Allergic pneumonitis is an acute or chronic hypersensitivity reaction of the lungs and small airways.

Etiology

An underlying etiology is rarely determined in pulmonary hypersensitivity reactions in dogs and cats. Type I or immediate hypersensitivity is probably the most common mechanism, although Type III and IV mechanisms may also be involved (see immunologic diseases, see The Biology of the Immune Systemet seq). The cellular infiltrate is typically eosinophilic; however, mixed inflammatory infiltrates consisting of mononuclear cells, eosinophils, and neutrophils, or predominantly lymphocytic infiltrates can be seen.

Pulmonary infiltration with eosinophilia (PIE, see Immunologic Diseases: Localized Anaphylactic Reactions) is a group of diseases associated with both pulmonary-associated and peripheral eosinophilia. Not all types of allergic pneumonitis, however, are associated with PIE. Causes of PIE include migrating parasites, reaction to microfilariae of heartworms, lungworms, chronic bacterial or fungal infections (eg, histoplasmosis, aspergillosis), viruses, external antigens, and unknown precipitating factors. Canine heartworm (see Heartworm Disease) pneumonitis occurs when dogs become sensitized to microfilariae. A similar reaction may be seen in cats with heartworms. Migrating intestinal parasites and primary lung parasites may induce either subclinical or mild signs of allergic pneumonitis.

Pulmonary nodular eosinophilic granulomatous syndrome is a rare, severe PIE-like syndrome occurring in dogs and most often associated with heartworm infection. In this condition, a severe granulomatous hypersensitivity reaction to microfilariae (or other antigen) results in mixed alveolar and interstitial pulmonary infiltrates plus variably sized, multiple pulmonary nodules scattered throughout the lung fields. Associated pathology may include eosinophilic granulomatous lymphadenitis, tracheitis, tonsillitis, splenitis, enteritis, gastritis, and pericholangitis. Pulmonary hypersensitivity also may be caused by drugs and reactions to inhaled allergens; however, this is poorly documented in small animals.

Clinical Findings

Chronic cough is the most common sign. It may be mild or severe, productive or nonproductive, and progressive or nonprogressive. Weight loss, tachypnea, dyspnea, wheezing, exercise intolerance, and occasionally hemoptysis may be seen. Severely affected animals may exhibit moderate to severe dyspnea and cyanosis at rest. Auscultation varies from unremarkable to increased breath sounds, crackles, or wheezes. Fever is usually absent. The degree of dyspnea and coughing is related to the severity of inflammation within the airways and alveoli.

Diagnosis

This is based largely on history and on radiographic and clinicopathologic findings. Thoracic radiographs frequently show irregular patchy alveolar infiltrates and increased bronchial and interstitial markings. Radiographic evidence of heartworm disease or parasitic pulmonary disease may suggest an underlying etiology. Typical hematologic changes are mild leukocytosis, variable peripheral eosinophilia (4–50%), and occasionally basophilia. Fecal analysis and an occult heartworm test are indicated when lung parasitism or heartworm disease is suspected. Bronchoalveolar lavage for cytologic analysis, culture, and detection of larval forms is often helpful. In allergic pneumonitis, bronchoalveolar lavage cytology generally reveals a predominance of eosinophils. Bacterial cultures of aseptically collected lavage specimens are commonly negative.

Treatment

When an underlying cause can be found, elimination of the offending agent and a short-term course of glucocorticoids resolves the problem. Prednisolone beginning at 1–2 mg/kg, PO, tapered over 10–14 days is often sufficient. When PIE is secondary to heartworm disease or pulmonary parasites, treatment with prednisolone before or during treatment for the parasite controls the pulmonary signs. When an underlying etiology cannot be determined, prolonged therapy with prednisolone for 3–12 wk is often required. When severe bronchoconstriction is suspected, bronchodilators or β2-agonists may be helpful. Severely dyspneic animals may require short-term oxygen therapy.

Last full review/revision March 2012 by Ned F. Kuehn, DVM, MS, DACVIM; Steven L. Marks, BVSc, MS, MRCVS, DACVIM