitis.

Variations of hypersensitivity pneumonitis and their antigens include:

• Farmer’s lung. Saccharopolyspora rectivirgula.
  
• Bagassosis. Thermoactinomyces sacchari.
  
• Grain handler’s lung. S rectivirgula, Thermoactinomyces vulgaris.
  
• Humidifier/air-conditioner lung. S rectivirgula, T vulgaris.
  
• Pigeon breeder lung. Avian or animal proteins.
  
• Cheese worker’s lung. Penicillium casei.
  
• Malt worker’s lung. Aspergillus clavatus.
  
• Paprika splitter’s lung. Paprika dust.
  
• Wheat weevil. Sitophilus granarius.
  
• Mollusk shell hypersensitivity. Sea snail shells.
  
• Chemical worker’s lung. Trimellitic anhydride, diisocyanate, methylene diisocyanate.

Symptoms of acute hypersensitivity pneumonitis usually occur after heavy exposure to an allergen and include:

• Cough
• Fever
• Chills
• Shortness of breath
• Chest tightness
• Headache
• Body aches
• Malaise (general ill feeling)
• Feeling rundown
• Nausea

These symptoms can persist for up to 18 hours, but usually resolve once the individual is away from the allergen for a period of time. However, re-exposure to the allergen is likely to trigger symptoms again.

Symptoms of chronic hypersensitivity pneumonitis are often initially milder than in the acute form of the disease. They are more likely to appear after repeated, prolonged exposure to lower doses of an allergen. They include:

• Feeling easily winded, particularly after exertion
• Shortness of breath
• Cough
• Dry cough
• Lack of appetite
• Weight loss

Although the symptoms initially appear mild in chronic hypersensitivity pneumonitis, the hidden damage can be severe and include scarring of the lungs (fibrosis). This is a largely irreversible condition that can lead to death. More advanced symptoms of chronic hypersensitivity pneumonitis also include substantial weight loss and muscle wasting.

A physician will conduct a physical examination and compile a patient medical history and list of symptoms in attempting to diagnose hypersensitivity pneumonitis.

Hypersensitivity pneumonitis can be easily mistaken for a case of viral pneumonia. However, hypersensitivity pneumonitis is suspected if the patient repeatedly displays symptoms after exposure to the same environment. Crackles (rales) from the lungs are often heard when a stethoscope is used to examine the chest of a patient with hypersensitivity pneumonitis.

Tests used to diagnose the disease include:

• Chest x-rays. Small, diffuse nodules may appear on an x-ray of a patient with symptoms of acute hypersensitivity pneumonitis. These will disappear once symptoms subside. Evidence of chronic hypersensitivity pneumonitis may be evident on x-rays in the form of scarred lung tissue.
  
  
• Pulmonary function tests. During a bout of hypersensitivity pneumonitis, test results are likely to show a marked decline in forced vital capacity, total lung capacity and preserved airflow.
  
  
• Complete blood count. This is a screening test often used to diagnose various disorders. The presence of hypersensitivity pneumonitis is likely to be indicated by an increase in immunoglobulins and white blood cells.
  
  
• Hypersensitivity pneumonitis antibody panels and skin testing. These can be done to identify the specific offending molds, bacteria and particles.
  
  
• Aspergillus precipitins test. A laboratory test that detects antibodies in the blood resulting from exposure to the fungus Aspergillus.
  
  
• High-resolution CT scan of the chest. A test that allows for multiple x-rays to be taken from different angles around the patient.
  
  
• Bronchoscopy with washings or transtracheal biopsy. This is a procedure in which a bronchoscope (a flexible fiber optic instrument used to view the airways) is inserted through the nose or mouth and lung-tissue specimens are collected.

In some cases, a challenge test may be performed to pinpoint the responsible allergen. A small amount of the suspected allergen will be introduced to the patient, and any reaction will be studied. Challenge tests can produce potentially dangerous allergic reactions and should only be performed in a proper healthcare setting under the guidance of a physician.

Once the allergen responsible for symptoms has been identified, avoidance will be the chief form of treatment. Patients should try to limit or eliminate exposure to the allergen. In some cases, this may require a change of occupation. In other cases, steps can be taken to reduce exposure, such as:

• Wearing a mask when exposed to the allergen
  
• Improving ventilation in an environment where the allergen is present
  
• Using an air filtration system
  
• Regularly cleaning humidifiers and heating and cooling systems

Chronic pneumonitis can be difficult to treat. Corticosteroids may be used to try and reduce inflammation. However, studies have shown that while this therapy helps to relieve symptoms initially, long-term use of corticosteroids has a negligible effect on symptoms.

Preparing questions in advance can help patients to have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions regarding hypersensitivity pneumonitis:

1. Do my symptoms indicate hypersensitivity pneumonitis?
   
   
2. What tests will you use to determine if I have hypersensitivity pneumonitis?
   
   
3. What type of hypersensitivity pneumonitis do I have?
   
   
4. What may have caused me to develop hypersensitivity pneumonitis?
   
   
5. What are my treatment options?
   
   
6. When can I expect my symptoms to subside?
   
   
7. Does this condition pose a danger to my overall health?
   
   
8. What steps can I take to prevent symptoms related to hypersensitivity pneumonitis?
   
   
9. Do you recommend that I change my occupation?