Atypical interstitial pneumonia
Atypical interstitial pneumonia (AIP) ('rye grass staggers', ‘fog fever’) is a multifaceted disease with several known causes or clinical presentations. Cases of AIP are often complicated with bacterial, viral, or mycoplasmal organisms.
Causes are attributed to cattle grazing on lush forages high in the amino acid tryptophan, resulting in toxic lung damage and pneumonia. The disorder occurs when cattle are abruptly moved from sparse dry summer pastures to lush irrigated pastures in the fall.
AIP commonly occurs as a herd outbreak with as many as 50% of cattle showing acute onset of respiratory distress. Outbreaks typically occur during the first 2 weeks of a pasture change, generally during the first few days.
Clinical signs include head tremors, stiff gait, and staggering. As the disease worsens, rapid, labored respiration is observed with head extended and held low. Adults are more susceptible than yearlings. Death occurs at any time during the course of clinical illness. Bacterial pneumonias consist of bronchopneumonia, fibrinous pneumonia, and pleuropneumonia as well as caseonecrotic, aspiration, and tuberculous pneumonias. Two major patterns of interstitial pneumonia are recognized in cattle, and verminous pneumonia is associated with Dictyocaulus viviparus infection.
During the acute phase, lesions consists of alveolar septal thickening and a massive outpouring of fibrin-rich proteinaceous fluid into alveolar spaces with formation of hyaline alveolar membranes mixed with few leukocytes (white blood cells). At about three days, proliferation of the epithelial cells (pneumocyte proliferation) lining alveoli commences and at this stage, there may be a mix of hyalin membranes and pneumocyte proliferation. As the lesions progress, pneumocyte proliferation dominates. During these stages, gross lesions include failure of the lungs to collapse when the chest cavity is opened; the lungs have a meaty texture, and edema and/or emphysema (gas bubbles) distend interlobular septae.
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