Pleural effusion is a common cause of dyspnea in dogs and cats. It is classically an expiratory or mixed dyspnea with absence of normal lung sounds ventrally. In some animals a "fluid line" can be detected, where lung sounds are absent ventral to an imaginary horizontal line and increased dorsal to the line. Clinical sings are more severe with larger volumes of fluids and when fluid accumulates acutely. Clinical approach is based on chest radiographs and thoracocentesis. Radiographs confirm the presence of fluid and subjectively access the amount, as well as potential causes (e.g., heart failure, neoplasia), whereas thoracocentesis provides a sample for analysis. Severely dyspneic patients should be taped before radiographs are obtained. Removal of even small amounts of fluid may substantially decrease the animal's ventilatory effort. Fluid inside the pleural cavity can be classified as transudate (protein < 2.5 g/dl, cells < 1,500/µl), modified transudate (protein: 2.5-4.0 g/dl, cells 1,000-7,000/µl, mostly mesothelial cells and macrophages), exudates (protein > 3 g/dl, cells > 7,000/µl, mostly degenerate neutrophils), chyle (protein variable, cells < 10,000/µl, mostly lymphocytes) or blood (protein, cells and PCV similar to peripheral blood). Cats with FIP can have a fluid with elevated protein (> 5 g/dl), but not very cellular (< 10,000/µl).

Pure transudate is more likely caused by hypoalbuminemia, but it may also occur in patients with bilateral heart failure or neoplastic effusions, although the latter two are usually present a modified transudate. A modified transudate can also occur in patients with effusions secondary to vasculitis, diaphragmatic hernia and lung lobe torsion. Exudate is found in patients with pyothorax, cats with FIP or in some cases of vasculitis. Multiple organisms are often cultured in patients with bacterial pyothorax. In cats, obligate anaerobes or Pasteurella spp. are most common, whereas in dogs obligate anaerobes or Actinomyces and Nocardia spp are frequently cultured. Chylothorax is observed when there is interference with lymphatic drainage or from increased lymphatic flow. Heart failure should be considered as a differential in patients with chylothorax, particularly in cats.

Therapy of pleural effusion consists of maintaining the chest free of fluids and addressing the underlying disease. Some patients may require placement of a chest tube. Antibiotics and pleural lavage are necessary in patients with pyothorax. Surgery may be needed if there is a foreign body or a lung abscess. Chylothorax can be managed medically by feeding a low-fat diet, performing needle thoracentesis as needed, and using a benzopyrene such as Rutin. Surgery is indicated in patients that do not respond to medical therapy.