Hypersensitivity pneumonitis is a condition that appears to be similar to "farmer’s lung disease" in people and occurs in both acute and chronic forms in adult cattle. The human and bovine forms of the disease may coexist on problem farms because of common exposure to moldy hay dust and spores of Micropolyspora faeni. Affected cattle are bright and alert but have severe tachypnea and coughing. Prognosis and response to treatment is usually poor. Prevention is largely achieved by limiting exposure to dust from moldy hay.
Another form of pulmonary hypersensitivity in cattle is caused by exposure to moldy hay, containing spores that elicit an allergic response in certain individuals. This syndrome most commonly occurs when feeding indoors, where ventilation is poor. Hypersensitized cattle exhibit dyspnea, coughing, and become less productive. If they can be transferred outdoors, signs may resolve, but sensitized animals will likely have recurrence throughout their lives whenever exposed to the moldy hay allergen. Severe interstitial fibrosis of the lung can result. There is no specific treatment.
Etiology of Hypersensitivity Pneumonitis in Cattle
Hypersensitivity pneumonitis occurs when sensitized individuals inhale antigens from thermophilic actinomycetes, commonly the spores of Micropolyspora faeni. The actinomycetes proliferate in vast numbers in hay, grain, or other vegetable material that has overheated to ~65°C (150°F) after damp storage (30%–40% moisture content). Dust that contains large numbers of spores is released when this moldy hay is shaken. The small size (1 mcm) of the spores allows them to reach the smallest airways and alveoli to provoke a reaction that has been termed a “hypersensitivity pneumonitis”; this is considered to be predominantly a Type III hypersensitivity reaction, although a Type IV hypersensitivity component is suspected ( see The Biology of the Immune System et seq).
Affected herds are found in areas where rainfall is usually appreciable during the haymaking season, suggesting that a clinical problem may arise only after repeated sensitization and challenge from the spores. Clinical disease tends to arise during the latter half of the winter feeding period and usually only when moldy hay is fed indoors. Under such circumstances, serum antibodies (usually detected by immunodiffusion) to M faeni are widespread among adult cattle by the end of each winter feeding period, and many apparently healthy cattle are seropositive. By contrast, few adult cattle are seropositive on other farms on which “good” hay or grass silage is fed.
Clinical Findings of Hypersensitivity Pneumonitis in Cattle
Cattle develop clinical signs to the acute form of hypersensitivity pneumonitis over a period of weeks. Usually, only the most severely affected cases are noticed. There is respiratory distress, anorexia, and agalactia in animals ≥ 5 years old. Coughing and pyrexia also occur, and adventitious sounds are occasionally heard on auscultation. Death is rare.
The chronic disease usually has a higher morbidity; in most instances, the clinical signs are weight loss, poor production, and persistent coughing. Affected cattle are fairly bright and eat reasonably well; however, tachypnea, hyperpnea, and coughing are widespread. Auscultation may reveal cranioventral crackles. Exercise intolerance may occur, and congestive cardiac failure can develop if pulmonary fibrosis is widespread.
Lesions on Post-Mortem Examination
The macroscopic lesions are often unremarkable; usually, there is mild peripheral lobular overinflation with diffusely scattered, small, gray, subpleural spots. Although transient pulmonary edema may be a feature of severe acute cases, the histologic lesions consistently found are interalveolar cellular infiltration, epithelioid granulomata, and bronchiolitis obliterans. In some chronic cases, small foci of alveolar epithelial hyperplasia and metaplasia with interstitial fibrosis are found. These areas may extend to include most, if not all, of the lung substance to produce cases clinically indistinguishable from diffuse fibrosing alveolitis. Circumstantial evidence suggests that some cases of diffuse fibrosing alveolitis are the end stage of hypersensitivity pneumonitis.
Treatment and Control of Hypersensitivity Pneumonitis in Cattle
Dexamethasone or NSAIDs
Avoidance of moldy hay
Because it is often impossible to completely shield cattle with hypersensitivity pneumonitis from further challenge, most recover only partially after dexamethasone treatment (1 mg/5–10 kg body weight). However, improvement is usually marked when cattle are turned out in the spring. Prevention is difficult in areas where hay is likely to be wet during the curing process and it is not possible to alter the feeding regimen.
Key Points
Hypersensitivity pneumonitis is a chronic respiratory disease in housed cattle.
Chronic exposure to spores in moldy dusty hay is associated with the disease.
Minimize dust and molds for prevention.
