CHAPTER 64 Granulomatous Lung Disease
Granulomatous lung disease in horses encompasses multiple causative disorders, including bacterial and fungal infection, environmental disease, disease caused by immune complexes generated in association with adverse events such as vaccine or drug reactions (including responses to immune stimulants), and neoplasia. Recently a condition known as equine multinodular pulmonary fibrosis was described that can initially appear as a granulomatous disease (see Chapter 65, Equine Multinodular Pulmonary Fibrosis).
In many horses with granulomatous lung disease, the condition is determined to be idiopathic, and failure to identify an underlying cause often results in random treatments that are subjective and can add to the overall poor long-term prognosis. In human medicine, granulomatous lung disorders have been more extensively defined and include conditions such as eosinophilic granuloma, Wegener’s granulomatosis, bronchocentric granulomatosis, and allergic granulomatosis, although idiopathic pulmonary fibrosis remains a more common finding. In veterinary medicine, few of the granulomatous lung diseases are common, with the exception of diseases that are endemic in a region (e.g., silicosis, hairy vetch toxicosis) or are suspected type IV hypersensitivities. Diagnostic testing often requires ancillary equipment and aggressive techniques such as lung biopsy followed by specialized histopathologic interpretation. Secondary bone disease such as hypertrophic osteoarthropathy or osteoporosis-associated pulmonary silicosis may be present or develop (see Chapter 66, The Silicosis and Osteoporosis Syndrome).
Granulomatous lung disease, whether from a known cause or idiopathic, usually is found in middle-aged to older horses that eventually develop chronic deep coughs, exercise intolerance, mucopurulent nasal discharge, and, terminally, weight loss associated with dyspnea. The presence of granulomatous lung disease or granulomas can be an early incidental finding, and months to years may pass before clinical compromise becomes significant. Pleural effusion is not a clinical feature of granulomatous lung disease. The lung disorder may be unilateral, and normal pulmonary function can be provided by the unaffected lung. Fever may be detected and can be a result of either primary or secondary infection. Weight loss is an inconsistent finding and usually is not observed until vital sign values have become elevated.