Merck Manual

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Pet Owner Version

Allergic Pneumonitis in Dogs


Ned F. Kuehn

, DVM, MS, DACVIM, Michigan Veterinary Specialists

Reviewed/Revised Jun 2018 | Modified Oct 2022

Allergic pneumonitis is an acute or chronic allergic reaction of the lungs and small airways. The lungs “overreact” to the presence of a parasite or other irritant, causing inflammation and a longterm cough. There is often a higher than normal number of white cells called eosinophils in the blood. The underlying cause is rarely determined in these reactions in dogs.

Eosinophilic bronchopneumopathy Eosinophilic Bronchopneumopathy (formerly known as PIE Syndrome [Pulmonary Infiltration with Eosinophilia]) In a Type I reaction, the animal has been previously exposed to an antigen and produces an excess of antibodies. If this antigen appears in the blood, the reaction can be either body-wide (such... read more , previously known as PIE syndrome, is a group of diseases associated with increased numbers of eosinophils in the blood and lungs. This group of diseases is one type of allergic pneumonitis. Causes of eosinophilic bronchopneumopathy include parasites, chronic bacterial or fungal infections, viruses, external antigens, and unknown factors.

Canine heartworm pneumonitis occurs when dogs become sensitized to the prelarval stage (microfilariae) of heartworms. Migrating intestinal parasites and primary lung parasites may cause mild signs of allergic pneumonitis. Pulmonary nodular eosinophilic granulomatous syndrome is a rare, severe allergic reaction occurring in dogs and is most often associated with heartworm Heartworm Disease in Dogs Heartworm disease is a potentially fatal, but preventable, infection caused by a worm parasite, Dirofilaria immitis. The organism is transmitted by mosquitoes, which carry the heartworm... read more infection. It may also occur with advanced and uncontrollable eosinophilic bronchopneumopathy. In this condition, a severe immune response causes masses to form throughout the lungs. The reaction can also affect other organs throughout the body.

A chronic cough is the most common sign of allergic pneumonitis. The cough may be mild or severe, and it may be dry (nonproductive) or contain secretions (productive). Weight loss, rapid or difficult breathing, wheezing, intolerance to exercise, and occasionally coughing up of blood may be seen. Severely affected animals may have bluish mucous membranes and trouble breathing when resting. The degree of labored breathing and coughing is related to the severity of inflammation within the airways and alveoli.

The diagnosis is based largely on the animal’s history and signs, chest x-rays, and laboratory tests. Evidence of heartworm disease or parasitic lung disease on x-rays may suggest these as an underlying cause of the allergic reaction. Blood tests show an increase in several types of white blood cells, indicating inflammation or infection. Fecal analysis and a heartworm test are performed when lung parasites or heartworms are suspected. A specialized procedure called a bronchoalveolar lavage may be recommended to collect samples (such as white blood cells, parasites, or bacteria) from the lungs.

When an underlying cause is identified, elimination of the offending agent and a short-term course of a corticosteroid usually resolves the problem. When heartworm disease or lung parasites appear to be the cause, corticosteroid treatment before or during treatment for the parasite controls the lung signs. If an underlying cause cannot be determined, prolonged corticosteroid therapy is often required. When severe airway constriction is suspected, bronchodilators or beta2-agonist medications may be helpful. Animals with severe shortness of breath may require oxygen therapy.

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