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 chronic 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.

Pulmonary infiltration with eosinophilia, known as PIE syndrome (see Immune Disorders of Dogs: PIE Syndrome (Pulmonary Infiltration with Eosinophilia)), is associated with allergic pneumonitis. Causes of PIE syndrome 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 infection (see Heart and Blood Vessel Disorders of Dogs: Heartworm Disease in Dogs).

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 at rest. 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.

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.

Last full review/revision July 2011 by Ned F. Kuehn, DVM, MS, DACVIM; Neil W. Dyer, DVM, MS, DACVP; Joe Hauptman, DVM, MS, DACVS; Steven L. Marks, BVSc, MS, MRCVS, DACVIM; Stuart M. Taylor, PhD, BVMS, MRCVS, DECVP